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Patterns of Brown-Headed Cowbird Parasitism in a Recently Invaded Area and Potential Mechanisms Limiting Cowbird Reproduction

Permanent Link: http://ufdc.ufl.edu/UFE0021542/00001

Material Information

Title: Patterns of Brown-Headed Cowbird Parasitism in a Recently Invaded Area and Potential Mechanisms Limiting Cowbird Reproduction
Physical Description: 1 online resource (129 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: ater, brood, cowbird, distribution, ejection, evolutionary, fecundity, florida, host, lag, molothrus, parasitism
Wildlife Ecology and Conservation -- Dissertations, Academic -- UF
Genre: Wildlife Ecology and Conservation thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Despite conservation implications surrounding recent Brown-headed Cowbird (Molothrus ater) expansion into the southeastern United States, there are few ecological data on cowbirds in Florida. Objectives for this project were to (1) quantify parasitism rates to determine the extent of brood parasitism and identity of host species and (2) test hypotheses explaining patterns of parasitism and cowbird distribution in Florida. I surveyed parasitism of 35 host species breeding in Florida through nest monitoring and observations of family groups with fledglings. Although cowbirds are moderately abundant in the study area, only 1.6% of 1117 nests and 6.1% of 278 family groups with fledglings were parasitized. While rates of parasitism are extremely low across the community and for individual hosts, my results suggest that cowbirds may preferentially parasitize a few host species nesting in the midstory and canopy. To help explain observed low cowbird parasitism rates, I tested egg rejection behavior of four common Florida songbird species known to reject parasitism elsewhere. I experimentally parasitized 78 nests using eggs obtained from a captive cowbird colony. Three of four species tested typically accepted experimental cowbird eggs, rejecting parasitism at significantly lower rates than those published in the literature for each species. Rejection behaviors do not appear to limit Brown-headed Cowbird reproduction in north-central Florida. I tested the hypothesis that reduced physiological fecundity of female Brown-headed Cowbirds contributed to low observed parasitism rates. I compared ovarian reproductive condition of breeding female cowbirds in Florida with those from Texas, where high parasitism rates have been recorded. Based on reproductive assays, I estimated that average female cowbirds in Florida and Texas have similarly high fecundity at 34 eggs per female per breeding season. Low observed cowbird parasitism frequencies combined with moderate abundance of cowbirds in north-central Florida presents a paradox that my data do not explain. Cowbird reproduction may be limited by high nest predation rates or limited diversity and abundance of host species. I present a summary of census and reproductive data that suggest that these mechanisms may contribute to observed patterns of cowbird parasitism in Florida.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Sieving, Kathryn E.
Local: Co-adviser: Robinson, Scott K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021542:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021542/00001

Material Information

Title: Patterns of Brown-Headed Cowbird Parasitism in a Recently Invaded Area and Potential Mechanisms Limiting Cowbird Reproduction
Physical Description: 1 online resource (129 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: ater, brood, cowbird, distribution, ejection, evolutionary, fecundity, florida, host, lag, molothrus, parasitism
Wildlife Ecology and Conservation -- Dissertations, Academic -- UF
Genre: Wildlife Ecology and Conservation thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Despite conservation implications surrounding recent Brown-headed Cowbird (Molothrus ater) expansion into the southeastern United States, there are few ecological data on cowbirds in Florida. Objectives for this project were to (1) quantify parasitism rates to determine the extent of brood parasitism and identity of host species and (2) test hypotheses explaining patterns of parasitism and cowbird distribution in Florida. I surveyed parasitism of 35 host species breeding in Florida through nest monitoring and observations of family groups with fledglings. Although cowbirds are moderately abundant in the study area, only 1.6% of 1117 nests and 6.1% of 278 family groups with fledglings were parasitized. While rates of parasitism are extremely low across the community and for individual hosts, my results suggest that cowbirds may preferentially parasitize a few host species nesting in the midstory and canopy. To help explain observed low cowbird parasitism rates, I tested egg rejection behavior of four common Florida songbird species known to reject parasitism elsewhere. I experimentally parasitized 78 nests using eggs obtained from a captive cowbird colony. Three of four species tested typically accepted experimental cowbird eggs, rejecting parasitism at significantly lower rates than those published in the literature for each species. Rejection behaviors do not appear to limit Brown-headed Cowbird reproduction in north-central Florida. I tested the hypothesis that reduced physiological fecundity of female Brown-headed Cowbirds contributed to low observed parasitism rates. I compared ovarian reproductive condition of breeding female cowbirds in Florida with those from Texas, where high parasitism rates have been recorded. Based on reproductive assays, I estimated that average female cowbirds in Florida and Texas have similarly high fecundity at 34 eggs per female per breeding season. Low observed cowbird parasitism frequencies combined with moderate abundance of cowbirds in north-central Florida presents a paradox that my data do not explain. Cowbird reproduction may be limited by high nest predation rates or limited diversity and abundance of host species. I present a summary of census and reproductive data that suggest that these mechanisms may contribute to observed patterns of cowbird parasitism in Florida.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Sieving, Kathryn E.
Local: Co-adviser: Robinson, Scott K.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0021542:00001


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PATTERNS OF BROWN-HEADED COWBIRD PARASITISM IN A RECENTLY INVADED
AREA AND POTENTIAL MECHANISMS LIMITING COWBIRD REPRODUCTION



















By

MATTHEW J. REETZ


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2008







































O 2008 Matthew J. Reetz




































To LeAnn, my favorite wife









ACKNOWLEDGMENTS

Invaluable field help was provided by Andrew Cook, Steve Daniels, Dan Dawson,

Elizabeth Bauman, Nia Haynes and Rebecca Hamel. I thank the Florida Department of

Environmental Protection, Danny Driver, David Armstrong, Joel McQuagge, Jesse Smalls, Steve

Coates, Mats Troedsson, and Bruce Christensen of the University of Florida (UF), and Geoff

Parks of the Gainesville Nature Operations Division for access to field sites. I am indebted to

Michael Avery, Eric Tillman, John Humphrey, Kandy Keacher and Michael Milleson of the

USDA National Wildlife Research Center for support with trapping and maintenance of captive

cowbirds. Brian Branciforte and Kate Haley of the Florida Fish and Wildlife Commission and

Monica Lindberg, Caprice McCrae, Laura Hayes and Sam Jones of UF provided valuable

logistical support. Research design of this project was improved through comments of various

avian discussion groups. My deepest thanks go to my committee members Michael Avery, Doug

Levey, David Steadman, and especially co-chairs Kathryn Sieving and Scott Robinson.

Research protocols D539 and D954 were approved by the UF Institutional Animal Care and Use

Committee. This work was made possible by the Florida Fish and Wildlife Conservation

Commission's program, Florida' s Wildlife Legacy Initiative, and the U.S. Fish and Wildlife

Service's State Wildlife Grants program (SWGO4-031). Finally, my thanks go to C. LeAnn

White for her steadfast support, especially when the light at the end of the tunnel grew dim.












TABLE OF CONTENTS


page

ACKNOWLEDGMENT S ............ ...... .__. ...............4....


LI ST OF T ABLE S ........._.___..... .__. ...............7....


LIST OF FIGURES .............. ...............8.....


AB S TRAC T ............._. .......... ..............._ 10...


CHAPTER


1 PATTERNS OF PARASITISM IN NORTH-CENTRAL FLORIDA, A REGION OF
RECENT COWBIRD EXPANSION ................. ......... ...............12......


Introduction................ ..... .. .. .... .......1
Patterns of Cowbird Expansion in the Southeast .............. .....................15
Obj ectives ................. ...............16.................
M ethods .............. ...............17....
Re sults ................ ...............18.................
D iscussion................ .. ..... ... .. .. ...............2
Cowbird Parasitism in North-Central Florida ............... ........... ..............2
Potential Limits on Parasitism Rates or Their Accurate Detection ............... ... .........._..22
Conclusions and Management Implications ................. ...............26................


2 RESPONSES OF FOUR SONGBIRD SPECIES TO EXPERIMENTAL COWBIRD
PARASITISM IN A RECENTLY INVADED AREA .............. ...............32....


Introduction............... ..............3
M ethods .............. ...............35....

Study Design .............. ...............35....
Selection of Study Species .............. ...............36....
Re sults............. ...............38___ .......
D iscussion.................. ......................3

Rej section Responses of Florida Hosts ...._.. ...._._._._ .........__. ...........3
Acceptance of Cowbird Eggs .............. ...............41....
Conclusions .............. ...............43....


3 DOES VARIATION IN FEMALE COWBIRD FECUNDITY EXPLAIN OBSERVED
LOW LEVELS OF PARASITIZATION? ................ ...............46................


Introduction.................. ... ...................4
Limitations to Cowbird Reproduction................. ......... .......4
Low Nest Parasitism in Florida: Low Cowbird Fecundity? ................ ........_._. ......48
Method s .................... ...............49.
Cowbird Collection .............. ...............49....













Reproductive As says .............. ...............51....
Clutch Size Estimates ................. ...............53........... ....

Fecundity Estimates............... ...............5
Re sults........._._..... ........_ ...............56....
Cowbird Collection .............. ...............56....

Reproductive As says .............. ...............57....
Clutch Size Estimates ................. ...............61........... ....

Fecundity Estimates................ ...............6
Additional Trapping Observations .............. ...............65....
D iscussion............... .... ............6
Clutch Size Estimates ................. ...............68._._. ......

Fecundity Estimates............... ...............6
Cowbird Collection .............. ...............71....
Conclusions .............. ...............73....


4 ALTERNATIVE HYPOTHESES FOR THE "SLOW" COWBIRD INVASION OF
FLORIDA ................. ...............84.................


Introduction............... .. ... ..........8
Host Community Composition ................. ...............86........... ....
Nest Predation ................. ...............92.................


APPENDIX


A STUDY SITE DESCRIPTION AND HO ST SPECIES SURVEYED ................. ...............1 02


LITERATURE CITED ................. ...............110......... ......


BIOGRAPHICAL SKETCH ................ ............. ............ 129...










LIST OF TABLES


Table page

1-1 Summary of nests and family groups for 32 species monitored in the study areas,
2004-2006.. ............ ...............30.....

2-1 Results of experimental parasitism of four common Florida species. ........._.... ..............45

3-1 Summary of means of morphometric data taken from breeding female Brown-headed
cowbirds collected in Florida and Texas, 2005-2007 and results of statistical
com prisons. ............. ...............75.....

3-2 Estimate of clutch size of 2005 Texas cowbirds. .............. ...............75....

3-3 Estimate of clutch size of 2006-2007 Florida cowbirds. ................ ................ ...._.76

4-1 Host species documented as breeders in Florida also known to have fledged Brown-
headed Cowbirds............... ...............10










LIST OF FIGURES


Figure page

1-1 Breeding Bird Survey data of Brown-headed Cowbirds detected per route in Florida
from 1966-2006............... ...............2

1-2 Estimated number of Brown-headed Cowbirds per Breeding Bird Survey route for
northern and southern Florida, 1966-2006 ................. ...............29...............

3-1 Location of Brown-headed Cowbird collection sites from April-August, 2005-2007. .....77

3-2 Number of female cowbirds collected per month at sites in north-central Florida in
2006 and 2007 ................. ...............77........... ....

3-3 Mean number of female cowbirds collected per trap day at sites in north-central
Florida from April to July, 2006-2007 ................. ...............78...............

3-4 Number of female cowbirds collected per month in Florida by trap and by pellet gun
from April-July 2007. ............. ...............78.....

3-5 Mean weekly mass of ovaries of all Florida and Texas female cowbirds collected
between 15 April and 15 July, 2005-2007. ............. ...............79.....

3-6 Mean weekly mass of oviducts of all Florida and Texas female cowbirds collected
between 15 April and 15 July, 2005-2007. ............. ...............79.....

3-7 Mean weekly mass of ovaries of breeding Brown-headed Cowbirds collected in
Florida, 2006-2007 ................. ...............80.................

3-8 Mean weekly mass of oviducts of breeding Brown-headed Cowbirds collected in
Florida, 2006-2007 ................. ...............80.................

3-9 Mean weekly mass of ovaries of breeding Brown-headed Cowbirds collected in
Texas, 2005. ............. ...............81.....

3-10 Mean weekly mass of oviducts of breeding Brown-headed Cowbirds collected in
Texas, 2005. ............. ...............81.....

3-11 Mass of female Brown-headed Cowbirds collected in Florida between 23 April and
12 July, 2006-2007................ ...............8

3-12 Mass of female Brown-headed Cowbirds collected in Texas between 19 April and 6
July, 2005. ............. ...............82.....

3-13 Mean area of post-ovulatory follicles (POFs) of Brown-headed Cowbirds collected
in Texas and Florida from April to June, 2005-2007................ ...............8










3-14 Estimated laying rate (eggs produced per day) of breeding cowbirds collected in
Florida and Texas during four periods of the cowbird breeding season. ................... ........83

4-1 Distribution of Brown-headed Cowbirds in Florida. Redrawn from Breeding Bird
Survey abundance map (1994-2003). ............. ...............97.....

4-2 Proportion of total abundance composed of species of varying quality as cowbird
hosts .............. ...............97....

4-3 Total number of host species of three types at locations in three regions. ................... .....98

4-4 Mean abundance of three types of Brown-headed Cowbird hosts in three regions. ..........98

4-5 Mean abundance of forest species that represent good quality cowbird hosts in
Illinois and Florida ................. ......... ........ ......... ........ ................99









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


PATTERNS OF BROWN-HEADED COWBIRD PARASITISM IN A RECENTLY INVADED
AREA AND POTENTIAL MECHANISMS LIMITING COWBIRD REPRODUCTION


By

Matthew J. Reetz

May 2008

Chair: Kathryn E. Sieving
Cochair: Scott K. Robinson
Major: Wildlife Ecology and Conservation

Despite conservation implications surrounding recent Brown-headed Cowbird (M~olothrus

ater) expansion into the southeastern United States, there are few ecological data on cowbirds in

Florida. Obj ectives for this proj ect were to (1) quantify parasitism rates to determine the extent

of brood parasitism and identity of host species and (2) test hypotheses explaining patterns of

parasitism and cowbird distribution in Florida.

I surveyed parasitism of 35 host species breeding in Florida through nest monitoring and

observations of family groups with fledglings. Although cowbirds are moderately abundant in

the study area, only 1.6% of 1 117 nests and 6. 1% of 278 family groups with fledglings were

parasitized. While rates of parasitism are extremely low across the community and for

individual hosts, my results suggest that cowbirds may preferentially parasitize a few host

species nesting in the midstory and canopy.

To help explain observed low cowbird parasitism rates, I tested egg rej section behavior of

four common Florida songbird species known to reject parasitism elsewhere. I experimentally

parasitized 78 nests using eggs obtained from a captive cowbird colony. Three of four species










tested typically accepted experimental cowbird eggs, rej ecting parasitism at significantly lower

rates than those published in the literature for each species. Rej section behaviors do not appear to

limit Brown-headed Cowbird reproduction in north-central Florida.

I tested the hypothesis that reduced physiological fecundity of female Brown-headed

Cowbirds contributed to low observed parasitism rates. I compared ovarian reproductive

condition of breeding female cowbirds in Florida with those from Texas, where high parasitism

rates have been recorded. Based on reproductive assays, I estimated that average female

cowbirds in Florida and Texas have similarly high fecundity at 34 eggs per female per breeding

season.

Low observed cowbird parasitism frequencies combined with moderate abundance of

cowbirds in north-central Florida presents a paradox that my data do not explain. Cowbird

reproduction may be limited by high nest predation rates or limited diversity and abundance of

host species. I present a summary of census and reproductive data that suggest that these

mechanisms may contribute to observed patterns of cowbird parasitism in Florida.









CHAPTER 1
PATTERNS OF PARASITISM IN NORTH-CENTRAL FLORIDA, A REGION OF RECENT
COWBIRD EXPANSION

Introduction

The Brown-headed Cowbird (M~olothrus ater) is one of three brood-parasitic cowbird

species in North America. It was prehistorically mainly found in the grasslands of the Great

Plains region of central North America where it foraged with bison herds and parasitized locally

breeding host populations. With the arrival of European settlers that caused habitat

fragmentation, destruction of bison populations, and establishment of permanent feeding areas,

cowbirds began expanding their range in response to habitat fragmentation and the replacement

of nomadic bison with largely sedentary cattle. Until the 1800s, Brown-headed Cowbirds were

uncommon in the eastern United States, probably because of a lack of suitable open feeding

habitats among large tracts of forest (Mayfield 1965). With the increase in human population,

however, cowbirds began to appear in states successively farther east (Lowther 1993) and local

records of cowbirds changed from rare or absent to common or abundant (Mayfield 1965).

Presently, the breeding range of the Brown-headed Cowbird includes the entire contiguous

United States and much of North America (Sauer et al. 2006).

Because of its recent, widespread expansion, the Brown-headed Cowbird has been of

great conservation concern. It is a generalist obligate brood parasite that continually parasitizes

new host species, and has a prodigious reproductive output with females laying 40 eggs per

season (Scott and Ankney 1980) in the nests of over 220 host species (Friedmann and Kiff

1985). Prior to human alteration of the landscape, cowbirds were naturally limited to host

species with which they were sympatric during the breeding season. The spread of the Brown-

headed Cowbird brought it into contact with a new suite of host populations and species, most of

which lacked recent historical exposure to brood parasitism. Significant songbird population









declines during the 20th century were thought to be due to the steady increase of cowbird

populations continent-wide (Mayfield 1977, Grzybowski et al. 1986, Robbins et al. 1989, Trail

and Baptista 1993).

Where Brown-headed Cowbird populations are well established, brood parasitism can

significantly reduce reproductive success of host species and communities (Friedmann et al.

1977, Robinson et al. 1995b). For example, Brown-headed Cowbird parasitism contributes to

negative net reproduction of some songbird populations, which are likely maintained only by

immigration from other areas (Robinson et al. 1995b). Cowbirds limit reproduction of hosts in a

variety of ways, including reduced clutch sizes (Mayfield 1961, Burhans et al. 2000, Hoover

2003) through removal and predation of host eggs or young (Friedmann 1963, Sealy 1992,

Arcese et al. 1996, Granfors et al. 2001, Hoover and Robinson 2007), or reduced hatching

(McMaster and Sealy 1998, Burhans et al. 2000) and host fledging success (McMaster and Sealy

1998, Burhans et al. 2000). Costs can also be incurred beyond the nest through increased

juvenile and adult mortality (Payne 1965, Hoover and Reetz 2006).

Overall costs of cowbird parasitism are particularly evident at local scales in contact zones

where cowbirds begin parasitizing novel hosts that are highly susceptible to parasitism.

Cowbirds spread rapidly throughout California beginning in the early 1900s, for example, and

brood parasitism has contributed to the extirpation of local populations of Least Bell's Vireo

(Vireo bellii) in much of its California range (USFWS 1998). Furthermore, parasitism by

Brown-headed Cowbirds has threatened the extinction of species with limited ranges such as

Kirtland's Warbler (Dendroica kirtlandii)tlt~t~l~ltlt~t~ in northern Michigan (Mayfield 1992). As a result,

Brown-headed Cowbirds became a regional management problem for particular songbird species

and were often aggressively controlled to improve reproductive success of hosts (Kelly and










DeCapita 1982, Beezley and Rieger 1987, Eckrich et al. 1999). Identifying situations where

management of cowbird populations is necessary to prevent such declines is of importance in

conservation of some populations of host species (Rothstein and Cook 2000), particularly since

the cowbird has spread over most of North America, coming into contact with new host species

and populations.

There are many reasons to monitor the reproductive success (and thereby, cowbird

parasitism rates) of songbirds for conservation and management of avian habitats, populations,

and species. Potential impacts of cowbird parasitism can be identified using rates of parasitism

as an initial cue, with further detailed work leading to conservation of affected host species.

Research to characterize Shiny Cowbird (M~olothrus bonariensis) expansion to Puerto Rico

(since the 1940s), for example, identified extremely high rates of parasitism (75-100%) on

multiple native species (Post and Wiley 1977, Wiley 1985). This work ultimately led to a

conservation program for the endangered, endemic Yellow-shouldered Blackbird (Agelaius

xanthomus), which had suffered population declines of >80% as a result of intense parasitism

(Wiley et al. 1991). A large-scale cowbird trapping program at Fort Hood, Texas reduced

parasitism of the federally endangered Black-capped Vireo (Vireo atricapilla) from 91% in 1987

to 8.6% ten years later (Eckrich et al. 1999). Cowbird management is also an ongoing and

critical component in the conservation of the threatened and endangered populations of

Southwestern Willow Flycatcher (Empidonax traillii extimus), Least Bell's Vireo, Black-capped

Vireo, California Gnatcatcher (Polioptila californica), Kirtland's Warbler, and Golden-cheeked

Warbler (Dendroica chrysoparia).

Where cowbirds have clear negative effects on host populations and communities, rates of

parasitism (percentage of nests/broods with cowbird eggs/chicks) can be as high as 80-100 %









(Norris 1947, Hatch 1983, Grzybowski et al. 1986, Robinson 1992) and often above 50% (Elliott

1978, Burgham and Picman 1989, Hahn and Hatfield 1995, Trine et al. 1998). Therefore,

monitoring songbird reproductive success and rates of cowbird parasitism can help determine the

potential for serious (or unimportant) consequences of cowbird activity in conservation planning.

This is particularly important in areas where cowbirds have most recently expanded their

breeding range, such as the southeastern United States.

Patterns of Cowbird Expansion in the Southeast

The first records of Brown-headed Cowbirds breeding in the southeastern United States

were in far western North Carolina in 1933 (Pearson et al. 1959) and far western South Carolina

in 1934 (Sprunt and Chamberlain 1970). Breeding cowbirds reached northern Georgia in the late

1940s (Parks 1950). By the mid-1960s, cowbirds were parasitizing nests in many parts of the

Carolinas but were still scarce in the eastern Piedmont and inner coastal plain regions, although

they were first reported during the breeding season near the coastal cities of Wilmington, NC and

Charleston, SC in 1967 and 1965, respectively (Potter and Whitehurst 1981).

Brown-headed Cowbirds were first documented breeding in Florida during the mid-1950s,

when a fledgling was sighted in Escambia County in the far western panhandle (Monroe 1957).

Cowbirds were subsequently confirmed breeding in Jacksonville in 1965 (Ogden 1965) and

south to Alachua County in 1980 (Edscorn 1980). By the end of the 1980s, the breeding range of

the Brown-headed Cowbird had expanded to incorporate all of Florida (Hoffman and

Woolfenden 1986, Paul 1989) and thus the entire southeastern portion of the United States.

Brown-headed Cowbirds are presently confirmed breeding in at least 50 of Florida' s 67 counties

(FFWCC 2007). Since the mid-1960s, the population of Brown-headed Cowbirds in Florida

grew slowly but steadily (Figure 1-1) and also increased in Breeding Bird Survey stratum three,

which includes northern Florida (Figure 1-2; Sauer et al. 2006). Cowbirds are roughly as









abundant in the northern portion of Florida as they are in much of the rest of the continent (Sauer

et al. 2006), so there is good reason to expect that they may represent a threat to breeding

songbird populations.

Objectives

Most of the approximately 50 songbird species breeding in Florida are known to be

parasitized by cowbirds in other parts of their ranges, and some are known as suitable hosts (i.e.,

have reared cowbird young) in other areas (Ortega 1998). Some of these species are locally rare

in north-central Florida (e.g., Hooded Warbler Wilsonia citrina, Yellow-breasted Chat Ictera

virens; FFWCC 2007) or are experiencing significant declines in the southeast (e.g., Red-eyed

Vireo Vireo olivaceus, Northern Parula Parula amnericana; Sauer et al. 2006) and would,

therefore, be particularly susceptible to any negative effects of brood parasitism they may incur.

Isolated records of parasitism of breeding songbirds in Florida suggest that many species may be

at risk to Brown-headed Cowbird parasitism (see review in Cruz et al. 1998). Despite

widespread concern over potential impacts of cowbird parasitism for native breeding songbirds,

only scattered data on parasitism were collected in Florida during the 60 years since cowbirds

began breeding in the state.

To determine potential impacts of Brown-headed Cowbird expansion into Florida, I

conducted a comprehensive survey of cowbird parasitism in songbird communities in north-

central Florida, including (1) monitoring understory bird nests for rates of occurrence of cowbird

eggs and nestlings in host nests and (2) surveying post-fledgling family groups (adults with

fledglings) of both understory and canopy songbird species to determine the rates of successful

brood parasitism by cowbirds among the species sampled. More than 30 species were included

in my study; each is known to have been parasitized elsewhere in their breeding range by Brown-









headed Cowbirds. This represents the first community-wide survey documenting cowbird

parasitism rates in both nests and post-fledging families of host species breeding in Florida.

Methods

I surveyed nests from a diversity of potential hosts to determine natural parasitism rates in

north-central Florida. Nest searches were conducted daily between 0700 and 1900 EST at sites

within 25 km of Gainesville, FL (ca. 29039' N, 82ol9'W) that included many different types of

habitats (Table A-1). Sites ranged from urban (downtown Gainesville) to rural (no or low nearby

housing density), from semi-natural (e.g., city parks) to natural (e.g., managed state preserves),

and from disturbed (e.g., regenerating rangeland) to quasi-natural (large natural communities in

state preserves). Nests were monitored every 1 to 3 days. Data were supplemented from nest

records from an avian monitoring proj ect (Sieving & Contreras, unpubl. data) conducted in

Myakka River State Park near the southwest Florida gulf coast in Sarasota and Manatee Counties

(ca. 27014' N, 82014' W) from April to July, 2001-2005. Cowbird abundance in this area is

approximately one-quarter of that in northern Florida (Sauer et al. 2006) but large mixed-sex

roosting flocks with 1500+ cowbirds have been seen during the breeding season in nearby St.

Petersburg, FL (D. Margeson, pers. comm.).

Data on nest success for canopy species such as Red-eyed Vireo, Northern Parula, and Pine

Warbler (Dendroica pinus) are rare or non-existent because their nests are generally high in the

canopy (>10m), and therefore difficult to monitor reliably using current technologies. Nests of

cryptic species (e.g., Carolina Wren Thi psthesin s/ ludovicianus) are also under-represented in

typical nest searching/monitoring samples due to low detection rates. Therefore, to account for

these biases in the data set, I conducted intensive surveys for family groups (adults and

fledglings) for all species that nest throughout the understory and canopy (Verner and Ritter









1983). Monitoring of adults feeding young outside of the nest is the main component of

reproductive indices used to estimate reproductive success (Vickery et al. 1992) and are useful

when nest monitoring is logistically infeasible (Bonifait et al. 2006).

Family groups were found by watching foraging adults and following adults carrying food

or by tracking fledgling begging calls. Fledglings of some species give more audible begging

calls, but I listened for any call that might represent fledgling feeding or presence. Family

groups of the study species that I encountered were followed for at least ten minutes to determine

if a cowbird fledgling was present. Multiple species have rarely been observed feeding a single

cowbird fledgling (Klein and Rosenberg 1986), so I assumed cowbird fledglings were being fed

by the species that raised them. I typically observed repeated feedings of cowbirds by the same

species and never witnessed a feeding of a given cowbird by more than one host species. Lower

nesting (understory) species tend to be better represented in standardized samples of forest bird

communities. To overcome any potential bias this may represent, I spent approximately 75% of

time conducting visual scans for adult birds and families in the midstory and canopy and 25% in

lower strata at forested sites. Many times, family groups for lower nesting species were located

opportunistically when searching for and monitoring nests.

Results

From March-August, 2004-2006, I monitored the parasitism status of 35 potential host

species breeding in Florida (Tables 1-1, A-2). Nest surveys in north-central Florida included

monitoring of 994 nests of 29 potential cowbird host species. Nesting data from Myakka River

State Park added 123 nest records of nine host species and included 26 records for three species

not monitored in this study (32 species total). Only 18 nests (1.6%) of eight potential host

species contained a cowbird egg or nestling. The most records of parasitism (N=6) were from









Northern Cardinal (Cardinalis cardinalis), but constituted only 2.5% of cardinal nests monitored.

Other species parasitized with at least Hyve nest records were Blue-gray Gnatcatcher (Polioptila

caerulea; 20%), Hooded Warbler (20%), Blue Grosbeak (Guiraca caerulea; 14.3%), White-eyed

Vireo (Vireo griseus; 10.5%), Bachman's Sparrow (Aimophila aestivalis; 9.1%), Eastern Towhee

(Pipilo erythrophthalmus alleni; 6.1%), and Northern Mockingbird (M\~imus polyglottos; 0.2%).

One of three (33%) Common Yellowthroat nests was parasitized. The only Yellow-throated

Vireo (Vireo flavifrons) nest contained a lone cowbird nestling. Of nests of the 10 species

parasitized, 2.4% (18 of 750) contained cowbird eggs or nestlings. Nest records of Northern

Mockingbird constituted the largest proportion of the sample of parasitized birds (N=397) and

may have biased average parasitism rate over multiple species. However, even after excluding

these records, the parasitism frequency of nests was still low at 5.7% (17 of 299) of nests.

Cowbirds fledged successfully from fiye nests (one each of Yellow-throated Vireo, Blue-gray

Gnatcatcher, Common Yellowthroat, Bachman's Sparrow, Northern Cardinal). Bachman's

Sparrow nests are known to be parasitized (Ortega 1998), but this represents the first report of

this species raising a cowbird to fledging and the first record of cowbird parasitism on the

Bachman's Sparrow in Florida (Reetz et al. 2008). Among the few nests of midstory and canopy

species that I was able to monitor, 1.7% (2 of 116) were parasitized compared to 1.6% (16 of

1001) of nests of the remaining species. Excluding Northern Mockingbird nests from the sample

of lower nesting species, 2.5% (15 of 592) of nests were parasitized.

I monitored 278 family groups of 20 species, including three species (Carolina Chickadee

Poecile carolinensis, Red-eyed Vireo, and Northern Parula) for which no nest data were

available. Seventeen (6.1%) of these family groups contained a cowbird fledgling (Table 1-1).

Of the six species with at least one parasitized family group, 10.6% (17 of 160) of families









contained a cowbird fledgling. Family groups of midstory or canopy nesting species were more

likely to be found feeding a cowbird young than lower nesting species (Log Odds Ratio=3.04,

CI=4.65-93.61). Indeed, of the 17 parasitized family groups monitored, 15 (88.2%) were of

midstory or canopy nesting species, and 17.9% (15 of 84) of midstory or canopy nesting family

groups observed were parasitized. For example, 3 of 5 (60%) Summer Tanager (Piranga rubra),

7 of 18 (38.9%) Northern Parula, 3 of 9 (33%) Blue-gray Gnatcatcher, 1 of 4 (25%) Red-eyed

Vireo, and 1 of 14 (7.1%) Pine Warbler family groups monitored were observed feeding a

cowbird fledgling. Northern Cardinals were the only lower nesting species that was observed

feeding a fledgling cowbird (2 of 93 family groups or 2.2%).

Discussion

Cowbird Parasitism in North-Central Florida

Despite the apparent abundance of cowbirds and their feeding habitat in northern Florida, I

detected very low rates of Brown-headed Cowbird parasitism across the community of potential

host species breeding in north-central Florida. Less than 2% of nests were found with cowbird

eggs and approximately 6% of family groups contained a cowbird fledgling. While this

represents the first community-wide survey of Brown-headed Cowbird parasitism in Florida,

some more narrowly focused reproductive studies present similarly low rates of cowbird

parasitism. For example, Prather and Cruz (1995) found no parasitism cases among 42 Prairie

Warbler (Dendroica discolor pahedicola) and 20 Yellow Warbler (Dendroica petechia

gundlachi) nests in south Florida, and only two cases in 108 nests of ten species on the

southwestern gulf coast (2002). During the six years (1986-1991) of the Florida Breeding Bird

Atlas, a collaborative proj ect designed to record breeding distributions of all avian species in

Florida, volunteers and researchers monitored more than 2400 nests of the same species surveyed









in this study. Less than 0.5% of nests were found with cowbird eggs or nestlings (FFWCC

2007). Species seen feeding fledgling Brown-headed Cowbirds during the Atlas proj ect were

similar to those in this survey and included Blue-gray Gnatcatcher, Red-eyed Vireo, Northern

Parula, Pine Warbler, and Summer Tanager, among others. Taken together, available monitoring

data of Florida host species show very limited Brown-headed Cowbird nest parasitism.

Results from parasitism surveys in this proj ect are nonetheless surprising given the relative

abundance of Brown-headed Cowbirds in north-central Florida, ranging from 3-10 birds per

Breeding Bird Survey route (1994-2003; Sauer et al. 2006). Such abundances are comparable to

those in most of the eastern and far western United States, where reproductive studies have

recorded much higher community-wide parasitism rates than reported here. For example, nest

parasitism rates of 36 host species in northeastern New Mexico are 20.8% of 850 nests (Goguen

and Mathews 1998) and of 32 host species in southeastern New York are 16.53% of 301 nests

(Hahn and Hatfield 1995). Parasitism frequency of common host species in recently-invaded

South Carolina and Georgia is lower at 8.2% of nests (Kilgo and Moorman 2003), but still

significantly higher than the frequency reported here.

Even species with high rates of parasitism elsewhere in their ranges seem to elude it in

Florida. Northern Cardinals had parasitism rates of 2% of nests and family groups in this study,

but suffer parasitism rates of 25-100% of nests in other parts of their breeding range (reviewed

in Ortega 1998). Rates of parasitism are also very high (40-83%) for Eastern Towhee and

White-eyed Vireo in other regions (Norris 1947, Young 1963, Goertz 1977, Hopp et al. 1995),

yet were only 5.3% and 10.5%, respectively, in north-central Florida. Finally, fewer than 2% of

nests of understory nesting species contained cowbird eggs, yet 13% of nests in a similar suite of

understory species were parasitized elsewhere in the southeastern U.S. (Whitehead et al. 2002).









Various procedural and ecological-evolutionary mechanisms may underlie the low rates of

parasitism detected in this study.

Potential Limits on Parasitism Rates or Their Accurate Detection

First, rates may have been affected by inclusion of certain species that are likely incidental

or inappropriate hosts. For example, rates of nest parasitism in southeastern Ontario, where

cowbirds are abundant, are only 6.7% across all 87 species for which parasitism has been

recorded at least once (Peck and James 1987). Many of these species, however, are accidental or

unsuitable cowbird hosts (e.g., Virginia Rail Rallus limicola) not known to fledge cowbirds

(Ortega 1998). Nine of the ten most parasitized species in Ontario, however, are appropriate

hosts and some averaged as high as 48% of nests parasitized (Peck and James 1997, 1998a, b).

Therefore, parasitism rates of at least marginally suitable hosts are likely significantly higher

than the 7% reported for the entire community of breeding birds. I surveyed a total of 3 5 host

species, all of which have been recorded as hosts of Brown-headed Cowbirds (Ortega 1998).

but some hosts included in the survey, (e.g., Mourning Dove Zenaida macroura, Blue Jay

Cyanocitta cristat, Northern Mockingbird, Brown Thrasher Toxostoma rufum and House Finch

Calrpodacus mexicanus) are inappropriate or poor hosts due to life-history traits (e.g., nestling

diet, defense behaviors) incompatible with cowbird parasitism. When all nests of the 14

unsuitable host species surveyed are removed from the sample, the parasitism rate of the

remaining 21 species is still only 3.6% of nests (18 of 486) and 7.5% of family groups (17 of

238; Table 1-1). Therefore, surveys of parasitism frequency do not appear to be biased by

selective inclusion of host species.

Second, many hosts have nests that are difficult to find (e.g., Common Yellowthroat) or

extremely difficult to monitor (e.g., Northern Parula and other canopy nesting species), and thus

parasitism may go undetected. For example, very low rates of nest parasitism (1%) were









detected across a host community in the Sierra Nevada of California (Verner and Ritter 1983), a

survey that included species heavily parasitized elsewhere in the region. Observations of family

groups still showed low parasitism rates (4%), but the authors' analyses suggested that parasitism

was biased toward species with nests that are difficult to monitor. Results from my surveys also

suggest that nest parasitism may go undetected due to preference for particular species infeasible

to survey. I was able to monitor few midstory and canopy nest, and parasitism rate (1.9%) in this

limited sample was similar to that for lower nesting species (1.5%). However, I note that the

vast maj ority (88%) of parasitized family groups I observed was of species nesting in the higher

strata, presenting an overall parasitism rate of approximately 20% for the nesting guild with the

lowest nest detectability in nest monitoring surveys. As a result, it appears that low observed

frequencies across the community of hosts is at least partially due to difficulties in obtaining

adequate samples for all potential species.

Third, estimates of cowbird parasitism that are based on the percent of parasitized nests

can underestimate actual parasitism levels if nests are abandoned, nest contents are readily

depredated (Robinson et al. 1995a), or cowbird eggs are ejected. Many species nesting in north-

central Florida practice egg rej section behaviors (i.e., egg ej section, egg burial, and abandonment

of nests) in some parts of their ranges. Many of the most abundant songbird species that breed in

Florida are known to readily ej ect cowbird eggs or abandon parasitized nests. For example,

Northern Mockingbirds and Brown Thrashers ej ect up to 69% (Peer et al. 2002) and 100%

(Rothstein 1975b, Haas and Haas 1998) of experimental cowbird eggs placed in their nests. If

anti-parasitism behaviors are prevalent in many of the host species breeding in a region, a large

portion of actual nest parasitism will go undetected leading to lower observed parasitism rates









overall. Egg rej section behaviors do not appear to be prevalent in common hosts in Florida

(Chapter 2).

Fourth, though Brown-headed Cowbird fecundity is usually very high with an average

female producing 40 eggs in a breeding season (Scott and Ankney 1980), there is significant

variation in fecundity over the species' range (Payne 1965, 1973a, 1976, Scott and Ankney

1980). It is possible, therefore, that while cowbirds are abundant in north-central Florida, host

communities may show reduced parasitism of nests simply because breeding female cowbirds

lay fewer eggs. Cowbird fecundity has been measured using a variety of methods, and limited

reproduction would provide a parsimonious explanation for low rates of parasitism in Florida.

Females breeding in north-central Florida show high fecundity (Chapter 3), with a rate of egg

production similar to the highest rate estimated for the species.

Finally, low parasitism rates may be related to the limitation of suitable hosts. Cowbirds

have been identified as "generalist" parasites because they are known to parasitize over 220

species (Ortega 1998). Because parasitism often affects many or sometimes most potential host

species in a community, parasitism may not be host-specific because eggs are spread among

hosts according to their abundance ("shotgun" hypothesis; Rothstein 1975b). In this case, 'good'

hosts (those able to successfully fledge cowbird young) are parasitized as frequently as 'poor'

hosts. Host-specific characteristics can affect the survival probability of cowbird young so that

selection may favor parasitism of hosts successful at raising cowbirds. As a result, parasitism of

nests may be biased toward a few select species ("host selection" hypothesis; Rothstein and

Robinson 2002). Host selection is supported by data showing nonrandom parasitism frequencies

of a suite of potential hosts (Woolfenden et al. 2003, Ellison et al. 2006). In areas where









cowbirds are very abundant, individuals are likely to employ both host selection and shotgun

tactics because competition for resources (nests of "good" hosts) is more intense.

For successful expansion and population growth of cowbirds in new regions such as

Florida, the availability of at least some suitable hosts that accept cowbird eggs and appropriately

nourish juveniles is critical. Resultant parasitism frequencies are often higher than those that

rej ect eggs (Sealy and Bazin 1995) or feed young a diet inappropriate to cowbirds (Middleton

1991), but patterns of parasitism across the host community have not typically been addressed in

areas where cowbirds have recently expanded. In a newly colonized area such as Florida, low

initial cowbird densities should favor host selection because of low competition for nests. In this

case, the availability (abundance/diversity) of quality hosts may represent a limiting factor to

reproduction.

Data presented in this chapter are consistent with host selection because certain species,

particularly those that nest in the midstory or canopy, appear to have been preferentially

parasitized. This result probably does not reflect a selection bias by cowbirds toward higher

nests, per se, but rather toward suitable host species. Lower nesting species such as Northern

Mockingbird, Northern Cardinal, and Carolina Wren are probably poor cowbird host species

because of anti-parasitism behaviors, nest site choice, or other life-history traits (Rothstein

1975b, Haggerty and Morton 1995, Halkin and Linville 1999). In north-Florida, these and

similar species constitute a relatively higher proportion of lower nesting species abundance than

in other regions with a more diverse and abundant community of quality understory nesters (e.g.,

emberizids, parulids, etc.). Species such as Indigo Bunting (Pa~sserina cyan2ea), Yellow-breasted

Chat and Orchard Oriole are quality understory hosts and are moderately parasitized in other

southeastern states where they are common (Whitehead et al. 2002, Kilgo and Moorman 2003).









However, north Florida represents the southern extent of their breeding range where they are

found in very low densities (Sauer et al. 2006). As a result, a subset of midstory and canopy

nesting species that is preferentially parasitized may be bearing the brunt of cowbird parasitism

in north-central Florida because it represents the highest abundance of quality hosts but has

relatively low species diversity.

Conclusions and Management Implications

Expansion of breeding Brown-headed Cowbirds into Florida was greeted with widespread

concern, particularly with the additional arrival of the Shiny Cowbird in south Florida. Atherton

and Atherton (1988:63) stated "it is conceivable that the breeding ranges of the two cowbird

species will meet within the next few years and that both Prairie Warbler and Black-whiskered

Vireo will be in grave danger of being extirpated from the state." The observation of a flock of

1000+ Brown-headed Cowbirds with six Shiny Cowbirds in south Florida in July 1988 (Paul

1988:1281) prompted the dramatic prediction that it represented "... the end of birding as we

know it in south Florida." To date, there has been only one published breeding observation (Paul

1988), and one breeding specimen collected (Chapter 3) that confirm reproduction of Shiny

Cowbirds in Florida. Furthermore, through monitoring of nests and family groups with

fledglings, I detected very low rates of Brown-headed Cowbird parasitism on the community of

potential host species breeding in north-central Florida. Therefore, brood parasitism by cowbirds

does not appear to currently represent an urgent threat to most of Florida' s breeding songbirds.

My nest parasitism surveys suggest, however, that a subset of host species, particularly

midstory and canopy nesting species, accounts for a disproportionate amount of recruitment of

juvenile cowbirds in north-central Florida; these species could be suffering negative

consequences of parasitism. Despite the logistical difficulties in locating and monitoring nests of

midstory and canopy nesting species, I stress a critical need nest data to determine parasitism









rates. In tandem, monitoring of populations of these species is needed to determine if declines

are occurring from negative effects of brood parasitism. In addition, the relationship between

cowbirds and their hosts should continue to be monitored across different host communities and

in areas with different abundances of cowbirds. Brown-headed Cowbird abundance is highest in

northwestern Florida, where diversity and abundance of good quality hosts also are the highest in

the state (Sauer et al. 2006). Finally, nest parasitism frequencies of species that are locally

abundant but have restricted distributions should be determined. In particular, the breeding

distribution of the Painted Bunting (Pa~sserina ciris) in Florida is concentrated in the northeastern

corner of the state with smaller populations scattered along the Atlantic coast (FFWCC 2007).

At a site in South Carolina, Painted Buntings had 35.3% of nests parasitized by Brown-headed

Cowbirds (Whitehead et al. 2002). Such a rate could threaten small, isolated populations

breeding in Florida.

Perhaps the most useful conclusion of this study to management is that monitoring of

family groups may provide a more comprehensive technique for assessment of the true incidence

of successful cowbird parasitism in any area affected by increasing cowbird populations. Of

individual species with reasonable sample sizes, the highest parasitism rates recorded were 39%

(Northemn Parula) and 33% (Blue-gray Gnatcatcher) of family groups. While these rates are

considered moderate among those nest parasitism rates published in the literature (Ortega 1998),

they do not consider cowbird eggs that do not hatch or are lost from nests, or increased nest

predation of parasitized nests (Burhans et al. 2002). On the other hand, these rates may also

overestimate relative parasitism due to sampling biases inherent in the methodology. For

example, cowbird fledglings are notoriously and relentlessly boisterous when begging for food

from host adults, which may make fledged families more conspicuous. I attempted to avoid









over-sampling of families with noisy cowbird fledges by finding and following family groups by

scanning for adults with food, or listening for begging of fledglings of all species. Fledglings of

some species have relatively soft calls and cryptic behaviors, making them more difficult to

detect from a distance than cowbird young, especially high in the canopy. Therefore, this

method is at least representative of parasitism in species' nests that successfully fledge young. In

any case, these data suggest that parasitism rates of selected species may be much higher than

community-wide nest surveys alone would detect. Therefore, in addition to cowbird population

monitoring and nest monitoring, family group surveys may be a particularly valuable means of

assessing the overall intensity of cowbird parasitism affecting songbird communities. It is also,

perhaps, the most efficient technique for identifying the host species most likely to recruit young

cowbirds into the breeding population.














O*




-- *'


5-


S4~
3-


1966


19172 19~78 1984 19~90 1996 2002


Year
Figure 1-1. Breeding Bird Survey data of Brown-headed Cowbirds detected per route in Florida
from 1966-2006.


25 m


-All Routes
~tNorthemn Florida
SSouthemn Florida


1966 1971 1976 1981 1986 1991 1996 2001 2006


Figure 1-2. Estimated number of Brown-headed Cowbirds per Breeding Bird Survey route for
northern and southern Florida, 1966-2006. BBS Routes considered Southern Florida
include all routes south of Marion County (ca. 28057'35" N), the county immediately
south of Alachua, where this study was conducted. Data presented represent raw
counts and only routes recording Brown-headed Cowbird at least once were included.




















U
U
U


3
52
12


Unparasitized Parasitized Unparasitized Parasitized
Category Nests Nests Families Families


Table 1-1. Summary of nests and family groups for 32 species monitored in the north-central Florida from March to August,
2004-2006. U: species nesting in the midstory to canopy, L: species nesting low or on ground. Nesting category based on
field observations and Ehrlich et al. (1988). Species totals for each nesting category are given for nests/family groups
monitored. Number of supplemental nests (number parasitized) from Contreras & Sieving (unpubl. data, 2000-2004) are:
MODO = 19, GCFL = 12, BLJA = 8, EABL = 7, NOMO = 18, COYE = 2 (1), EATO = 43 (3), BACS = 11 (1), EAME =
3. Species with an asterisk are considered incidental or poor hosts.


Common Name
Eurasian Collared Dove*
Mourning Dove*
Great-crested Flycatcher*
Acadian Flycatcher
Loggerhead Shrike*
White-eyed Vireo
SYellow-throated Vireo
Red-eyed Vireo
Blue Jay*
Carolina Chickadee*
Eastern Tufted Titmouse*
Carolina Wren
Blue-gray Gnatcatcher
Eastern Bluebird*
Wood Thrush
Northern Mockingbird*
Brown Thrasher*
Northern Parula
Pine Warbler


Latin Name
Streptopelia decaocto
Zenaida macroura

M~yiarchus crinitus
Empidonax virescens
Lanius ludovicianus
Vireo griseus
Vireo flavifCrons
Vireo olivaceus

Cya~nocitta cristat
Poecile carolinensis
Baeolophus bicolor
Thil yesthesi its\ ludovicianus
Polioptila caerulea
Sialia sialis

Hylocichla mustelina
M~imus polyglottos
Toxostoma rufum
Parula americana
Dendroica pinus










Table 1-1. Continued.


Unparasitized
Nests


Parasitized
Nests


Unparasitized
Families


Parasitized
Families


Common Name
Prothonotary Warbler
Common Yellowthroat
Hooded Warbler
Yellow-breasted Chat
Summer Tanager
Bachman's Sparrow
Eastern Towhee
Northern Cardinal
Blue Grosbeak
Indigo Bunting
SEastern Meadowlark*
Red-winged Blackbird
Common Grackle*
Boat-tailed Grackle*
Orchard Oriole
House Finch*
TOTALS
Midstory/Canopy Species
Lower Nesting Species
Quality Hosts


Latin Name
Protonotaria citrea
Gl'lthy/pli\ trichas
Wilsonia citrina
Icteria virens
Piranga rubra
Aimophila aestivalis
Pipilo erythrophthalmus alleni
Calrdinalis cardinalis
Guiraca caerulea
Pa~sserina cyan2ea
Sturnella magna
Agelaius phoeniceus
Quiscalus quiscala
Quiscalus major
Icterus spurius
Calrpodacus mexicanus


Category


U
L
L
L
L
L
L
L
U
U
U
L
35
17/10
15/10


1
10
54
237
6
2
3
81
2
2
2
29
1099
114
985
469









CHAPTER 2
RESPONSES OF FOUR SONGBIRD SPECIES TO EXPERIMENTAL COWBIRD
PARASITISM IN A RECENTLY INVADED AREA

Introduction

Multiple direct and indirect costs are associated with brood parasitism by Brown-headed

Cowbirds that reduce survival and fecundity of host species (see Chapter 1). As a result, nest

parasitism can impose strong selection on hosts for behaviors to reduce the number of cowbird

eggs that host females incubate (Rothstein 1990). Two predominant behaviors commonly

expressed by hosts that rej ect parasitism are removal of cowbird eggs from the nest and desertion

of parasitized clutches. Indeed, quantification of the frequency of egg ejection and clutch

desertion behaviors, via experimental emulation of cowbird nest parasitism (egg additions), is

used to determine the prevalence of rej section responses within a species or host community

(reviewed in Peer and Sealy 2004a). In this way, host species can be categorized as either

ej ecters (species that physically remove parasitic eggs from their nest) or accepters. Species that

initially accept eggs into their nests may still rej ect parasitism by abandoning the parasitized

clutch; either by deserting the nest (Rothstein 1975b, reviewed in Ortega 1998:192) or by

burying the eggs under a new nest lining (Sealy 1995). Despite its apparent benefit, only a

relatively small subset of species (ca. 29 of >220) that are parasitized exhibit at least intermediate

egg ejection frequencies (Table 2.1 in Ortega 1998). Furthermore, many species that do not eject

cowbird eggs from their nests do not subsequently abandon nests or bury cowbird eggs.

Two maj or explanations are proposed for why potential host species accept nest parasitism.

Under the evolutionary equilibrium hypothesis, nest parasitism is tolerated as a result of

conflicting selection pressures (Zahavi 1979, Rohwer and Spaw 1988). Costs of ejection errors

or abandonment of nests, representing losses of host eggs and energy expended in replacement

nesting, may outweigh the costs of raising parasite young; thus acceptance could be









evolutionarily favored (Lotem and Nakamura 1998). Under conditions of coexistence between

cowbirds and host species that accept parasitism under this scenario, selection simultaneously

favors cowbird traits that maximize efficiency of host use and host traits that minimize costs of

parasitism (Robertson and Norman 1976, Dawkins and Krebs 1979). Under the evolutionary lag

hypothesis (Rothstein 1975b, Dawkins and Krebs 1979, Davies and Brooke 1989), acceptance of

parasitic eggs occurs because adaptations that eliminate or reduce the frequency of brood

parasitism have not yet developed, spread, or become Eixed in the host population (Rothstein

1975a). As a result, host populations are not able to recognize parasitic eggs (Rothstein 1982) or

females (Smith et al. 1984, Bazin and Sealy 1993), or lack appropriate responses to foreign eggs.

In host populations in recent contact with expanding populations of Brown-headed

Cowbirds, evolutionary lag is the most reasonable explanation for a general lack of adaptive

response to novel brood parasitism (Rothstein 1975b, Burgham and Picman 1989). In such

situations, naive host populations may be at increased risk of decline due to high rates of nest

failure (Cruz et al. 1989) if they are unable to recognize or respond to nest parasitism (Rothstein

et al. 1980). Despite having only come into contact with Shiny Cowbirds in the 1990s, however,

Gray Kingbirds (Tyrannus dominicensis) in the Bahamas are known to ej ect artificial cowbird

eggs (Baltz and Burhans 1998), a behavior that is also expressed where the two species have

been sympatric for longer periods (Cruz et al. 1985, Post et al. 1990). In populations no longer

sympatric with cowbirds, multiple species have shown similar retention of cowbird egg rej section

behaviors displayed in populations still exposed to parasitism (Rothstein 2001, Peer et al. 2007).

Therefore, retention of behavioral defenses against brood parasites in some species and

populations may buffer them from immediate negative consequences of recent cowbird

expansion.










Colonization of Florida by Brown-headed Cowbirds has occurred over the last 50-60 years

(see Cruz et al. 2000) and was first thought to represent a significant threat to many breeding

songbirds (Cruz et al. 1998). However, observed parasitism frequencies for individual hosts and

across an entire host community are unusually low despite moderate Brown-headed Cowbird

abundance (Chapter 1). Elsewhere in the southeastern United States, many host species are

known to have displayed anti-parasitism behaviors during the recent southward expansion of

cowbirds (Whitehead et al. 2002). These behaviors are likely maintained by gene flow from

regions where cowbirds have consistently been breeding over the last two centuries (Rothstein

1990). In addition, fossil evidence suggests that Brown-headed Cowbirds (Hamon 1964, Ligon

1966, Emslie 1998) and another extinct cowbird species (Pandanerus floridana;~d~~d~~d~ Brodkorb 1957)

were common in Florida in the late Pleistocene. During this time rej section behaviors may have

evolved in some species in response to heavy brood parasitism and may be retained in current

populations in the absence of cowbirds. Indeed, at least one species endemic to Florida, the

Florida Scrub Jay (Aphelocoma coerulescens), is known to readily ej ect artificial cowbird eggs

(Fleischer and Woolfenden 2004). Species known as ej ecters (e.g., Northern Mockingbird,

Brown Thrasher) and deserters (e.g., Northern Cardinal) elsewhere within their breeding

distribution are among the most abundant songbird species currently breeding in Florida.

I tested the hypothesis that the prevalence of egg rej section behaviors in Florida hosts

contributes to limiting the reproduction of Brown-headed Cowbirds. I experimentally parasitized

four common host species that are known to vary in the degree of their rej section response, but

that commonly respond adaptively to parasitism in some part of their range. I predicted that

parasitism rej section behaviors in populations of Florida songbird species would be comparable in

frequency to populations exhibiting moderate to high rej section frequencies in other parts of their










range. If rej section behaviors are generally less prevalent or absent in Florida study species,

reflecting a typical evolutionary lag in adaptive responses to cowbird expansion, factors other

than effective host rej section of parasitism are more likely to be contributing to limited Brown-

headed Cowbird reproduction in Florida. While most studies of egg ej section have used artificial

cowbird eggs to examine host responses (e.g., Rothstein 1975b, Baltz and Burhans 1998, Robert

and Sorci 1999), nests in this experiment were parasitized with actual cowbird eggs obtained

from a captive colony raised for this purpose. The goal was to simulate parasitism as accurately

as possible and avoid potential complications inherent in the use of artificial eggs (Rothstein

1976, 1977, Martin-Vivaldi et al. 2002).

Methods

Study Design

In September 2004, I captured 40 hatch-year cowbirds (20 male, 20 female) in Alachua

County, FL using baited drop-down decoy traps. From March-August 2005 and 2006, 10 pairs

were separated for breeding in individual outdoor pens (4 x 2 x 2m) at the USDA National

Wildlife Research Center, Florida Field Station in Gainesville, FL under an approved animal use

protocol (IACUC D539). Birds were fed a specialty diet that met caloric and dietary needs for

reproduction (Holford and Roby 1993). Each pen contained two or more open-cup or domed

artificial nests and an occasional inactive Northern Cardinal nest. Some artificial nests contained

a single wooden artificial egg that was similar in size, appearance, and weight to the egg of a

Red-eyed Vireo. To improve the probability that a captive female cowbird would lay eggs,

cowbird pairs were housed with Zebra Finches (Taeniopygia guttata and Society Finches

(Lonchura striata domestica. These finches bred readily in captivity and provided stimuli

associated with active bird nests (e.g., nest-building, incubation behavior, warm eggs). Pens

were monitored daily for the presence of cowbird eggs.









From 28 April to 14 July, 2004 and 2005, eggs obtained from captive birds were used to

experimentally parasitize four primary species that are abundant during the breeding season in

Florida. Nests were searched for daily in a variety of habitats at sites within 25 km of

Gainesville, FL (Chapter 1). Sites included nature preserves (Ordway-Swisher, Bolen Bluff,

Split Rock), wetlands (Newnan's Lake) and numerous Gainesville city parks and neighborhoods.

A single experimental cowbird egg was placed in each host nest between the egg-laying stage

and 2-3 days following initiation of incubation as determined by candling of eggs (Lokemoen

and Koford 1996). In one case, a Northern Cardinal nest received an experimental egg before

the female laid her first egg. Eggs were warmed slightly in the hand prior to being placed in the

nest. To mimic the common removal of a host egg by a female cowbird (Nolan 1978), a single

host egg was often removed (75% of cases) from nests containing at least two host eggs. Nests

were visited daily and host responses were categorized as "rej ected" or "accepted" based on

Rothstein (1975b). Two types of rejection behavior were distinguished. Eggs were considered

"ej ected" if the cowbird egg or fragments were found outside the nest, or if the egg was missing

with no apparent decrease in numbers of host eggs. Nests were considered "abandoned" if eggs

were cold and nests undisturbed with no sign of nest activity for three consecutive days. If no

change occurred on subsequent days and the nest remained active, the egg was considered

"accepted". To assure that cowbird eggs were missing due to ej section and not partial nest

predation, I excluded experimental nests that showed partial or complete host egg loss within

two days following apparent cowbird egg ej section.

Selection of Study Species

Study species chosen are known to respond to cowbird parasitism through ej section or nest

abandonment behaviors in other regions: Brown Thrasher, Northern Mockingbird, Northern









Cardinal, and Red-winged Blackbird. Study species were categorized as either ejecters or

accepters based on criteria in Peer et al. (2002) using ejection rates of experimentally parasitized

nests in populations with at least recent exposure to cowbird parasitism. Brown Thrashers were

considered ej ecters (75-100% ej section rate) whereas Northemn Cardinal and Red-winged

Blackbirds were categorized as accepters (0-20% ejection rate). Northern Mockingbirds were

categorized as intermediate ej ecters (21-74%) because ejection rates vary significantly among

populations. Nests of these species were the focus of search efforts in this experiment and

represented an array of generally high to low parasite rej section rates allowing determination of

relative rej section rates in the study. Observed frequencies of rejection and acceptance behaviors

for each study species were compared to expected frequencies based on artificial parasitism

studies in other regions. Expected responses from multiple studies were compared to observed

responses using two-tailed Fisher' s Exact Probability Test. Expected responses (number of cases

of rej ection/acceptance) were 62/28 for Brown Thrasher (Rothstein 1975b, Haas and Haas 1998)

29/26 for Northern Mockingbird (Rothstein 1975b, Friedmann and Kiff 1985, Peer et al. 2002).

For Red-winged Blackbird and Northemn Cardinal, I derived two expected frequencies, a

conservative estimate and one incorporating the highest frequencies reported for each species

(Graham 1988, Clotfelter and Yasukawa 1999). Conservative expected frequencies were 16/143

for Red-winged Blackbird (Rothstein 1975b, Ortega 1991, Prather and Cruz 2006), and 4/60 for

Northern Cardinal (Rothstein 1975b, Eckerle and Breitwisch 1997, Whitehead et al. 2002). High

expected values were 97/284 for Red-winged Blackbird and 33/85 for Northemn Cardinal. If

rej section behaviors contribute to limiting cowbird reproduction in Florida, I predicted that

observed rej section frequencies for each species would be similar or higher in Florida populations

than in representative populations from other regions. If observed frequencies are significantly









lower than expected, the hypothesis will be rej ected indicating that rej section of parasitism does

not explain low observed parasitism of hosts in Florida.

Results

Ten pairs of captive cowbirds produced 34 eggs in 2005 and 122 eggs in 2006 (mean

mass=2.88g, range 1.16-3.54g). At least some experimental eggs were viable because three

eggs that were accepted into nests subsequently hatched, but 47% (27 of 58) of accepted eggs did

not hatch. All cowbirds from hatched eggs were depredated in the nest by natural predators. All

other experimental eggs were ejected by hosts, abandoned in nests, or depredated. For a limited

number of Brown Thrasher nests (N=9), I made observations of behaviors following artificial

parasitism. Egg ejection always occurred within twenty minutes after the return of the female,

and some females that eventually ej ected the egg initially sat on the parasitized clutch.

Eggs obtained from the cowbird colony were used to artificially parasitize 75 nests of the

four selected cowbird hosts (Table 2-1). Brown Thrasher was the only species tested that

consistently rejected cowbird parasitism of their nests while all other species typically accepted

cowbird eggs. Northern Mockingbirds, Northern Cardinals and Red-winged Blackbirds accepted

cowbird eggs in 85%, 95% and 100% of cases, respectively. Observed rejection frequencies

were higher than expected based on the literature for Brown Thrasher, and lower for the other

three study species. Northern Mockingbird showed significantly different rejection frequencies

than expected (P<0.01; Table 2-1). Rejection frequencies for Northern Cardinals and Red-

winged Blackbirds were not significantly different than conservative expected values, but were

significantly lower than high expected values (P<0.05).









Discussion


Rejection Responses of Florida Hosts

The hypothesis that egg rej section behaviors of Florida hosts explain low observed

parasitism frequencies was not supported by the data. Results show a low prevalence of egg

rej section in three of four common cowbird hosts that were experimentally parasitized. Northern

Mockingbird, Northern Cardinal and Red-winged Blackbird accepted cowbird eggs placed in

their nests in the overwhelming maj ority (93%) of cases. Results are consistent with similarly

high acceptance rates of artificial cowbird eggs found for Red-winged Blackbirds in southern

Florida (Prather and Cruz 2006). Northern Cardinals breeding elsewhere in the southeast have

been shown to abandon 50% of naturally parasitized nests (N=4; Whitehead et al. 2002). This

pattern was not observed in north-central Florida where cardinals accepted eggs in 95% of nests

experimentally parasitized, many of which were parasitized before completion of the clutch. In

the one case in which a lone experimental egg was placed in a cardinal nest before host eggs

were laid, the host female still laid an entire clutch in the nest. Surprisingly, Northern

Mockingbirds ejected very few experimental eggs, accepting parasitic eggs in most cases.

Mockingbirds breeding elsewhere show intermediate responses with frequencies of cowbird egg

ejection ranging from 25-100% (Rothstein 1975b, Post et al. 1990, Peer et al. 2002). Frequency

of egg ej section by Northern Mockingbirds in this experiment is the lowest recorded for this

species. Therefore, while all four study species display rejection and abandonment behaviors in

other parts of their distributions, three of four study species do not appear to have recently

developed or retained these behaviors in Florida populations.

Brown Thrashers have only recent exposure to cowbird parasitism in Florida but ej ected

nearly all experimental cowbird eggs and abandoned one parasitized nest. Other studies have

shown a high prevalence of ej section behaviors in populations no longer exposed to brood










parasitism (Rothstein 2001, Peer and Sealy 2004b, Peer et al. 2007). For example, Gray Catbirds

(Dumetella carolinensis) introduced to Bermuda retain ej section behaviors despite an absence of

brood parasites there (Rothstein 2001). There are three possible explanations for observed high

ejection frequency of experimental cowbird eggs by thrashers in this experiment. First,

Rothstein (1975a) estimated that it would take 20-100 years for some parasitized populations to

switch from primarily accepting to ej ecting cowbird eggs. As the overall intensity of parasitism

on the host community in north-central Florida is probably not significant (Chapter 1), rapid

evolution of rej section in Brown Thrashers is unlikely. Second, Florida populations may be

receiving genes for egg rej section from areas with longer exposure to cowbirds (Rothstein 1990).

All four study species, however, have sedentary populations and are year-long residents in

Florida. Sedentary populations have been shown to have significantly lower gene flow than

migratory populations (Arguedas and Parker 2000) suggesting gene flow of "rejection alleles" is

unlikely for Florida populations. Finally, some species may retain rejection behaviors for

extremely long periods of time because the rejection response is selectively neutral in the

absence of parasitism (Rothstein 1975a). For example, Boat-tailed Grackles (Quiscalus major)

may have retained ej section behaviors for as long as 800,000 years without exposure to brood

parasitism (Peer and Sealy 2004b). Fossil evidence suggests that all four study species were

sympatric with cowbird species in central Florida at least 10,000 years ago (Hamon 1964, Emslie

1998). During this time rejection behaviors may have evolved in all of the study species but

were retained only by Brown Thrashers when cowbird species went extinct or changed their

geographic distribution.









Acceptance of Cowbird Eggs

Northern Mockingbirds, Northern Cardinals, and Red-winged Blackbirds typically

accepted experimental parasitism of their nests in this experiment. These species may have

accepted cowbird eggs because they were unable to discriminate cowbird eggs from their own

(Rothstein 1982). Northern Cardinals eggs are similar in size, color, and patterning to Brown-

headed Cowbird eggs whereas the other study species have eggs that differ significantly in

appearance. However, the degree of dissimilarity between host and parasite eggs would not be

important in populations lacking the ability to discriminate eggs. In these populations, any egg

recognition errors may result in hosts damaging or ej ecting their own egg when attempting to

remove cowbird eggs (Rothstein 1977). Therefore, costs associated with host egg loss or

abandonment of unparasitized nests would outweigh the costs associated with retaining rej section

behaviors, and would select for acceptance of eggs in populations with absent (Rarskaft et al.

2006) or limited parasitism pressure, as in Florida.

Many species may accept parasitism because they have ratios of bill length to egg-width

too small to eject cowbird eggs by grasping them (Rothstein 1975b). Some smaller species are

known to eject cowbird eggs by puncturing (Rothstein 1976, 1977, Sealy 1996) but this behavior

is not widespread. Perhaps the unusual strength of cowbird eggs (Picman 1989) makes them

difficult to puncture, resulting in incidental damage to host eggs (Spaw and Rohwer 1987). The

species experimentally parasitized in this experiment are all similar in size or larger than Brown-

headed Cowbirds with bills large enough to grasp cowbird eggs. Therefore, study species did

not accept eggs simply because of morphological constraints. Indeed, Rothstein (1975b)

artificially parasitized four accepter species with miniature cowbird eggs capable of being grasp-

ejected, but eggs were still accepted in 87% of nests. However, many of the species that

constitute the host community in north-central Florida are smaller species (e.g., parulids,









vireonids) that may accept parasitism due to inability to remove larger cowbird eggs, and may

instead rely on other behavioral defenses to cowbird parasitism. For example, early nest

attentiveness (Neudorf and Sealy 1994) and active nest defense (Gill and Sealy 1996) by Yellow

Warblers may deter female cowbird intruders.

Acceptance of cowbird eggs may have occurred in the study species because cues

associated with a female cowbird intruder were not simulated. For example, Meadow Pipits

(Anthus pratensis) ej ect experimental European Cuckoo (Cuculus canorus) eggs more frequently

when a model cuckoo is placed at the nest (Moksnes et al. 1991). However, other studies have

found no correlation between parasite presence and rejection behaviors (Moksnes and Roskaft

1988, Sealy 1995, Soler et al. 2000). In this experiment, lack of response to cowbird eggs in the

nest was likely not due to the host missing the cue of female cowbird presence at the nest.

Responses occurring before cowbird eggs are placed in the nest, however, may be important to

reducing overall cowbird parasitism. Red-winged Blackbirds, for example, often mob cowbirds

and chase them from nesting colonies (Robertson and Norman 1976). Density of nesting Red-

winged Blackbirds has been negatively correlated with parasitism rate suggesting that group

defense may be an important cowbird deterrent (Freeman et al. 1990). Northern Cardinals and

Northern Mockingbirds are also known to practice aggressive behaviors toward many types of

nest intruders (Breitwisch 1988, Nealen and Breitwisch 1997), which may limit the number of

eggs that female cowbirds are able to place in their nests.

The two species known to commonly abandon parasitized nests, Northern Cardinal and

Red-winged Blackbird, did not do so in this experiment. Nest abandonment was not observed

perhaps because it is not strictly a response to cowbird parasitism in some populations. For

example, adults often only abandon nests if too many host eggs are removed (Rothstein 1982,









Rothstein 1986). Furthermore, Least Bell's Vireos only abandoned nests when removal of a host

egg took place one day before it was replaced with a cowbird egg (Kosciuch et al. 2006). These

studies suggest that adults may choose to desert a nest in response to the selection pressure of

nest predation, not necessarily brood parasitism. During experimental parasitism of nests in this

study, a single host egg was removed and replaced by a cowbird egg only in cases when two or

more host eggs were present. Therefore, it is not known if Florida birds would abandon nests as

a result of partial clutch loss instead of addition of cowbird eggs. Individuals in other

populations of Northern Cardinal and Red-winged Blackbird abandon naturally parasitized nests

at significantly higher rates than unparasitized nests (Graham 1988, Ortega 1991, Clotfelter and

Yasukawa 1999, Whitehead et al. 2002). Florida populations in this study appear to lack the

ability to abandon nests strictly in response to brood parasitism, but may employ abandonment

behavior in response to other pressures.

Conclusions

In several species, loss of rej section behavior has been demonstrated in populations that are

no longer exposed to parasitism. For example, American Robins (Turdus migratorius) eject

cowbird eggs only in areas where Brown-headed Cowbirds have historically bred (Briskie et al.

1992). Cruz and Wiley (1989) showed that the prevalence of egg ej section in Village Weavers

(Ploceus cucullatus) declined significantly in a population introduced to Hispaniola where the

selective pressure of brood parasitism was absent. Interestingly, ejection frequencies in the same

population increased from 14% to 83% in 16 years, perhaps due to novel parasitism pressures

exerted by colonizing Shiny Cowbirds (Robert and Sorci 1999). In Florida, I observed low

rej section frequencies for three of the four study species and yet also observed relatively low

natural parasitism frequencies of each species and the entire host community, overall. Results










reported here suggest that selective pressure currently imposed by Brown-headed Cowbirds on

ej section behaviors of certain hosts in Florida is likely weak.

I assumed that the prevalence of rej section behaviors in hosts commonly known to practice

them would be representative of the community of hosts in north-central Florida. However,

cowbirds are known to show preferences for specific hosts (Woolfenden et al. 2003, Ellison et al.

2006) on which they may exert more intense selective pressure for development of anti-

parasitism behaviors. Indeed, observations of family groups feeding fledglings indicate that

species such as Northern Parula and Blue-gray Gnatcatcher may be preferred hosts in north-

central Florida (Chapter 1). Small species such as these that are incapable of ejecting eggs may

instead rely on abandonment behaviors or active defense. Indeed, some species show a positive

relationship between nest defense and degree of parasitism (Robertson and Norman 1976,

Strausberger and Burhans 2001). While results of this study suggest that rejection responses of

some species do not explain parasitism frequencies in north-central Florida, behavioral responses

of preferred host species may be more important to limiting Brown-headed Cowbird

reproduction in the region.










Table 2-1. Results of experimental parasitism of four common Florida species. All species are known to ej ect cowbird eggs or
abandon parasitized nests. Brown Thrashers consistently ej ected cowbird eggs whereas the other species tested generally
accepted cowbird eggs experimentally placed in their nests. P-values indicate results of Fishers Exact Tests comparing
observed frequency of rejection behavior with expected values for each species derived from the literature. For Red-
winged Blackbird and Northern Cardinal, p-values represent comparisons with both conservative and high expected values.
Asterisks (*) indicate comparisons in which observed rej section frequencies were significantly lower than expected for each
species.
Species N Accepted Ejected Abandoned % Rej ected P-value
Brown Thrasher 17 2 14 1 88.2 0.1433

Northern Mockingbird 20 17 3 0 15.0 0.0038*

Red-winged Blackbird 19 19 0 0 0.0 1.00, 0.0058*

Northern Cardinal 19 18 1 0 5.3 0.2433, 0.0427*









CHAPTER 3
DOES VARIATION INT FEMALE COWBIRD FECUNDITY EXPLAIN OB SERVED LOW
LEVELS OF PARASITIZATION?

Introduction

Many ultimate morphological, physiological, and ecological factors are thought to

determine reproductive life-history traits such as clutch size (Moreau 1944, Skutch 1949, Lack

1954, Ashmole 1963, Cody 1966). Intraspecific clutch size increases with latitude (Koenig

1984, Jarvinen 1989, Dunn et al. 2000) and longitude (Johnston 1954, Bell 1996), and decreases

with elevation (Badyaev and Ghalambor 2001, Johnson et al. 2006). These gross geographic

patterns are altered by regionally specific proximate factors. For example, fecundity of Spotted

Owls (Strix occidentalis) was negatively affected by weather patterns prior to and during the

breeding season (LaHaye et al. 2004), perhaps because decreased precipitation and temperature

altered the abundance of food or water resources. Under experimental conditions, breeding birds

given supplemental water (Coe and Rotenberry 2003) or food (Arcese and Smith 1988, Boutin

1990, Nager et al. 1997) will lay larger clutches than birds in control groups. Food availability

and other nutritional factors, therefore, may underlie regional variation in reproductive

parameters such as clutch size or fecundity.

Because both additional energy and specific nutrients are required for egg production, diet

quality and availability of suitable foods may be equally or more important than food abundance

in determining fecundity, clutch size and overall reproductive output. Reproduction of captive

Zebra Finches given food resources ad libitum, for example, was constrained more by nutrient

reserves, energy expenditure (Deerenberg et al. 1996), and diet quality (Semel and Sherman

1991) than by food abundance. Calcium, in particular, is not available in quantities sufficient for

egg production in many environments (Barclay 1994, Graveland and van Gijzen 1994) and birds

often supplement calcium intake during the breeding season (Simkiss 1975, St. Louis and









Breebaart 1991, Graveland 1996). Indeed, Great Tits (Parus major) supplemented with calcium

increased clutch sizes (Tilgar et al. 1999). Songbirds breeding in areas with limited calcium

have reduced reproductive output (Graveland 1990, Graveland et al. 1994, Graveland and Drent

1997). Because the abundance of exchangeable calcium in the soil shows extensive geographic

variation, reproductive indices such as clutch size or fecundity may be correlated with calcium

availability over large spatial scales (Patten 2007).

Brown-headed Cowbirds have the highest known fecundity of any passerine in North

America (Smith and Rothstein 2000). Females can produce as many as 77 eggs in 89 days of

breeding (Holford and Roby 1993). This high rate of egg production is made possible because

the Brown-headed Cowbird is the only known songbird species that lacks ovarian regression

following clutch completion (Lewis 1975) and can start new clutches in as little as one day (Scott

and Ankney 1980). Brown-headed Cowbird fecundity (number of eggs laid per breeding

season), however, shows dramatic geographic variation. Female cowbirds, for example, are

estimated to lay 2-8 eggs per breeding season in New York, 10-12 eggs in Michigan (Payne

1965, 1976), 24-30 eggs in Oklahoma and California (Payne 1965, 1973a), and 40 eggs in

southeastern Ontario (Scott and Ankney 1980). This extreme intraspecific variability in

fecundity suggests that the impacts of cowbird parasitism on hosts may also be highly variable.

In this chapter, I explore the possibility that limited reproduction of cowbirds may underlie low

observed parasitism rates.

Limitations to Cowbird Reproduction

Various nutritional factors have been implicated in regional differences in reproductive

values of Brown-headed Cowbirds over their breeding distribution. Cowbirds exhibit high

fecundity in areas where grain-based agriculture is widespread (e.g., California) probably









because food sources are accessible and abundant. Where food resources are not abundant or are

sparsely distributed, food acquisition may increase reproductive costs. For example, egg-laying

rate of Brown-headed Cowbirds was significantly lower in birds that commuted long rather than

short distances between feeding and breeding areas (Curson and Mathews 2003). Differences

between West Indian islands in abundance of Shiny Cowbirds are attributed to availability of

food rather than hosts (Post et al. 1990). Finally, as the most fecund North American songbird

species, Brown-headed Cowbirds require large amounts of calcium for egg synthesis. As a

result, cowbirds are known to supplement calcium through consumption of removed host eggs

(Scott et al. 1992) and a variety of other calcareous sources (Ankney and Scott 1980). Any

variation in supplemental calcium availability can thus explain regional differences in fecundity,

as well as observed parasitism rates (Holford and Roby 1993). Furthermore, as nest parasites

cowbirds may be uniquely limited by the abundance of nests of suitable hosts. Cowbirds in

northern Michigan do not breed until approximately a month later than those in southern

Michigan, because suitable hosts have not yet begun breeding (Payne 1965).

Low Nest Parasitism in Florida: Low Cowbird Fecundity?

Despite relatively high breeding season abundance of Brown-headed Cowbirds in north-

central Florida (3-10 birds per Breeding Bird Survey Route; Sauer et al. 2006), observed

parasitism frequency is unusually low; only 1.6% of nests of 29 potential host species were

parasitized (Chapter 1). Rates of parasitism on specific host species are also lower than in other

regions, even those that cowbirds have recently invaded. Parasitism rates on Northern Cardinals

(Calrdinalis cardinalis), for example, are 20.1% in Ontario (Peck and James 1998b), 7.5% in

Texas (Barber and Martin 1997), 5.4% in South Carolina (Whitehead et al. 2000), and only 2.3%

in north-central Florida.









To help explain low observed parasitism frequencies, I tested the hypothesis that

reproductive output of Brown-headed Cowbirds was limited in north-central Florida by low

fecundity. To determine differences in reproductive parameters that may influence patterns of

parasitism, I analyzed female reproductive organs to estimate fecundity and clutch size using a

methodology that has been performed for a variety of species (Hannon 1981, Kennedy et al.

1989, Arnold et al. 1997, Pearson and Rohwer 1998, Lindstrom et al. 2006), including Brown-

headed Cowbirds (Payne 1965, 1973a, 1976, Scott and Ankney 1980, 1983). Reproductive

condition of female cowbirds from Florida was compared with cowbirds from Fort Hood, Texas,

an area with high rates of cowbird parasitism (e.g., Barber and Martin 1997). I predicted that

cowbird fecundity, in particular, would be lower in Florida than in Texas but did not specifically

test any mechanism that would explain observed differences.

Methods

Cowbird Collection

Brown-headed Cowbirds were collected at sites in both north-central Florida and east-

central Texas (Figure 3-1). Sites were located at roughly similar latitude to control for potential

biases. Cowbirds breeding in Florida are the subspecies M. ater ater which tend to be larger

(Lowther 1993) than those breeding in Texas, which are typically the dwarf cowbird subspecies

M. a. obscurus (Summers et al. 2006). Texas specimens were trapped or shot at Fort Hood in

Bell and Coryell Counties in central Texas (ca. 31011' N, 97040' W) between April and July,

2005 by the Nature Conservancy according to protocol used by Summers et al. (2006). Cowbird

control has been conducted at Fort Hood since 1988 (Eckrich et al. 1999) due to negative

impacts of parasitism on native breeding birds. Cowbirds were either captured using a series of

baited decoy traps typically placed near free-ranging cattle or shot from within 30 m using a 20-

gauge shotgun, full choke, number 9 lead shot and 2-3/4 inch shotshells. Birds were shot









between 0800-1100 CST when foraging in groups on the ground. Potential local breeders were

distinguished from migrant females based on morphometric, phenotypic and physiological

characters (Summers et al. 2006).

Females were captured in Florida from April-July of 2006 and 2007 in baited drop-down

decoy traps. Traps were baited daily with F-R-M Game Bird Starter (Flint River Mills,

Bainbridge, GA) placed on a mesh tray below trap openings. In 2006, two trap sizes were used.

Large traps (2.4 x 2.4 x 1.8 m) owned by the USDA National Wildlife Research Center, Florida

Field Station were constructed of wood and wire mesh. Medium-sized traps (1.7 x 1.4 x 1.4 m)

were based on the "pick-up trap" design of the Texas Parks and Wildlife Department (2008) and

were primarily constructed of one-inch hardware cloth. Small traps (1.45 x 1.2 x 1.2 m) of

similar design were used in 2007. Live cowbird decoys placed in traps typically included one

female and one or more male cowbirds collected in September 2004 and housed at the USDA

Wildlife Research Center in Gainesville, FL. Traps were checked daily from 0900-1100 or

1400-1600 EST and females were euthanized humanely on site via cervical dislocation

according to IACUC-approved protocol. Trapping effort ranged from one to eight open traps per

day at up to three sites. In 2006, trapping was conducted at the University of Florida (UF) Dairy

Research Unit (DRU), Beef Research Unit (BRU), Horse Teaching Unit, and Santa Fe Beef

Research Unit in Alachua County, FL. In 2007, trapping was conducted at the same sites in

addition to the UF Large Animal Clinic and Beef Teaching Unit (BTU).

With the onset of the breeding season in May, the number of female cowbirds that can be

trapped often drops significantly (Beezley and Rieger 1987, Summers et al. 2006). Shooting of

females in feeding areas allows a steady collection of specimens throughout the breeding season

(Summers et al. 2006). Trapping of cowbirds may also be biased toward individuals of poorer









condition (Dufour and Weatherhead 1991). Therefore, trapping was supplemented in Florida in

2007 with collection of individual birds using pellets and a Crosman 664 Powermaster

pneumatic air rifle. Birds were shot opportunistically between 1400 and 1800 EST at DRU,

BRU, and BTU sites in or near fields where they foraged with cattle. l used playback of female

chatter calls in some cases to improve shooting efficiency as recommended by Rothstein et al.

(1987). All collected birds were labeled and frozen until analyzed.

Reproductive Assays

All females collected were analyzed at the Florida Museum of Natural History in

Gainesville, FL. Breeding condition of females was assessed using pre-ovulatory and post-

ovulatory follicles (POFs; Scott and Ankney 1983). Pre-ovulatory follicles represent developing

ova that are subsequently released into the abdominal cavity and taken into the oviduct for egg

production. After ovulation, POFs remain attached to the ovary and are reabsorbed over time.

The condition of the oviduct was examined for presence of an oviducal egg or signs that an egg

had passed (straight vs. convoluted). Ovaries were examined with a dissection probe, #5

tweezers, and Bausch & Lomb Stereo Zoom 4 (7-30x magnification) dissecting microscope

because macroscopic counts of follicles may be unreliable (Arnold et al. 1997).

I recorded the number and size of pre-ovulatory follicles (ones that could potentially

produce the yolk of an egg) in each specimen. The widths of pre-ovulatory follicles were

measured to the nearest 0.01mm using a Mitutoyo Absolute Digimatic caliper (model CD6:CS).

Round, orange-yellow pre-ovulatory follicles were distinguished from degenerating or atretic

follicles which are irregular in shape and cream-colored. Pre-ovulatory follicles generally grow

quickly and sequences of developing follicles may represent eggs of the same clutch. K-means

hierarchical cluster analysis (maximum 10 iterations) using SPSS (2003) was used to group









follicles into categories A, B, or C based on size (sensu Payne 1965, Scott 1978) to identify

distinguishable sequence breaks representing separate clutches. A follicles are those that would

likely have ovulated on the next day following collection, B follicles two days from collection

and so on. Although smaller pre-ovulatory follicles (e.g., D follicles) were sometimes present in

birds in the sample, some follicles are known to regress before ovulation (Payne 1976).

However, Scott (1978) found only 3 cases of atresia in 69 follicles >5mm indicating that larger

follicles (i.e., A and B) are unlikely to regress before releasing the yolk. Therefore, analysis was

limited to three clusters to improve the probability of including only follicles that would reach

ovulation.

I also recorded the number and size of all recognizable POFs which are flat and oval-

shaped and have an open slit (stigma) through which ova are released into the abdominal cavity

during ovulation. Only those follicles with ruptured stigma were considered POFs. POFs were

distinguished from burst atretic follicles (Pearson and Rohwer 1998) by examining color and

other differentiating characteristics (Scott and Ankney 1983). Burst atretic follicles are ruptured

but typically contain a cream-colored residue that is lacking in POFs. The area of each flattened

POF was estimated as the product of width and maximum lengths measured to the nearest

0.01mm.

Ovaries, oviducts, and oviducal eggs were removed and weighed to the nearest 0.0001g

using a Mettler Toledo Analytical Balance (model AB54-S). Average start and end dates of the

cowbird breeding season were determined separately for Texas and Florida based on presence of

enlarged oviducts, large pre-ovulatory follicles (i.e., A follicles) and oviducal eggs. Collected

birds found in reproductive condition with enlarged oviducts (>0.7g), oviducal eggs, or the

presence of one or more large pre-ovulatory (A follicles) or post-ovulatory follicles are









"breeding". All other birds are considered "non-breeding" birds. Breeding birds that were

collected with a developing egg anywhere in the oviduct are called "laying" birds while all other

breeding birds are called "non-laying" (sensu Scott and Ankney 1980).

Shapiro-Wilk tests of normality indicated that body mass, ovary mass, and oviduct mass

variables from each collection location were normally distributed (P>0.15 for all). Therefore,

measures of reproductive condition of breeding birds were compared between Florida and Texas,

and laying and non-laying cowbirds using two-tailed t-tests assuming unequal variances in SPSS

(2003). Change in body mass over the course of the breeding season at each collection location

was analyzed using simple linear regression. Finally, as a result of the change in size of

reproductive organs with the beginning and end of the breeding season, I used regression

analysis to test if mass of oviducts and ovaries of breeding birds differed between Florida and

Texas. PROC GLM (SAS Institute Inc. 2003) with Type IV sum of squares was used to fit a

general linear model to the data because it allows for both continuous and categorical predictor

variables. The response variables in each of the two models were oviduct mass and ovary mass

with collection week and location (Florida and Texas) as predictor variables. A quadratic

regression function was applied to the week variable in both models due to the observed growth

and regression curve of reproductive organs. Residuals plots were used to assure that there was a

good fit between the model and the data.

Clutch Size Estimates

Average clutch size for Brown-headed Cowbirds has been estimated based on sequences of

developing pre-ovulatory follicles and regressing post-ovulatory follicles (Payne 1965, Scott

1978). There are, however, some limitations to using this method. Some developing follicles

may regress prior to ovulation (Payne 1976), released ova may get reabsorbed before entering

the oviduct (Meyer et al. 1947) and post-ovulatory follicles may be difficult to distinguish from









burst atretic follicles (Payne 1965, Lindstrom et al. 2006). As a result, clutch size can be

overestimated when relying on assumed follicle sequences (Davis 1942, Payne 1965, Scott 1978,

Arnold et al. 1997). Clutch size can also be underestimated using post-ovulatory follicles if they

are reabsorbed quickly (Arnold et al. 1997). For these reasons, I estimated clutch size by

determining numbers of post-ovulatory follicles observed in breeding birds (Scott and Ankney

1983).

The method used relies on two clutch size estimates based on numbers of post-ovulatory

follicles in (1) laying birds and (2) non-laying birds. First, I estimated the total number of eggs

laying birds would have laid in their current clutch. This number was derived by finding the

total number of clearly-identifiable POFs in the sample population of laying birds, which should

represent the average midpoint of laying sequences. That is, a sample of 10 laying cowbirds

with an average clutch size of 5 would lay 50 eggs and develop 50 POFs over the laying period.

At the midpoint of their sequences they should have 30 POFs because the third POF represents

the midpoint of a 5-egg clutch. Thus, the midpoint number of POFs can be used to determine the

total number of eggs the sample would have laid by multiplying it by 2, and subtracting the

sample size (i.e., (30 x 2)-10=50). Because size breaks in regressing follicle sequences indicate

distinct clutches (Payne 1976, Hannon 1981), I only included POFs that were clearly in the same

laying sequence.

Second, clutch size of non-laying birds was estimated. Non-laying birds do not have

oviducal eggs by definition, so they are likely in between clutches. Therefore, recent clutch sizes

were estimated based on total number of POFs divided by the sample size. In some cases, non-

laying birds did not have clearly identifiable POFs so were assumed to have laid a clutch earlier.

This assumption is supported by the observation that all non-laying birds showed evidence of









egg production (e.g., enlarged and convoluted oviducts). Because it is assumed that these birds

had the same frequency distribution of clutch sizes, the contribution of these birds was estimated

as the same proportion of POFs recorded in all other non-laying birds. For example, if a sample

of 50 non-laying birds included 40 birds with 60 POFs (1.5 POFs per bird) the remaining 10

individuals without identifiable POFs would be estimated to have 15 POFs (10 birds x 1.5). To

estimate average clutch size, the sum of (1) total number of eggs that laying birds would have

produced and (2) the estimated number of POFs of non-laying birds, were divided by the total

number of birds in the sample. Estimates were performed separately for Florida and Texas

samples.

Inter-clutch interval (number of days between distinct clutches) was estimated using the

proportion of non-laying birds in a particular portion of the non-laying period (Scott and Ankney

1979). For example, some birds without an oviducal egg may have finished laying their clutch

on the morning before collection. These birds would have large POFs that are less than a day old

indicating ovulation occurred on the day prior to collection. This would represent the first day of

the non-laying period for these females. Similarly, large pre-ovulatory follicles (A follicles) in a

cowbird without an egg in the oviduct would represent the end of the non-laying interval since

ovulation was likely to occur on the following day. The reciprocal of the proportion of non-

laying birds in each of these stages should represent the inter-clutch interval. For example, in a

sample of non-laying birds with an inter-clutch interval of two days, roughly one-half of the

birds should be in one of the two non-laying days. These two non-laying states were used to

estimate the non-laying interval in Florida and Texas female cowbirds.

Fecundity Estimates

Fecundity in this study is defined as the number of eggs that an average female cowbird

lays over a breeding season of average length. Average annual cowbird fecundity in each study









area was determined as the product of daily fecundity (laying rate) and duration of the cowbird

breeding season (Scott and Ankney 1980). Daily fecundity was calculated using the proportions

of breeding birds collected that had an egg in the oviduct (Payne 1973b). Average dates of the

breeding season in Texas and Florida were estimated as the first and last days when

approximately half of birds had enlarged oviducts, large pre-ovulatory follicles or oviducal eggs.

Only breeding birds that were collected between respective breeding season dates in Texas and

Florida were included in fecundity estimates. I assumed that all birds had an equal probability of

being collected regardless of whether or not they were carrying a developing egg. Scott &

Ankney (1979) found that the proportion of collected birds carrying an egg was not affected by

variables such as collection location, method, or time of day, among others.

Results

Cowbird Collection

A total 142 female cowbirds were collected from Texas between 22 March and 6 July,

2005. The majority (N=94) of these specimens were collected by shooting. A total of 71 and 63

female cowbirds were collected in Florida between 23 April and 12 July, 2006 and 2007,

respectively (Figure 3-2). Total effort (traps open x days open) was 146 trap days in 2006 and

463 in 2007. In general, more females were caught per trap day in large traps (0.415) than in

small and medium traps combined (0.061; Figure 3-3). All 2006 specimens were collected in

traps, the maj ority (59%) of which was trapped in July when a higher proportion of individuals

was no longer in breeding condition. Trapping success was low from April to May in both years

but remained low in June only in 2007 (Figure 3-3). Trapping success increased from May to

June in 2006 but the maj ority of these birds (74%) was collected after June 21 when some birds

were in declining or non-reproductive condition (see below). Shooting of individual female

cowbirds in 2007 greatly increased sample sizes during May and June (Figure 3-4).










Body mass of breeding Florida cowbirds was significantly lower in trapped than shot birds

(35.2 vs. 38.5g, t=-4.581, P<0.001) but there was no difference when mass of oviducal eggs was

removed (34.3 vs. 31.6g, t=1.115, P=0.272). Ovaries (0.3250 vs. 0.5217g; t=-0.3838, P<0.001)

and oviducts (1.0253 vs. 1.4126g, t=-4.495, P<0.001) were also significantly lighter in trapped

birds. However, wing chord (t=0.013, PO.990) and tail length (t=-0.767, P=0.448) were similar

in trapped and shot birds in Florida. Among 20 Florida cowbirds collected by shooting during

the breeding season, 17 had an oviducal egg. Furthermore, laying birds that were shot were

more likely to have an oviducal egg than trapped birds (Log Odds Ratio=2.76, CI=3.63-68.41).

Reproductive Assays

Reproductive assays were conducted on all female cowbird specimens collected. Of 142

Texas females collected, 79 had enlarged oviducts, POFs, large pre-ovulatory follicles, or

oviducal eggs and were categorized as breeding birds. Of breeding birds, 40 were found with an

egg in the oviduct and were called laying birds, and the remaining 39 were categorized as non-

laying birds. 134 specimens were collected in Florida and 56 were called breeding birds. Of

Florida breeding birds, 29 were laying birds and 27 were non-laying birds.

It was not possible to test for any effect of year on reproductive measures due to small

sample sizes for part of the breeding season in Florida in 2006. However, there were no

differences (independent samples t-test, P>0.20 for all) in the number of pre- and post-ovulatory

follicles, body mass, or mass of oviducts and ovaries of samples collected during 1 June to 21

June in 2006 (N=8) or 2007 (N=16). These variables were also not significantly different when

samples from 2006 were paired with randomly-chosen samples from similar collection dates (+ 2

days) in 2007 (paired samples t-test, P>0.30 for all, N=8). Therefore, all data presented for

Florida birds represent pooled data over both collection years.










Ovary measurements (f SE) of Texas cowbirds collected between 22 March and 6 April

showed no signs of breeding season development (26.89 f 1.21lmm2, 0.027 f 0.002g, N=40).

Oviducts were similarly small and undeveloped during this time (0.043 f 0.009g, N=7). For

both Florida and Texas females, growth and maturation of ovarian follicles led to a dramatic (ca.

14-fold in Texas) increase in mean ovary mass in mid- to late-April (Figure 3-5). Similarly,

oviducts of collected specimens increased significantly (ca. 28-fold in Texas) in mid- to late

April (Figure 3-6). Mean ovary and oviduct mass remained fairly consistent from May to early

June, although there was much individual variation due to the dynamic nature of ovarian follicle

growth and yolk release during ovulation (Figures 3-7 to 3-10).

On average, laying birds in Florida had significantly heavier ovaries (t=2.480, P=0.017)

and oviducts (t=3.429, P=0.001) than non-laying birds (Figures 3-7, 3-8). However, breeding

birds without oviducal eggs still had large oviducts (i.e., mean mass >1.0g), probably because

they recently completed clutches or were preparing to ovulate. Laying birds in Texas had

significantly larger ovaries (t=2.091, P=0.041), but not oviducts (t=1.263, P=0.212) than non-

laying birds (Figures 3-9, 3-10). Ovaries and oviducts began to regress in the second week of

June as resource allocation to reproduction was likely waning with the end of the cowbird

breeding season. Breeding females from Texas and Florida had similar ovary mass (t=0.805,

P=0.422) over the course of the breeding season (Table 3-1) but oviducts were significantly

heavier in Texas than Florida females (t=-2.304, P=0.023). Because growth and regression of

reproductive organs roughly followed a bell-shaped curve, I applied a general linear model to the

data to determine if oviduct and ovary mass differed between Florida and Texas cowbirds as a

function of week of the breeding season. The regression model indicated that collection location

was not a significant predictor of ovary mass (parameter estimate=0.037, F=0.88, P=0.350;









Figure 3-7). However, the model indicated that collection site was a significant predictor of

oviduct mass (parameter estimate=-0. 129, F=5.17, P=0.025) with Texas females having heavier

oviducts than Florida females.

Body mass of Florida females declined significantly from 23 April until 12 July (R2=0.161,

P<0.001, N=1 10; Figure 3-11). Mass of breeding birds in Florida tended to decrease over time

but not significantly (R2=0.024, P=0.263, N=53) especially when the mass of the oviducal egg

was removed from body mass (R2=0.006, P=0.585). Mean mass of breeding females (R2<0.001,

P=0.990, N=58) and all cowbirds collected in Texas between 19 April and 6 July (R2=0.001,

P=0.826, N=79) did not change significantly with time (Figure 3-12). Similarly, no body mass

change was observed over time when oviducal egg mass was removed from body mass of laying

birds (R2=0.012, P=0.429). Overall body mass decline from April to July in Florida birds was

likely due to mass differences between pre-breeding season birds prepared for reproduction and

post-breeding birds with regressing reproductive organs. That mass declined over the entire

collection period in Florida but not Texas is due to a significant difference in sample size of birds

collected after 25 June (Florida=81, Texas=6) when most birds are no longer in breeding

condition. Although the eastern subspecies of cowbird is generally larger than the dwarf

cowbird of Texas (Lowther 1993, Pyle 1997), mass breeding Florida and Texas females was not

different (t=-1.1 13, P=0.268). Similarly, wing chords of Florida breeders also were not

significantly different from those of Texas cowbirds (Table 3-1) collected at Fort Hood during

previous breeding seasons (Summers et al. 2006).

Increase in ovarian mass was associated with the development of pre-ovulatory follicles in

late April. Oocytes (immature ova) were generally ca. 1.0mm in diameter in March and early

April whereas breeding birds had one or more large orange-yellow yolky follicles representing









potential eggs. A follicles (those likely to ovulate the next day) were as large as 10.50mm in

diameter in Texas and 9.89mm in Florida. Sequences of developing follicles were determined

using cluster analysis. For Florida birds, A, B, and C follicles were those that were larger than

8.57, 5.85, and 3.39mm diameter, respectively. Similarly, Texas birds cluster cutoff values for

A-C follicles were 8.72, 6.38 and 3.37mm diameter, respectively. These values are similar to

those reported by Scott (1978) who found that the three largest pre-ovulatory follicles clustered

at >8mm, and ca. 6 and 4mm. Earliest and latest A follicles were found in cowbirds collected on

26 April and 28 June in Texas and 26 April and 27 June in Florida.

All but four breeding individuals had at least one post-ovulatory follicle (POFs), although

birds without POFs were still clearly in reproductive condition. All laying birds had one large

POF that was associated with the egg in their oviduct and which released the yolk one day prior

to collection (Day -1). Laying birds had as many as four POFs that showed a graded series in

size. Mean areas (f SE) of the sequence of POFs from largest (Day -1) to smallest (Day -4) in

laying birds in Florida were 22.89 f 0.72, 13.01 f 0.47, 7.80 f 0.44, and 6.57 f 1.50mm2,

respectively. In Texas cowbirds, mean areas of POFs in this sequence were 25.89 f 0.85, 25.42

f 0.73, 12.01 f 0.75, and 10.43 f 1.38mm2. Day -2 POFs that were identified as likely being

part of the same sequence showed ca. 40% decrease in size from the most recent follicle. This

indicated that POFs regressed rapidly and that follicles older than four days would likely be

difficult to identify. Furthermore, the three most recent POFs (Days -1, -2, and -3) of laying

birds were significantly larger in Texas than in Florida (t-tests, P<0.01 for all; Figure 3-13).

POFs from Days -1 and -2 were ca. 13% and 18.5% smaller in laying Florida specimens, a result

which may be caused by differences in the timing of specimen collection between the two study

areas. The sample of Florida cowbirds included individuals collected throughout daylight hours









whereas Texas cowbirds were all captured before 1100 hours. Ovulation in Brown-headed

Cowbirds occurs around 0700 hours (Scott and Ankney 1983) after which the follicle begins

regressing. I was not able to specifically test effect of time of collection on size of POFs because

appropriate data were not collected. However, all birds that were shot were collected in the

afternoon. Among laying birds collected in Florida, the largest POF was significantly smaller

(t=2.653, P=0.017) in birds collected by shooting (N=18) versus trapping (N=10). This may

explain observed differences between Florida and Texas in POF size. However, the total number

of POFs counted in laying birds in Florida was the same regardless of collection method (t=-

3.760, P=0752) suggesting that all POFs that were present following ovulation in the morning

were recognizable in birds collected later in the day.

Clutch Size Estimates

Clutch size and inter-clutch interval estimates were determined using the sample of

breeding birds from each collection location. In Texas, 40 laying females had between one and

four readily identifiable POFs and most commonly three (Table 3-2). The mid-point of the

laying sequence for these birds was estimated as 108 total POFs. Two non-laying birds were

excluded from counts due to the poor condition of their ovaries. A total of 29 non-laying

females had 66 recognizable POFs (2.3 POFs/bird). The remaining eight non-laying birds

without POFs were therefore estimated to contribute 18 POFs (8 birds x 2.3). Based on these

Eigures, average clutch size of breeding birds in Texas is estimated as 3.3 8 eggs per female

(Table 3-2).

In Florida, one laying bird was excluded from clutch size estimates because the ovary was

damaged when collected. A total of 28 laying females also had between one and four POFs,

with a mode of three (Table 3-3). These females produced 83 POFs. Twenty-three non-laying

birds had 46 distinct POFs (2.0 POFs/bird) so that the remaining four birds without POFs were









estimated to have eight POFs. Average clutch size of breeding birds in Florida is estimated as

3.49 eggs per female (Table 3-3).

Inter-clutch intervals were estimated using two methods that use the proportion of non-

laying birds with (1) size A pre-ovulatory follicles and (2) POFs large enough to have ovulated

an egg on the day before collection. In Texas, 13 of 37 non-laying birds had size A pre-ovulatory

follicles that would likely have ovulated on the next day. Of 27 non-laying birds in Florida, 12

had size A ovulatory follicles. The reciprocals of these proportions results in estimates of inter-

clutch interval of 2.85 and 2.25 for Texas and Florida, respectively.

For method 2, I determined the size of the smallest POF in all laying birds that could have

released the egg found in the oviduct. I assumed any non-laying bird with a POF larger than this

size ovulated on the previous day. Since POFs regress approximately 50% in 12 hours following

ovulation (Scott and Ankney 1983), I used the smallest POF of a laying bird collected in the

afternoon to establish a cutoff size for POFs in non-laying birds. These values are 18.82mm2 and

15.01lmm2 for Texas and Florida, respectively. Twenty of the 37 non-laying birds in Texas had

POFs that indicated that the last day of ovulation was one day prior to collection. Of 27 non-

laying Florida birds, 14 had large POFs. The reciprocals of these two proportions result in inter-

clutch interval estimates of 1.85 and 1.92 days for Texas and Florida, respectively. Based on

both estimation methods, the average non-laying period of female Brown-headed Cowbirds is

approximately 2-3 days in Texas and 2 days in Florida.

In five captive Florida Brown-headed Cowbird females used to generate eggs for

experimental parasitism research (Chapter 2), sequences of actual egg-laying in artificial nests

resulted in a mean estimate (f SE) of clutch size (3.68 f 0.57 eggs) and inter-clutch interval

(1.80 f 0.23 days) similar to those estimated above for wild-breeding populations.









Fecundity Estimates

Average fecundity of cowbirds was calculated based on the method of Scott & Ankney

(1980) using length of the breeding season and daily laying rate. Average start and end dates to

the laying season were estimated using the presence of enlarged oviducts, large pre-ovulatory

follicles and oviducal eggs. Of the eight Florida birds collected from April 23 to 29, three had

oviducal eggs and three of the five non-laying birds had large POFs and mid-sized (B) pre-

ovulatory follicles and were therefore in breeding condition. The earliest bird that had an

oviducal egg was collected on 26 April but had three distinct POFs indicating the ovulation

probably began as early as 23 April. I was not able to collect an adequate sample of birds prior

to this date. The earliest specimen was collected 23 April and had no POFs but some developing

follicles. Therefore, I assume that the average female begins breeding during the final week of

April. Therefore I estimate an average start date to the cowbird breeding season in Florida to be

26 April.

Females were collected in Florida as late as 17 July but the last female with an oviducal

egg was collected on 6 July. This female is likely an outlier because only one of the remaining

17 females collected in the first week of July had post-ovulatory follicles that indicated laying of

an egg could have occurred in early July. One of 46 birds collected after this date had a single

small post-ovulatory follicle, indicating that laying had likely ceased after the first week of July.

To assign an average end date to the cowbird breeding season in Florida, I estimated the

last date when ca. 50% of the birds were in breeding condition. From 25 June to 1 July, only one

of 18 birds collected had an oviducal egg. Of the remaining 17, six were likely still in breeding

condition due to the presence of pre-ovulatory follicles larger than 5mm in ovaries with little or

no atresia. Therefore, 38% of birds were still in laying condition. From 18-24 June, four of









seven birds (57%) were in breeding condition (one with an oviducal egg). Central dates for each

of these weeks are 21 June and 28 June respectively. If it is assumed that the percentage of birds

in breeding condition decreases steadily between these dates, the first date when less than 50% of

birds were in laying condition is estimated to be 24 June (48.9% of birds).

In Texas, birds were collected beginning in March and the first bird with an oviducal egg

was collected on 26 April. The presence of two large POFs indicated the ovulation could have

begun as early as 24 April. However, of the other six birds collected during 23-29 April none

had ovaries with developing follicles so were not in breeding condition. Because the sample

from Texas does not include sufficient sample sizes of identified breeders from mid-late April to

early May to deduce an average start date, I estimate a similar start date for Texas birds as those

from Florida. Reproductive behaviors of Brown-headed Cowbirds begin as early as late March

but few cowbirds are collected with an oviducal egg until mid-April (S. Summers, pers. comm.).

Since the start date estimates when an average bird is laying (i.e., 50% of the population), 26

April represents at least a conservative estimate of the beginning of the breeding season in Texas.

The latest oviducal egg-bearing female captured at Fort Hood was collected 12 July, but

for most years the latest is typically around 9 July (S. Summers pers. comm.). Consistent with

these observations, the last Texas specimen analyzed that had an oviducal egg was collected on 6

July. From 25 June to 1 July, four of nine birds collected had an oviducal egg. Of the remaining

five, none were likely still in breeding condition because they lacked developing follicles.

Therefore 44% of birds were still in laying condition. From 18-24 June, 10 of 16 birds were in

breeding condition, seven of which had an oviducal egg. Using central dates and the assumption

of a steady decrease in breeding condition, the first date when less than 50% of birds were in

laying condition is estimated to be 26 June (49.6% of birds).









In sum, I estimate the average breeding season length as 60 days from 26 April to 24 June

in Florida and 62 days from 26 April to 26 June in Texas. Both starting and ending dates are

earlier than those reported for Brown-headed Cowbirds breeding in southern Ontario (Scott and

Ankney 1980) but estimated breeding seasons for Florida and Texas are 4-6 days longer.

Nine birds from Florida and seven birds from Texas that were in reproductive condition

were excluded from estimates of daily laying rate because they were collected after the average

breeding season end dates. Of excluded birds, two Florida and five Texas females had

developing eggs in their oviducts. Daily laying rate was estimated as the proportion of breeding

birds with eggs in the oviduct (Payne 1973b). In Texas, 40 of 71 breeding birds collected

between 26 April and 26 June had oviducal eggs resulting in an average daily laying rate of 0.56

eggs per female per day. In Florida, 27 of 47 breeding birds collected between 26 April and 24

June resulting in an average daily laying rate of 0.57 eggs. Small sample sizes precluded

analysis, but daily laying rate showed some minor variation during the breeding season (Figure

3-14). Using average laying rates and the respective lengths of the breeding season, it is

estimated that each female Brown-headed Cowbird in both Texas and Florida lays an average of

34-35 eggs in a breeding season (TX: 62 days x 0.56 eggs/day = 34.93, FL: 60 days x 0.57

eggs/day = 34.47). Because these estimates only include breeding individuals, they only

represent average fecundity for birds in reproductive condition and not fecundity of the entire

population of birds present in the breeding area.

Additional Trapping Observations

During collection of Brown-headed Cowbirds in Florida, we collected four specimens of

Shiny Cowbirds, three female and one male. Because there are few diagnostic features that

distinguish females of Shiny and Brown-headed Cowbird (see Pyle 1997), tissues from each









female specimen were sent to the Bell Museum in Minneapolis for genetic analyses to confirm

species identification. DNA was extracted from the samples and PCR was used to amplify two

different loci. The first locus was a 486-base-pair fragment of the cytochrome b gene from the

mitochondrial genome. The second locus was a 565-base-pair fragment from the third intron of

the MUSK gene on the Z chromosome. Sequences were compared to known M. ater and M~

bonariensis sequences deposited in GenBank and youchered tissue museum samples to

determine species identity. All three specimens were confirmed as being Shiny Cowbirds.

These contribute to only a handful of reported cases of Shiny Cowbirds in north-central Florida,

all occurring since 2000. Furthermore, one of the females collected (UF 46077) also had a

developed ovary with three large pre-ovulatory follicles and a single potential post-ovulatory

follicle. The oviduct was large (~1g) and convoluted indicating that it recently passed an egg.

These observations represent the best evidence to date of Shiny Cowbirds breeding in Florida.

Discussion

Based on assays of ovarian and oviducal condition, an average female Brown-headed

Cowbird breeding in north-central Florida and Texas lays approximately 35 eggs over the course

of an 8-9 week breeding season. This estimate of fecundity is slightly lower than the 40 eggs

determined for birds breeding in Manitoba (Scott and Ankney 1980) and higher than the 30 eggs

derived for cowbirds in California (Payne 1965, Fleischer et al. 1987). Estimated fecundity and

clutch size of Florida cowbirds were nearly identical to values derived for females breeding in

east-central Texas, where nest parasitism has led to critical declines of some species of native

songbirds (Eckrich et al. 1999). Similarly, clutch sizes and inter-laying intervals were similar

between female cowbirds breeding in Florida and Texas. Therefore, the hypothesis that low

fecundity of cowbirds in Florida explains their observed low rates of parasitism is not supported.









Factors that have been shown to affect reproductive indices in other studies do not appear

to significantly affect Brown-headed Cowbirds in Florida. For example, calcium availability can

limit reproduction in some populations. While some of the soils in north-central Florida are

underlain with sand or clay and are depauperate in calcium (Abrahamson and Hartnett 1990),

availability of calcium for egg production is probably not limited. The physiographic regions of

north and central Florida are characterized by many areas with limestone at or near the surface

(Brown et al. 1990), providing a rich source of calcium carbonate. Furthermore, the Floridan

Aquifer is one of the most productive carbonate aquifers in the world (USGS 1990) and

dissolved limestone delivers large concentrations of calcium into a water system that feeds the

entire state (USGS 1990). Therefore, calcium is at least secondarily available through calcareous

stone, or through bone and mollusc shells often eaten by cowbirds (Ankney and Scott 1980). In

addition, Schoech and Bowman (2003) found no difference in plasma calcium levels between

suburban and natural populations of Florida Scrub Jays (Aphelocoma coerulescens) breeding in

central Florida. The authors assumed that natural calcium sources would be highly limited in

natural scrub habitats due to low pH levels, yet jays were able to find ample calcium for

production of eggs.

Individual female Brown-headed Cowbirds breeding in north-central Florida and Texas

also appear to be able to get ample food resources for production of eggs. In 2005, Florida

ranked seventeenth in the United States in the total number of cattle (USDA 2002b) and Alachua

County currently has nearly 49,000 head of beef and dairy cows (USDA 2007) on approximately

86,000 acres of agricultural land (USDA 2002a). Therefore, there are numerous feeding

locations in the study area where female cowbirds can acquire resources for egg production.

Cowbirds were observed foraging at all trapping locations as well as on road sides, at city parks,










and private bird feeders. I did not specifically measure abundance of cowbirds at these locations.

However, abundance at feeding areas where collection was performed varied by site and time of

the breeding season. At collection sites with the largest numbers of dairy and beef cows

(200-800), relatively few females were captured or seen foraging with cattle. Two sites located

near the largest urban area in the county (Gainesville) had large mixed-sex flocks with as many

as 100-150 individuals during peak foraging hours. Therefore, major nutritional factors thought

to determine cowbird fecundity or clutch size in other regions or in other species, do not appear

to be limiting fecundity in Florida cowbirds.

Clutch Size Estimates

Clutch sizes in this study were estimated using the number of readily recognizable post-

ovulatory follicles that were in the same laying sequence. Clutch size estimates for Florida (3.68

eggs) and Texas (3.38 eggs) cowbirds were smaller than those (3.91-4.60 eggs) derived using a

similar methodology for cowbirds in various locations in the United States (Payne 1976, Scott

and Ankney 1983). Clutch sizes are known to increase with latitude (Koenig 1984, Jarvinen

1989, Dunn et al. 2000) which may partially explain lower observed clutch sizes than those from

Michigan and Ontario (Payne 1976, Scott and Ankney 1983).

Lower clutch size estimates for the cowbirds in this study may also have been due to subtle

differences in methodology or study populations. Post-ovulatory follicles in Brown-headed

Cowbirds regress rapidly with 50% decrease in size every 12 hours in the two to three days

following ovulation (Scott and Ankney 1983). Furthermore, the opening of ruptured post-

ovulatory follicles closes in 3-4 days post-ovulation (Scott and Ankney 1983). As a result, it is

often difficult to categorize follicles older than 3-5 days as having released an ovum. On the

other hand, Payne (1965) was able to count post-ovulatory follicles of Michigan cowbirds for as










many as twelve days past ovulation. In the present study, the highest number of flaccid follicles

of any kind (post-ovulatory, atretic, burst prior to ovulation) found in a single specimen was six.

Assuming some of these follicles were misclassified and indeed all post-ovulatory, it still

represents only half the maximum number that Payne counted. Therefore, the clutch sizes I

estimated may be lower than those in the literature because I conservatively included only

follicles that could reliably be distinguished as post-ovulatory, and cowbirds I collected may

have absorbed post-ovulatory follicles relatively more quickly.

While an average wild female cowbird lays for 3-4 days in a row, captive females have

laid eggs for up to 13 consecutive days in Florida (Chapter 2), and are capable of laying 67 eggs

in as many days (Holford and Roby 1993). Therefore, the method used in this study probably

generally underestimates clutch size because some birds will lay for many days in a row but

older post-ovulatory follicles will be difficult or impossible to identify. For birds such as Brown-

headed Cowbirds that are indeterminate layers and not constrained by energetic or time demands

of incubating eggs and caring for young, clutch size may be of little significance. That is, length

of the laying sequences is probably less important than the daily laying rate, for example,

because a female cowbird can have a small clutch with short inter-clutch interval and lay as

many eggs in the same time span as a female with a large clutch. Therefore, overall fecundity is

likely a more appropriate and consistent measure of Brown-headed Cowbird reproduction in an

area.

Fecundity Estimates

Estimates of daily laying rate in this study were only slightly lower than those in other

published studies of cowbird reproduction. Based on proportions of birds with oviducal eggs, I

estimated that breeding female cowbirds in both Florida and Texas laid approximately 0.56 to









0.57 eggs per day. Other studies have estimated average daily laying rates from 0.64 to 0.73

eggs per day using similar methodologies (Scott and Ankney 1980, Fleischer et al. 1987).

Differences in daily laying rate, however, may be related to the variation in the length of the

cowbird breeding season geographically. Other studies have shown a decreasing avian breeding

season length with increasing latitude (Baker 1939, Johnston 1954). Average cowbird breeding

season length is longer in Florida (60 days), Texas (62 days), and Oklahoma (8-10 weeks; Payne

1976) than in Ontario (56 days; Scott and Ankney 1980) and Michigan (5-6 weeks; Payne

1976). Cowbirds breeding in areas with short seasons, whether due to abiotic (e.g., climate,

photoperiod) or biotic (e.g., host availability) factors, may have relatively higher average laying

rates to maximize overall fecundity. I assumed all birds stayed on site throughout the breeding

season and that daily laying rate was similar over the course of the breeding season. These

assumptions are likely violated because cowbirds are known to leave study areas (Woolfenden et

al. 2003) and because laying rate typically changes throughout the breeding season (Payne

1973a, Fleischer et al. 1987, Woolfenden et al. 2001, but see Scott and Ankney 1980). The

estimate represents an average over the course of the breeding season and therefore does not

consider temporal variation.

Fecundity was defined in this study as the number of eggs produced by an average female

cowbird during the breeding season. Some females that were in the study area likely laid fewer

than 3 5 eggs over the course of the 8-week season, and may not have bred at all. Of female

Brown-headed Cowbirds trapped in Ontario during the height of the cowbird breeding season,

only 2% of individuals were not in breeding condition (Scott 1978). However, Summers et al.

(2006) detected enlarged ovaries in only 18% of female cowbirds categorized as potential local

breeder in Texas, although the sample included birds collected outside of the breeding season. In









this study, 47 female cowbirds were collected in Florida between estimated beginning and end

dates of the cowbird breeding season in 2006 and 2007. Only three (6.4%) of these females were

not in reproductive condition as they lacked pre- or post-ovulatory follicles, or enlarged ovaries

and oviducts. Therefore, the majority of female cowbirds collected during the breeding season in

this study had recently laid eggs, were ovulating, or preparing to lay oviducal eggs. It is also

possible that the three individuals found in non-reproductive condition had already laid eggs

prior to collection since they were all captured in mid- to late-June. Finally, many of the females

collected in this study may have laid more eggs than the 35 estimated for an average bird.

Therefore, it is likely that the fecundity value derived for Florida Brown-headed Cowbirds in this

study is at least a conservative estimate of the number of eggs a typical female lays during

breeding.

Cowbird Collection

Trapping of Brown-headed Cowbirds has been used extensively to control populations

where they have become problematic (Kelly and DeCapita 1982, Beezley and Rieger 1987,

Eckrich et al. 1999). Trapping has been shown to more effective than shooting at removing large

quantities of breeding female cowbirds over the course of a breeding season (Summers et al.

2006). However, it is less efficient overall than shooting, and the effectiveness of trapping and

shooting has been shown to vary over the course of the breeding season (Summers et al. 2006).

Cowbirds are easily trapped in March and April, but may become trap-shy after the onset of the

breeding season (Dufour and Weatherhead 1991). As a result, birds were collected in Florida

using a combination of trapping and shooting. Trapping was most effective at capturing birds at

the end of the breeding season when many birds were in declining reproductive condition,

although birds entered traps throughout the entire collection period. On the other hand, shooting

was effective in collecting birds during May and June when trapping efficiency was low.










Many factors may contribute to differences in the efficiency of each collection method.

For example, trapping relies on a "local enhancement" of a food resource (Turner 1964) and

conspecific attraction to decoys by females using foraging areas. Breeding females are known to

shift their diet during the breeding season from seeds to insects (Ankney and Scott 1980),

presumably for production of eggs. Therefore, birds that are attracted to grains and seeds in traps

may represent individuals that are food stressed and therefore generally in poorer condition

(Dufour and Weatherhead 1991). Body mass, oviduct mass, and ovary mass of Florida breeding

cowbirds were significantly lighter in trapped birds than shot birds. However, I detected no

differences in morphological measurements such as wing chord and tail length between trapped

and shot birds. Other research has also found similar body condition between trapped birds and

those collected using other methods (Scott and Ankney 1979, Davis 2005). Furthermore, body

mass of trapped and shot females in this study were not significantly different when the mass of

oviducal eggs was removed. Therefore, mass differences of trapped versus shot birds may not

reflect a condition bias (e.g., Weatherhead and Greenwood 1981, Dufour and Weatherhead

1991), per se, but rather a sampling bias. Indeed, shot birds were more likely to have an oviducal

egg than trapped birds (Log Odds Ratio=2.76, CI=3.63-68.41) so that trapping may bias toward

individuals in between clutches rather than toward birds in poorer condition. It is not clear why

birds in between clutches would be more likely to be trapped than those that are ovulating. Egg

production can be energetically costly (Carey 1996, Williams 2005) so that non-laying birds may

be more attracted to easily-attainable, but nutritionally inferior food resources than laying

individuals. Some cowbird populations have been shown to be promiscuous (Strausberger and

Ashley 2003), so female cowbirds preparing for another bout of egg-laying may also have been

looking for extra pair copulations with male cowbird decoys in traps.









While trapping conducted in this proj ect may have been biased toward non-laying birds, it

is also possible that the shooting protocol was biased toward individuals in laying condition.

Birds were singled out and shot when foraging within and near feed lots and pastures. Collecting

females by shooting individual birds foraging in these areas is likely selective toward breeding

birds because they are exhibiting breeding behavior within and near host habitat (Summers et al.

2006). Furthermore, I often collected females that were foraging alone or in small flocks of a

few individuals. Scott & Ankney (1980) observed that females foraging in larger flocks (groups

of 10 or more) were less likely to be in laying condition. Indeed, 17 of the 20 breeding-season

cowbirds in reproductive condition that were collected via shooting had oviducts containing

developing eggs. If female cowbirds had been collected solely using this method over the course

of the field season, average female fecundity would be estimated as 5 1 eggs per female per

breeding season. With trapping alone, the estimate would be 22 eggs. Therefore, trapping or

shooting methodology, when used alone, may bias fecundity estimates that rely on the presence

of oviducal eggs and should be used in tandem when sampling cowbird populations.

Conclusions

Studies of Brown-headed Cowbird reproduction have distinguished potential fecundity

from effective (Hahn et al. 1999) or realized fecundity (Alderson et al. 1999, Woolfenden et al.

2003) because some cowbird eggs are likely ej ected by host adults, laid outside of nests, or

placed in nests that cannot be monitored. For example, Woolfenden (2003) used genetic analysis

to assign eggs found in nests to individual female cowbirds. Realized fecundity was

substantially lower than potential fecundity as measured by proportion of birds with oviducal

eggs, a result partially due to female cowbirds laying eggs outside of the study area. Results

from the present study indicate that potential cowbird fecundity is high in both Florida and Texas

populations. Therefore, fecundity is not physiologically limited in north-central Florida and does









not help explain low observed parasitism rates across the host community. However, fecundity

may be ecologically limited (reduced realized fecundity) through multiple mechanisms such as

egg rejection behaviors (Chapter 2), community interactions (e.g., nest predation), or the

abundance and diversity of suitable host species (Chapter 4).










Table 3-1. Summary of means (N, 95% confidence interval) of morphometric data taken from
breeding female Brown-headed cowbirds collected in Florida and Texas, 2005-2007
and results of statistical comparisons (independent samples t-tests). Data for wing
chord length of Texas birds are taken from Summers et al. (2006).
Ovary Mass Oviduct Mass Body Mass (g) Wing Chord (mm)
Florida 0.41 1.20 36.0 94.98

(54, 0.35-0.46) (55, 1.11-1.29) (54, 35.14-36.86) (62, 94.82-95.14)
Texas 0.39 1.38 36.61 94.95

(74, 0.33-0.45) (73, 1.29-1.46) (59, 35.95-37.26) (959, 94.34-95.56)
P-value 0.422 0.023 0.268

Table 3-2. Estimate of clutch size of 2005 Texas cowbirds using number of post-ovulatory
follicles (POFs) clearly identified to have a ruptured stigma. Laying birds are those
with oviducts containing a developing egg while non-laying birds are individuals in
breeding condition (presence of enlarged oviducts, pre-ovulatory follicles, or post-
ovulatory follicles) but without an oviducal egg. Number of POFs for 10 non-laying
birds without clearly identifiable POFs (-) is estimated using the proportion of POFs
in all other non-laying birds (66/29=2.3 POFs/bird). Method derived from Scott &
Ankney (1983).
Laying Birds Non-laying Birds
Number of Est. Clutch Number of
No. POFs Birds Total POFs Size Birds Est. POFs
1 2 2 8 18
2 13 26 1 8 8
3 20 60 2 7 14
4 5 20 3 12 36
4 2 8
Total 40 108 37 84

Clutch size = [(108 x 21 40] + 84 = 3.38 eggs
40 + 37










Table 3-3. Estimate of clutch size of 2006-2007 Florida cowbirds using number of clearly
identifiable post-ovulatory follicles (POFs). Number of POFs for non-laying birds
without clearly identifiable POFs (-) is based on the proportion of2.0 (26/23)
POFs/bird in all other non-laying birds.
Laying Birds Non-laying Birds
Certain Number of Est. Clutch Number of
Total POFs Est. POFs
POFs Birds Size Birds
1 1 1 4 8


Total


Clutch size = [(83 x 2) 28] + 54
28 + 27


3.49 eggs





























_I _I


Figure 3-1. Location of Brown-headed Cowbird collection sites from April-August, 2005-2007.
Site 1 is Fort Hood, Texas, and Site 2 is north-central Florida. Site 1 included
collection of female cowbirds in 2005 and Site 2 included cowbird collection in 2006
and 2007.


35 -1 2006
30 0 2007


21


April


May


June


Figure 3-2. Number of female cowbirds collected per month at sites in north-central Florida in
2006 and 2007. Sample sizes are given above each bar.


1


17


6
1











-~2006
-e2007
~-Small & Medium Traps
SLarge Traps


April May June July

Figure 3-3. Mean number of female cowbirds collected per trap day at sites in north-central
Florida from April to July, 2006-2007. Total trap days were 146 and 463 in 2006
and 2007, respectively. Total trap days were 392 and 217 for small/medium and large
traps, respectively. Trap efficiency data presented by trap size includes both 2006
and 2007 samples.


20 0 raped


April May June July


Figure 3-4. Number of female cowbirds collected per month in Florida by trap and by pellet gun
from April-July 2007. Sample sizes are given for each collection method.











0.70


-* Florida
18 | o Texas


0.60

0.50

S0.40

S0.30

0.20

0.10

0.00


4/15 4/29 5/13 5/27 6/10 6/24 7/8


Week Beginning


Figure 3-5. Mean weekly mass (f SE) of ovaries of all Florida and Texas female cowbirds
collected between 15 April and 15 July, 2005-2007. Samples sizes for each week are
given next to each point.


2.00
1.80 -9 5 (FL) 19
1.60 -3 6
1 3
S1.40 -1
18 11
1.20 6 4
1.0-2 6 8
0.80 8
0.60 -*- Florida52
0.40 -o Texas
0.18 21
0.20 -1/ 2 4
0.00
4/15 4/29 5/13 5/27 6/10 6/24 7/8
Week Beginning


Figure 3-6. Mean weekly mass (f SE) of oviducts of all Florida and Texas female cowbirds
collected between 15 April and 15 July, 2005-2007. Sample sizes for each week are
given next to each point.











0.80
0.70
0.60
0.50
0.40
0.30
0.20


-o Laying


-* Nnlain

4/15 4/29 5/13 5/27 6/10 6/24


0.10
0.00


Week Beginning


Figure 3-7. Mean weekly mass (f SE) of ovaries of breeding Brown-headed Cowbirds collected
in Florida, 2006-2007. Laying birds are those with an oviducal egg while non-laying
birds are in reproductive condition but lack an oviducal egg. Sample sizes for each
collection week are given next to each point.


2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00


365


3 2


3 5


-o Laying
-*-Non-laying|


4/15 4/29 5/13 5/27 6/10 6/24


Week Beginning


Figure 3-8. Mean weekly mass (f SE) of oviducts of breeding Brown-headed Cowbirds collected
in Florida, 2006-2007. Laying birds are those with an oviducal egg while non-laying
birds are in reproductive condition but lack an oviducal egg. Sample sizes for each
collection week are given next to each point.











0.70
-o Laying
0.60
-* Non-laying
0.50 i 1

S0.40

S0.302

0.20

0.10


0.00
4/15 4/29 5/13 5/27
Week Beginning


6/10 6/24


Figure 3-9. Mean weekly mass (f SE) of ovaries of breeding Brown-headed Cowbirds collected
in Texas, 2005. Laying birds are those with an oviducal egg while non-laying birds
are in reproductive condition but lack an oviducal egg. Sample sizes for each
collection week are given next to each point.


2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00


-o Laying
-* Non-laying


4/15 4/29 5/13 5/27 6/10 6/24 7/8

Week Beginning


Figure 3-10. Mean weekly mass (f SE) of oviducts of breeding Brown-headed Cowbirds
collected in Texas, 2005. Laying birds are those with an oviducal egg while non-
laying birds are in reproductive condition but lack an oviducal egg. Sample sizes for
each week are given next to each point.





45.0


40.0


35.0


30.0


0 *
a ~ o



e. o & o 8~ .
oo

0 AllBirds a o
9 OO O
*Breeding Birds


25.0
4/15 4/29 5/13


5/27 6/10 6/24


7/8 7/22


Figure 3-11i. Mass of female Brown-headed Cowbirds collected in Florida between 23 April and
12 July, 2006-2007. Mass of all birds decreased significantly over time (hashed line,
R2=0. 16, P<0.001, N=110). Mass of breeding birds did not significantly change over
the course of the breeding season (solid line, R2=0.024, P=0.263, N=53).




50.0oo All Birds
45.0 Breeding Birds


*, 'oo *
so


40.0


35.0


* *


30.0 01


25.0


4/15 4/29 5/13 5/27 6/10 6/24 7/8



Figure 3-12. Mass of female Brown-headed Cowbirds collected in Texas between 19 April and 6
July, 2005. Mass of all birds (hashed line, R2_0.001, P=0.826, N=79) and of
breeding birds (solid line, R2<0.001, P=0.990, N=58) did not change significantly
over time.










30.0


25.0

20 z.0

8 15.0

O 10.0


-1 -2
Collection Day


Figure 3-13. Mean area (+ SE) of post-ovulatory follicles (POFs) of Brown-headed Cowbirds
collected in Texas and Florida from April to June, 2005-2007. Sample presented
includes only birds with oviducal eggs. Day -1 represents POFs associated with
oviducal eggs and which released yolk one day prior to being collected. Asterisks
indicate comparisons between Florida and Texas that are statistically different (t-tests,
P<0.01). Sample sizes are given with each bar.


MFlorida
O Texas


7/14 20/39


9/13


10/20


S0.6


S0.4

0.2

-


4/26-5/10 5/11-5/25


5/26-6/9 6/10-End*


Figure 3-14. Estimated laying rate (eggs produced per day) of breeding cowbirds collected in
Florida and Texas during four periods of the cowbird breeding season. Laying rate is
based on the proportion of reproductively active birds that have oviducal eggs.
Proportions are next to each bar. *End of cowbird breeding season for Florida and
Texas are 24 June and 26 June, respectively.


-









CHAPTER 4
ALTERNATIVE HYPOTHESES FOR THE "SLOW" COWBIRD INVASION OF FLORIDA

Introduction

Brood parasitism by Brown-headed Cowbirds is thought to be a significant threat to

particular songbird populations and communities, especially in areas where cowbirds have only

recently invaded. Research on cowbird expansion to new areas has typically focused on effects

of cowbirds on breeding avian species and communities (e.g., Mayfield 1965, Braden et al. 1997,

Strausberger and Ashley 1997, Payne and Payne 1998), but not much attention has been paid to

the reverse. That is, cowbirds have been depicted as "agents of extermination" (Mayfield 1977)

that steamroll into new areas wreaking havoc on native bird populations. Yet, regional and local

dynamics of the host community have the potential to significantly influence patterns of cowbird

range expansion through their effect on reproductive success. Cowbird fecundity, for example,

has been estimated to be extremely high in some regions where an average female lays 40 eggs

per breeding season (Scott and Ankney 1980). Potential fecundity (total number of eggs that are

laid), however, may not reflect actual fecundity (total number of eggs incubated or young

surviving; Alderson et al. 1999, Hahn et al. 1999, Woolfenden et al. 2003). Multiple local

factors may reduce effective fecundity and the overall reproductive success of cowbirds. Host

adults, for example, may eject cowbird eggs from their nests or abandon parasitized nests,

thereby reducing the percent of parasitism detected and the number of cowbird young that are

produced.

The arrival of two cowbird species to Florida was thought to herald a conservation crisis

for many species of potential hosts. Shiny Cowbirds may have reached Florida as early as 1971

(Ogden 1971), but were officially recorded in south Florida in the mid-1980s (Smith and Sprunt

1987). By the mid 1990s Shiny Cowbirds were recorded in numerous localities (primarily









coastal) throughout Florida (Cruz et al. 2000). Arrival of the Shiny Cowbird to Florida was of

particular concern because of impacts on songbirds on recently colonized Caribbean islands and

because Shiny Cowbirds are known to colonize cavity nesters that are rarely if ever parasitized

by the Brown-headed Cowbird (Wiley 1985, Wiley et al. 1991). Shiny Cowbirds also often

prefer only one host (Post and Wiley 1977) so that populations of Black-whiskered Vireos (Vireo

altiloquus), Cuban Yellow Warblers and Florida Prairie Warblers confined to south Florida were

thought to be at risk to extinction through brood parasitism (Atherton and Atherton 1988). So

far, however, these dire predictions have not come to pass and there are only two cases

suggesting breeding by Shiny Cowbirds in Florida (Smith and Sprunt 1987, see Chapter 3).

Brown-headed Cowbirds were first documented breeding in Florida in the mid-1950s (Monroe

1957) and have since spread through most of the state where they are found at moderate levels of

abundance relative to other regions of North America (Sauer et al. 2006). Surveys of Brown-

headed Cowbird parasitism of host nests and family groups feeding fledglings show unusually

low frequencies both for individual species and for the entire community of potential hosts in

north-central Florida (Chapter 1).

In previous chapters of this dissertation, I explored hypotheses that might explain the

seemingly slow invasion of Florida by cowbirds. First, I tested the hypothesis that the low

frequency of Brown-headed Cowbird parasitism observed in north-central Florida resulted from

host defenses, i.e., cowbird eggs being removed by adults or nests being abandoned before the

active nest could be monitored (Chapter 2). I monitored over 1 100 nests of 32 potential host

species and noted no cases of natural cowbird egg ejection. Furthermore, parasitism experiments

revealed that parasitism rej section behaviors do not appear to be prevalent in many common

Florida hosts and are certainly no more prevalent in Florida than in areas where cowbird invasion










proceeded more rapidly (Rothstein 1994). Second, reproductive assays of female Brown-headed

Cowbirds breeding in north-central Florida revealed that potential fecundity is very high, with an

average female laying 35 eggs per breeding season (Chapter 3). Therefore, these hypothesized

limitations to cowbird reproduction do not appear to have influenced the observed rates of

cowbird parasitism in the region. In this chapter, I explore two more hypotheses that may

explain either observed low rates of parasitism in north-central Florida or the seemingly slow

invasion of Florida by cowbirds: host community composition and nest predation. The first

hypothesis is that the community composition of hosts in Florida slows invasion because they are

dominated by poor cowbird hosts that either have defenses against parasitism or are poor at

raising cowbird young. I test this hypothesis by comparing community composition of hosts in

areas where cowbirds have been successful and in Florida where the invasion is ongoing. The

second hypothesis is that host communities in Florida are themselves less productive than those

elsewhere in the cowbirds' range as a result of high rates of nest predation. I test these data

using simple models for source-sink dynamics and data from two regions where data exist on

cowbird productivity and host nest predation rates.

Host Community Composition

The occurrence and abundance of Brown-headed Cowbirds over local landscape scales has

been shown to be influenced by several factors, including the diversity (Chace 2004, Purcell

2006), abundance, and population density of hosts (Johnson and Temple 1990, Robinson and

Wilcove 1994, Tewksbury et al. 1999, Jensen and Cully 2005a, but see Evans and Gates 1997).

Abundance of Brown-headed Cowbirds in three vegetation types in western Montana, for

example, was positively correlated with the density of potential hosts (Tewksbury et al. 1999,

Robinson et al. 2000). Levels of parasitism may also be influenced by the availability of suitable

host species (Barber and Martin 1997, Brodhead et al. 2007, but see Jensen and Cully 2005b).









Parasitism of Black-capped Vireos, for example, was lower in areas with lower cumulative

density of four other potential host species (Barber and Martin 1997). Therefore, cowbird

abundance and distribution may be affected by the distribution and abundance of host species at

local scales.

Patterns of cowbird and host occurrence are more difficult to discern at larger (e.g.,

regional) scales because many factors can affect species distributions. Research has shown a

strong trend of diminishing faunal diversity (Means and Simberloff 1987) and density (Emlen

1978) from base to tip along Florida's peninsula. Several mechanisms have been proposed to

explain this general pattern, including changes in immigration and extinction rates (Cox 2006),

past climate change and sea-level fluctuations (Robertson and Kushlan 1974), and differences in

availability of different habitats (Emlen 1978, Wamer 1978) that would provide suitable

resources. One likely limiting factor on cowbird reproductive success should be the availability

of suitable hosts to raise cowbird young. Breeding Brown-headed Cowbirds are distributed

throughout Florida but are primarily located in the northern third of the state (Figure 4-1). Of the

ca. 50 host species known to breed in Florida that have raised cowbird young (Table 4-1), only

19 species have statewide distributions whereas 28 of the remaining 3 1 species are either found

only in north Florida (Cox 2006) or have distributions primarily in the northern half of the state

(FFWCC 2007).

Taken together, these data suggest that the spread of the cowbird into south Florida may be

limited by the availability of quality hosts. To test this hypothesis, I categorized species

breeding in Florida into one three categories based on their quality as cowbird hosts (Table 4-1)

based on criteria in Grzybowski and Pease (1999), Robinson et al. (unpubl. data) and data from

the literature. High quality host species are those that are often highly parasitized elsewhere and









that raise cowbird young to fledging. Marginal quality species are those that accept cowbird

eggs but are rarely parasitized and raise few cowbird young. Poor quality hosts represent

cowbird egg sinks due to rej section of parasitism or life-history traits (e.g., nestling diet)

incompatible with brood parasitism. In Florida, the maj ority of species that are distributed

statewide are abundant but are probably marginal hosts (e.g., Northern Cardinal), or poor hosts

(e.g., Mourning Dove, Eastern Meadowlark) for a variety of reasons (Table 4-1). On the other

hand, cowbird distribution roughly coincides with the highest diversity and abundance of good

quality hosts (see Figure 1). These data are generally consistent with the hypothesis that

cowbirds are most abundant in areas where good hosts are also found.

If Brown-headed Cowbirds are located in areas in Florida with the highest diversity and

abundance of suitable hosts, why are observed parasitism rates in north-central Florida so low?

To qualitatively address whether the composition of the host community contributes to this

pattern, I examined Breeding Bird Survey (BBS) data from 1997-2006 at routes within 75km of

three study areas where parasitism rates have been recorded. Study areas included (1) north-

central Florida (city of Gainesville, N=10 routes), (2) southwestern South Carolina (Savannah

River Site near Aiken, N=8), and (3) southern Illinois (city of Anna, N=7). BBS routes in each

region incorporated numerous habitat types but primarily forest, pasture, row crops, grassland

and wetland habitats (Sauer et al. 2006). Routes for southern Illinois included more pasture and

row crops (59%) on average than routes near Florida (14%) and South Carolina sites (37%).

Illinois routes included more deciduous forest (21%) than South Carolina (12%) and Florida

(0%), but routes in these latter areas also included significant evergreen forest (24-3 8%).

Documented rates of parasitism are lowest in Florida (Chapter 1), low to moderate in South

Carolina (Whitehead et al. 2002, Kilgo and Moorman 2003), and high in Illinois (Robinson










1992, Strausberger and Ashley 1997, Hoover 2003). Species breeding in each study area were

categorized as good, marginal, or poor based on their suitability as cowbird hosts. Total

abundance of species in each host category was compared among study areas using one-way

ANOVA and Dunnett's post-hoc comparisons assuming unequal variances. As in Gryzbowksi

and Pease (1999), I excluded species that are known to be cowbird hosts, but have various traits

that may limit parasitism of their nests (e.g., parids, bluebirds, doves).

In both Florida and South Carolina, the mean abundance of good quality hosts constitutes a

relatively higher proportion of the overall bird abundance than in Illinois (Figure 4-2). This may

suggest that Florida has a high availability of suitable hosts. However, the diversity of all

potential hosts is 91 and 121% higher in South Carolina and Illinois, where there are

approximately two times as many quality host species as in north-central Florida (Figure 4-3).

Furthermore, there were significant differences among study areas in the abundances of good

(F=33.73, P<0.001), marginal (F=391.26, P<0.001), poor (F=160.25, P<0.001), and all

(F=341.52, P<0.001) host species. The abundances of all three classes of hosts as well as total

host abundance are significantly lower in Florida than in both other regions (Dunnett' s post-hoc

comparisons, P<0.001; Figure 4-4). Kilgo and Moorman (2003) reported low-to-moderate rates

of nest parasitism for 24 species breeding in the southeastern coastal plain of South Carolina and

Georgia. Among the species with the highest average parasitism rates were Prairie Warbler

(13.6%), Hooded Warbler (14.4%), Yellow-breasted Chat (18.6%), Blue Grosbeak (18.8%) and

Indigo Bunting (16.5%). Relative abundances of these species are between 1.6 and 6.4 times

higher in South Carolina than in Florida, where most of them reach their southern range limit

(Sauer et al. 2006).









The Breeding Bird Survey (BBS) data summarized above are based on surveys conducted

primarily along roadsides, which may not adequately sample forested habitat (Bart et al. 1995,

Betts et al. 2007) that cowbirds may prefer (Hahn and Hatfield 1995). As a result, BBS surveys

may not adequately represent forest-interior songbirds that may be preferred hosts of cowbirds. I

compared census data obtained for forested habitats in Illinois (Robinson and Robinson 1999)

and Florida (S.K. Robinson unpubl. data) to better estimate differences in abundance of quality

forest hosts between regions. Illinois censuses were conducted in the Trail of Tears State Forest

from May-July, 1989-1992 and Florida censuses were conducted at various locations primarily

within 100km of Gainesville from May-July in 2004 (Stracey and Robinson 2008). Abundance

data obtained from 50-m radius point counts in Illinois were transformed to compare with 100-m

radius count data obtained in Florida. Only data from species considered quality cowbird hosts

were included.

The abundance of quality forest hosts was much higher in Illinois than in Florida.

Abundance of quality canopy and understory hosts was 5.2 and 3.2 times higher in Illinois than

Florida, respectively (Figure 4-5) although the number of species in each host type was similar

between regions. Rates of parasitism in these same Illinois forests are very high (Robinson and

Robinson 2001) and have more than 10 times as many cowbirds as Florida forests (Figure 4-5).

Cowbird numbers, however, may not be solely related to the overall abundance of the host

community but may depend primarily on abundances of a small number of common species that

act as good hosts (Rothstein and Robinson 2000). As a result, monitoring of high quality host

species with abundant Florida populations such as Red-eyed Vireo, Northern Parula and Pine

Warbler may be needed to determine how they are contributing to population growth of cowbirds

in Florida. Thus, BBS data and forest census data, in particular, suggest that quality hosts are









actually less abundant in Florida than they are in areas where cowbirds invaded long ago and

where cowbird populations have stabilized or even declined.

Based on these crude and correlative analyses, Brown-headed Cowbirds breeding in

Florida may suffer from a reduced availability of hosts. Lack of suitable hosts may in turn

reduce effective fecundity of cowbirds. If cowbirds are unable to locate suitable hosts when eggs

are ready to be laid, females may lay eggs outside of nests or in those of inappropriate hosts,

which are often not monitored in studies of brood parasitism. From 2004-2006, I monitored

approximately 1 100 nests of 35 potential host species breeding in Florida (Chapter 1), many of

which are inappropriate hosts because of life-history traits incompatible with cowbird parasitism

(e.g., Mourning Dove, Great-crested Flycatcher, Blue Jay). Of the limited cases of nest

parasitism observed in this study, however, none included nests of these inappropriate hosts. If

nests of preferred hosts are not available, females may also resorb the developing egg before it

enters the oviduct (Payne 1998). Indeed, Payne (1965) suggested that atresia of developing

ovarian follicles may be due to a shortage of suitable nests, although captive cowbirds do not

appear to limit laying based on availability of artificial nests (Holford and Roby 1993). Finally,

Hahn et al. (1999) suggested that a female cowbird unable to find an appropriate nest for her egg

may consume it to retain important nutrients.

Finally, host suitability is based on multiple factors such as nestling diet, breeding

phenology, nest-site choice, nest defense and rejection behavior. While many of these traits may

be retained among populations over their entire distribution, regional differences in the

expression of these traits may alter the suitability of some populations as cowbird hosts. For

example, some populations of Warbling Vireos (Vireo gilvus) accept cowbird parasitism of their

nests whereas females in other populations puncture-eject cowbird eggs (Sealy 1996). Data for









Florida, however, are lacking for estimating reproductive parameters influencing cowbird

population growth (e.g., cowbird fledging success, proportion of eggs accepted).

Nest Predation

Brood parasitism and nest predation are the two primary causes for reduced reproduction

in many songbird populations (Mayfield 1977, Wilcove 1985, Martin 1992, Robinson et al.

1995, Arcese and Smith 1999). Nest predation, in particular, can account for >90% of nest

losses in some populations also exposed to brood parasitism (Marller 1988, Hoover et al. 1995,

Martin 1998) and significantly limit annual reproductive success of songbird populations

(Robinson 1992, Pease and Grzybowski 1995). Schmidt & Whelan (1999) developed a model to

examine the relative contributions of nest predation and parasitism to songbird seasonal

fecundity. Results indicated that nest predation had a greater influence on reproductive success

than parasitism. Furthermore, high rates of nest predation under some circumstances could lead

to sink populations even with re-nesting and double-brooding. The authors suggest that

management practices that reduce nest predation may be more important in some populations

than management of brood parasitism through cowbird control.

While effects of nest predation (and parasitism) on host communities have been commonly

studied, less attention has been paid to general effects of nest predation on reproduction of

cowbirds. Winfree (2004) examined the possible effects of host community and habitat type on

the population growth of Brown-headed Cowbirds breeding in southern Illinois. Cowbird

offspring survival rates were 2-3 times lower in old fields than in forested habitats, the latter of

which represents a more recently invaded habitat type and are characterized by generally lower

rates of nest predation. These results suggest that invasion success of Brown-headed Cowbirds

may be facilitated by habitat types that reduce nest predation pressures that could slow cowbird










population growth. Winfree et al. (2006) even speculated that high nest predation rates in the

scrubbier habitats that cover much of Florida may be slowing their rate of spread because such

habitats tend to be associated with very high rates of nest predation.

Nest failure rates of select understory nesting species breeding in north-central Florida are

very high. For example, Mayfield daily survival rates for Northern Cardinal and Northern

Mockingbird in north-central Florida are as low as 0.9187 (Reetz et al. unpubl. data) and 0.8932

(C.M. Stracey pers. comm.), respectively. Host populations may compensate for high nest

failure rates through re-nesting or multiple broods (Pease and Grzybowski 1995) over long

breeding seasons. Indeed, both Northern Cardinals and Northern Mockingbirds have very short

re-nesting intervals following nest failure (Zaias and Breitwisch 1989, Filliater et al. 1994) and

begin breeding as early as February (M. Reetz pers. obs), which may alleviate high predation

pressures on these species. Because cowbirds only breed for a part of these long seasons,

however, high rates of re-nesting may not actually increase the availability of hosts to parasitize

(e.g., Morrison and Bolger 2002).

By modifying a well-known source-sink equation (Pulliam 1988: Equation Sa), Winfree

(2004) calculated upper and lower values (Tcrat) for egg-to-fledgling transition success of

cowbirds that would be required for a stable population. Values derived were based on

published data for adult female cowbird survival (Woolfenden et al. 2001), juvenile survival

(Ricklefs 1973, Greenberg 1980, Smith et al. 2002), and female cowbird fecundity (Scott and

Ankney 1980, Woolfenden et al. 2003). Lower and upper bounds of cowbird egg-to-fledgling

transition success needed to achieve cowbird replacement were calculated as 0.03-0.22. In

north-central Florida, an average female cowbird is estimated to produce 34 eggs per season

(Chapter 3) resulting in modified Tent values of 0.04-0. 11. To generate Tvalues derived from









field data, the following equation can be used (Winfree 2004), T= Ux Vx W, where Uis the

probability of egg acceptance by the host, Vis the probability of survival over the nesting

interval, and Wis the probability that the cowbird will survive to fledging in a successful nest.

Based on this equation, Winfree determined that certain habitats and species generate Tvalues

below that needed to sustain cowbird populations. Using field data on cowbird egg acceptance

(Chapter 2) and nest interval survival rates (Reetz et al. unpubl. data, C.M. Stracey pers. comm.)

for three species breeding in Florida, values for U and V would be 0.85 and 0.20 for Northern

Mockingbird, 0. 12 and 0.16 for Brown Thrasher, and 0.95 and 0.15 for Northern Cardinal.

Assuming an average Vvalue of 0.79 based on those determined by Winfree, T estimates of

cowbird egg-to-fledgling transition success for these three species in Florida are 0.13, 0.02, and

0. 11. Two of these values are near the higher estimate of Tent (0. 11) needed to sustain cowbird

populations and one (Brown Thrasher) is below the lower estimate (0.04). Thus, even with

liberal estimates of some of these values, these hosts could not sustain cowbird populations.

These species, however, are probably not the most suitable hosts in the songbird community in

north-central Florida. Indeed, a large sample of nests (N=826) of these three species was

monitored with only six cases of parasitism recorded. Nevertheless, the rate of nest parasitism

over the entire sample of species monitored was also low. It is possible that parasitism

frequency was affected by nest predation occurring prior to monitoring of nests. This would

require partial nest predation where the cowbird egg(s) were removed and one or more host eggs

remained to be found and monitored, an unlikely event as there were only a few cases of partial

clutch loss in all unparasitized nests monitored.

The most heavily parasitized species in northern Florida are likely those that nest in the

forest canopy. Family groups of these species were far more likely to contain a cowbird young









than family groups of understory species. Percent of family groups of canopy nesting species

with cowbird young were 39% for Blue-gray Gnatcatcher and 33% for Northern Parula, which

would be considered moderate as parasitism rates of nests. These estimates of parasitism

frequency are based on the assumption of complete survival of cowbird young in nests that

fledge host young, which is very unlikely. If we use the same Vvalue (cowbird survival in a

successful nest) of 0.79 for canopy species and assume cowbirds lay one egg per nest, as many

as 49% and 42% of nests of these two species may be parasitized. Furthermore, cowbird

parasitism may facilitate or increase nest predation (Dearborn 1999, Burhans et al. 2002, Zanette

et al. 2007) perhaps due to loud begging vocalizations of cowbird young. This would result in

family group parasitism rates that also underestimate actual parasitism of nests. Therefore, rates

of nest parasitism on midstory and canopy nesting species may be higher than family group

surveys indicate.

If Brown-headed Cowbirds are preferentially laying the maj ority of their eggs in nests of

preferred host species that are difficult to find or monitor (e.g., canopy nesting species), rates of

nest failure combined with behaviors that reduce the number of eggs that are incubated (e.g.,

abandonment) in these species would be more important in affecting overall cowbird production.

For example, Red-eyed Vireos are common in Florida (Sauer et al. 2006) and have been

recorded to abandon 34% of parasitized nests (Graber et al. 1985). If the probability of cowbird

egg acceptance (U) is 0.66 and cowbird survival in a successful nest (V) is 0.81 (Winfree 2004),

the interval survival rate would need to be 0.21 (daily survival of 0.937) to result in net positive

cowbird reproduction in Florida (Tcra=0. 11). This rate is lower than most data from the literature

for this species (daily survival of 0.932-0.963: Burke and Nol 2000, Duguay et al. 2001, Winfree

2004), which suggests that they may be a "source" host where they are found in sufficient









numbers. Despite logistical difficulties in collecting the data, comprehensive nest surveys of

midstory and canopy nesting species are required to determine if both parasitism frequencies and

rates of nest failure. Some evidence also suggests that variation in cowbird parasitism is

negatively correlated with interspecific differences in nest predation, indicating that the

probability of nest failure can influence host choice by cowbirds (Aviles et al. 2006). High nest-

loss rates in the limited subset of preferred Florida host species (as well as across the community

of alternative lower quality hosts), however, would primarily limit their capacity to act as

cowbird "generators" and would partially explain why Brown-headed Cowbirds have not

invaded Florida as successfully as they have other parts of North America. Alternatively, the

abundance of ideal "source" hosts, those that accept cowbird eggs and do not suffer high rates of

nest predation, may also determine the rate at which cowbirds can spread.
















































Figure 4-2. Proportion of total abundance composed of species of varying quality as cowbird
hosts. Classifications roughly based on criteria in Grzybowski and Pease (1999).
Data are summarized for Breeding Bird Survey routes within 75km of study sites in
three regions.


Florida


20%


South Carolina


20%


Illinois


18%


Figure 4-1. Distribution of Brown-headed Cowbirds in Florida. Redrawn from Breeding Bird
Survey abundance map (1994-2003). Legend indicates count of birds per route.


Good Hosts


SPoor Hosts


Marginal Hosts



















S30

20


10

-


Good


Marginal Poor


Figure 4-3. Total number of host species of three types at locations in three regions.


O Florida
H South Carolina
H Illinois


400 L

300

200 -

100

0-


A


Marginal


Poor


Good


Figure 4-4. Mean (+ SE) abundance of three types of Brown-headed Cowbird hosts in three
regions. Bars within each host category that do not share a letter are significantly
different using average abundance values from 1997-2006 (One-way ANOVA,
Dunnett' s post-hoc test assuming unequal variances, P<0.05).


o Florida
South Carolina
Illinois










14
111
12 O Illinois

10 -I I Florida






,A 6




Canopy Understory Cowbird

Figure 4-5. Mean (+SD) abundance of forest species that represent good quality cowbird hosts in
Illinois and Florida. Birds per count represents abundance of birds within 100-m
radius point counts. Values above bars indicate number of species recorded for each
host type. Data are from Robinson and Robinson (2000) for Illinois, and from
unpublished data (S.K. Robinson). Cowbird abundance includes individuals of both
sexes.









Table 4-1. Host species documented as breeders in Florida (FFWCC 2007) also known to have fledged Brown-headed Cowbirds.
Species are arranged in descending order of relative abundance (Sauer et al. 2006). Florida distribution category based on
Cox (2006) and Florida Breeding Bird Atlas (FFWCC 2007). Abbreviations indicate that species occur, SW: Statewide, N:
only in northern half, S: only in southern half, PN: primarily in northern half. Suitability is based on criteria in Grzybowski
and Pease (1999) and Robinson et al. (unpubl. data). G: good quality hosts; M: marginal hosts, P: poor hosts.
Florida Relative
Common Name Scientific Name Distribution Suitability Abundance
Red-winged Blackbird Agelaius phoeniceus SW M 49.94
Northern Mockingbird M~imus polyglottos SW P 49.11
Northern Cardinal Cardinalis cardinalis SW M 44.03
Mourning Dove Zenaida macroura SW P 38.74
Eastern Towhee Pipilo erythrophthalmus alleni SW G 30.93
Carolina Wren Thil yetei itsll / ludovicianus SW P 25.49
Eastern Meadowlark Sturnella magna SW P 24.04
Common Yellowthroat Gl'lrls ib/i\ trichas SW G 13.18
White-eyed Vireo Vireo griseus SW G 12.36
European Starling Sturnus vulgaris SW P 11.64
House Sparrow Pa~sser domesticus SW P 9.74
Northern Parula Parula americana PN G 6.40
Pine Warbler Dendroica pinus SW G 6.37
Loggerhead Shrike Lanius ludovicianus SW P 4.60
Brown Thrasher Toxostoma rufum SW P 4.08
Eurasian Collared Dove Streptopelia decaocto SW P 4.07
Summer Tanager Piranga rubra PN G 3.92
Eastern Bluebird Sialia sialis SW P 3.76
Carolina Chickadee Poecile carolinensis N P 3.52
Indigo Bunting Pa~sserina cyan2ea N G 3.42
Blue Grosbeak Guiraca caerulea N GM 3.26
Orchard Oriole Icterus spurious N G 2.99
Blue-gray Gnatcatcher Polioptila caerulea SW GM 2.91
Eastern Kingbird Tyrannus tyrannus SW P 2.79
Yellow-breasted Chat Icteria virens N G 2.35










Table 4-1. Continued.


Florida
Di stributi on
PN
SW
N
PN
N
PN
PN
N
S
N
N
N
N
N
N
N
PN
S
N
N
N
N
N
S
N


Relative
Abundance
1.82
1.62
1.39
1.30
1.19
1.16
0.91
0.85
0.84
0.73
0.48
0.40
0.27
0.17
0.13
0.10
0.04
0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01


Common Name
Red-eyed Vireo
Bachman's Sparrow
Wood Thrush
Prothonotary Warbler
Yellow-throated Vireo
Barn Swallow
House Finch
Hooded Warbler
Florida Prairie Warbler
Acadian Flycatcher
Yellow-throated Warbler
Eastern Wood-Pewee
Field Sparrow
Painted Bunting
Kentucky Warbler
Swainson's Warbler
Gray Catbird
Yellow Warbler
American Robin
Eastern Phoebe
Louisiana Waterthrush
Worm-eating Warbler
American Redstart
Grasshopper Sparrow
Chipping Sparrow


Scientific Name
Vireo olivaceus
Aintophila aestivalis
Hylocichla nustelina
Protonotaria citrea
VireoflavifCrons
Hirundo rustica
Calrpodacus nzexicanus
Wilsonia citrina
Dendroica discolor pahedicola
Enapidonax virescens
Dendroica dominica
Contopus virens
Spizella pusilla
Pa~sserina ciris
Oporornis forntosus
Liinnothy/pisl~i swainsonii
Dunsetella carolinensis
Dendroica pe techia
Turdus naigratorius
Sayornis phoebe
Seiurus zotacilla
Helmitheros vernzivorus
Setophaga ruticilla
Ananodra~nus savannarunt
Spizella passerina


Suitability
G
GM
G
G
G
P
P
G
GM
G
G
G
M
G
G
G
P
GP
P
G
G
G
G
G
GM









APPENDIX A
STUDY SITE DESCRIPTION AND HOST SPECIES SURVEYED

These tables include general descriptions of study sites (Table A-1) and the number of

nests and family groups of host species monitored at each site (Table A-2) during Brown-headed

Cowbird parasitism surveys in north-central Florida from March-August, 2004-2006.

Approximate area (hectares) of each site includes only locations where field work was

conducted. Coordinates represent the approximate center of each field site. Minimum and

maximum distances (meters) from nests at each site to cowbird feeding areas (livestock pastures)

were determined using plant community and landcover GIS data (Kautz et al. 1993) and satellite

imagery. Distance values in parentheses are to urban areas that provide cowbird feeding

opportunities. Gainesville Public Areas include city parks, cemeteries, and parking lots and

Gainesville Suburban Areas include private homes and neighborhoods in the city of Gainesville,

Florida. General habitat categories where nests were located at each site were determined using

GIS vegetation data. Habitat categories generally follow Kautz et al. (2007) and are 1 = Scrub, 2

= Sandhill, 3 = Dry Prairie, 4 = Upland Forest, 5 = Pineland, 6 = Freshwater Marsh, 7 = Shrub

Swamp, 8 = Forested Wetland, 9 = Shrub and Brushland, 10 = Grassland or Pasture, 11 =

Di sturbed/Transitional, 12 = Urban/Developed.










Table A-1. Description of study sites surveyed for cowbird parasitism in north-central Florida, 2004-2006.
Minimum Maximum


Site
Bolen Bluff State Park
Gainesville Public Areas
Gainesville Suburban Areas
Gum Root Nature Park
Loblolly Woods Nature Park
Myakka River State Park
Newnan's Lake
Ordway-Swisher Biological Station
Payne's Prairie State Preserve
San Felasco Hammock State Park

Split Rock/Sugarfoot Nature Preserves
University of Florida Campus
USDA National Wildlife Research Center


Area
160
400
300
124
14
1880
44
1400
1000
3800
122
670
11


Latitude
29031'49"N
29039'07"N
29039'07"N
29040'48"N
29039'34"N
27014'37"N
29037'23"N
29041'57"N
29031'49"N
29044'00"N
29037'57"N
29038'47"N
29039'l3"N


Longitude
82017'O2"W
82020'21"W
82020'21"W
82014'l18"W
82021'53"W
82015'48" W
82015' 01"W
81058'32"W
82017'O2"W
82027'O6"W
82024'l16"W
82020'45"W
82017'l14"W


Di stance
200

(0)
(0)
<100
<100
200
900
1500
<100
200
800

(50)
(<50)


Di stance
1000

(50-100)
(50-100)
650
500
4500
1400
4400
1400
2200
1500

(200)
(250)


Habitats
4, 7, 10
4, 5, 9, 10, 12
4, 5, 12


4, 5, 6,
4, 5, 6,
1, 3, 4,
6, 7, 8
2, 4, 5,
4, 5, 6,
2, 4, 5,
4, 5, 6,
4, 5, 6,
4, 5, 9


8
8
5, 6, 7, 8


6, 7, 8, 9, 10, 11
7, 8, 10
6, 8, 9, 10
7, 8
8, 9, 10, 12










Table A-2. Number of nests and family groups of host species monitored at study


sites in north-central Florida, 2004-2006.
Nests Families
Parasitized Total Families Parasitized


Study Site
Bolen Bluff State Park








Gainesville Public Areas










Gainesville Suburban Areas











Gum Root Nature Park


Species
White-eyed Vireo
Blue-Gray Gnatcatcher
Brown Thrasher
Northern Parula
Eastern Towhee
Northern Cardinal
Mourning Dove
Loggerhead Shrike
Carolina Wren
Eastern Bluebird
Northern Mockingbird
Brown Thrasher
Northern Cardinal
Mourning Dove
Eastern Tufted Titmouse
Carolina Wren
Northern Mockingbird
Brown Thrasher
Northern Parula
Summer Tanager
Northern Cardinal
White-eyed Vireo
Red-eyed Vireo


Total Nests
1










Table A-2. Continued.


Nests
Parasitized


Families
Parasitized


Study Site


Species
Northern Mockingbird
Brown Thrasher
Northern Cardinal
Carolina Wren
Northern Parula
Northern Cardinal
Mourning Dove
Great-crested Flycatcher
Blue Jay
Eastern Bluebird
Northern Mockingbird
Common Yellowthroat
Eastern Towhee
Bachman's Sparrow
Eastern Meadowlark
Red-winged Blackbird
Mourning Dove
White-eyed Vireo
Yellow-throated Vireo
Blue Jay
Eastern Tufted Titmouse
Blue-Gray Gnatcatcher
Eastern Bluebird


Total Nests
1
1
5
1


4
19
12
8
7
18
2
43
11
3
61
2
4


Total Families


Loblolly Woods Nature Park



Myakka River State Park












Newnan's Lake
Ordway-Swisher Biological Station










Table A-2. Continued.


Nests
Parasitized


Families
Parasitized


Study Site


Species
Northern Mockingbird
Brown Thrasher
Northern Parula
Pine Warbler
Common Yellowthroat
Summer Tanager
Eastern Towhee
Northern Cardinal
Mourning Dove
White-eyed Vireo
Carolina Chickadee
Carolina Wren
Blue-gray Gnatcatcher
Brown Thrasher
Northern Parula
Pine Warbler
Prothonotary Warbler
Common Yellowthroat
Summer Tanager
Eastern Towhee
Northern Cardinal
Blue Grosbeak

Indigo Bunting


Total Nests
1
6


Total Families


Payne's Prairie State Preserve










Table A-2. Continued.


Nests
Parasitized


Families
Parasitized


Study Site


Species
Red-winged Blackbird
Orchard Oriole
Mourning Dove
Acadian Flycatcher
White-eyed Vireo
Red-eyed Vireo
Carolina Wren
Blue-gray Gnatcatcher
Eastern Bluebird
Wood Thrush
Northern Parula
Pine Warbler
Hooded Warbler
Summer Tanager
Eastern Towhee
Northern Cardinal
Blue Grosbeak
Common Grackle
Mourning Dove
White-eyed Vireo
Yellow-throated Vireo
Red-eyed Vireo
Carolina Wren


Total Nests
9


Total Families


San Felasco Hammock State Park
























Split Rock/Sugarfoot Preserves










Table A-2. Continued.


Nests
Parasitized


Families
Parasitized
1


Study Site


Species
Blue-gray Gnatcatcher
Northern Mockingbird
Northern Parula
Common Yellowthroat
Yellow-breasted Chat
Northern Cardinal
Blue Grosbeak

Indigo Bunting
Eurasian Collared Dove
Mourning Dove
Loggerhead Shrike
White-eyed Vireo
Carolina Wren
Northern Mockingbird
Brown Thrasher
Northern Cardinal
Red-winged Blackbird
Common Grackle
Boat-tailed Grackle
Orchard Oriole
Mourning Dove
Carolina Wren
Northern Mockingbird


Total Nests
1


Total Families
2


University of Florida Campus

















USDA Wildlife Research Center










Table A-2. Continued.


Nests
Parasitized


Families
Parasitized


Study Site


Species
Brown Thrasher
Eastern Towhee
Northern Cardinal


Total Nests


Total Families










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BIOGRAPHICAL SKETCH

Matthew J. Reetz was born on July 30, 1973 in Willoughby Hills, Ohio. From 1991 to

1995, he attended the University of Illinois in Urbana-Champaign, earning a bachelor' s degree

in biology with concentration in ecology, ethology and evolution. Repeated field seasons

working in southern Illinois on proj ects dealing with songbird nesting biology encouraged the

pursuit of a higher degree. In the fall of 1997, he entered the University of Florida' s graduate

program to study ecology and ornithology and earned a Master of Science degree from the

Department of Wildlife Ecology and Conservation in August of 2000. Matthew earned an

Alumni Fellowship which facilitated dissertation research resulting in a doctorate in wildlife

ecology and conservation in May 2008. In August 2008, Matthew will be an Assistant Professor

in the Natural Sciences Division at Franklin College where he will teach biology, mentor

undergraduate proj ects, and conduct research in community ecology.





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PATTERNS OF BROWN-HEADED COWBIRD PARASITISM IN A RECENTLY INVADED AREA AND POTENTIAL MECHANISMS LIMITING COWBIRD REPRODUCTION By MATTHEW J. REETZ A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008 1

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2008 Matthew J. Reetz 2

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To LeAnn, my favorite wife 3

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ACKNOWLEDGMENTS Invaluable field help was provided by Andrew Cook, Steve Daniels, Dan Dawson, Elizabeth Bauman, Nia Haynes and Rebecca Hame l. I thank the Florida Department of Environmental Protection, Danny Dr iver, David Armstrong, Joel McQuagge, Jesse Smalls, Steve Coates, Mats Troedsson, and Bruce Christensen of the University of Florida (UF), and Geoff Parks of the Gainesville Nature Operations Divisi on for access to field sites. I am indebted to Michael Avery, Eric Tillman, John Humphrey, Kandy Keacher a nd Michael Milleson of the USDA National Wildlife Research Center for s upport with trapping and maintenance of captive cowbirds. Brian Branciforte and Kate Haley of the Florida Fish and Wildlife Commission and Monica Lindberg, Caprice McCrae, Laura Haye s and Sam Jones of UF provided valuable logistical support. Research design of this project was impr oved through comments of various avian discussion groups. My deepest thanks go to my committee members Michael Avery, Doug Levey, David Steadman, and especially co-cha irs Kathryn Sieving and Scott Robinson. Research protocols D539 and D954 were approved by the UF Institutional Animal Care and Use Committee. This work was made possible by the Florida Fish and Wildlife Conservation Commissions program, Floridas W ildlife Legacy Initiative, and the U.S. Fish and Wildlife Services State Wildlife Grants program (SWG04 -031). Finally, my thanks go to C. LeAnn White for her steadfast support, especially when th e light at the end of the tunnel grew dim. 4

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TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................7 LIST OF FIGURES .........................................................................................................................8 ABSTRACT ...................................................................................................................................10 CHAPTER 1 PATTERNS OF PARASITISM IN NORTH -CENTRAL FLORIDA, A REGION OF RECENT COWBIRD EXPANSION.....................................................................................12 Introduction .............................................................................................................................12 Patterns of Cowbird Expansion in the Southeast ............................................................15 Objectives ........................................................................................................................16 Methods ..................................................................................................................................17 Results .....................................................................................................................................18 Discussion ...............................................................................................................................20 Cowbird Parasitism in North-Central Florida .................................................................20 Potential Limits on Parasitism Rates or Their Accurate Detection .................................22 Conclusions and Management Implications ....................................................................26 2 RESPONSES OF FOUR SONGBIRD SPECIES TO EXPERIMENTAL COWBIRD PARASITISM IN A RECE NTLY INVADED AREA..........................................................32 Introduction .............................................................................................................................32 Methods ..................................................................................................................................35 Study Design ...................................................................................................................35 Selection of Study Species ..............................................................................................36 Results .....................................................................................................................................38 Discussion ...............................................................................................................................39 Rejection Responses of Florida Hosts .............................................................................39 Acceptance of Cowbird Eggs ..........................................................................................41 Conclusions .....................................................................................................................43 3 DOES VARIATION IN FEMALE COWBIRD FECUNDITY EXPLAIN OBSERVED LOW LEVELS OF PARASITIZATION?..............................................................................46 Introduction .............................................................................................................................46 Limitations to Cowbird Reproduction .............................................................................47 Low Nest Parasitism in Florida: Low Cowbird Fecundity? ............................................48 Methods ..................................................................................................................................49 Cowbird Collection .........................................................................................................49 5

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Reproductive Assays .......................................................................................................51 Clutch Size Estimates ......................................................................................................53 Fecundity Estimates .........................................................................................................55 Results .....................................................................................................................................56 Cowbird Collection .........................................................................................................56 Reproductive Assays .......................................................................................................57 Clutch Size Estimates ......................................................................................................61 Fecundity Estimates .........................................................................................................63 Additional Trapping Observations ..................................................................................65 Discussion ...............................................................................................................................66 Clutch Size Estimates ......................................................................................................68 Fecundity Estimates .........................................................................................................69 Cowbird Collection .........................................................................................................71 Conclusions .....................................................................................................................73 4 ALTERNATIVE HYPOTHESES FOR TH E SLOW COWBIRD INVASION OF FLORIDA...............................................................................................................................84 Introduction .............................................................................................................................84 Host Community Composition ...............................................................................................86 Nest Predation.........................................................................................................................92 APPENDIX A STUDY SITE DESCRIPTION AND HOST SPECIES SURVEYED.................................102 LITERATURE CITED ................................................................................................................110 BIOGRAPHICAL SKETCH .......................................................................................................129 6

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LIST OF TABLES Table page 1-1 Summary of nests and family groups for 32 species monitored in the study areas, 2004 2006.........................................................................................................................30 2-1 Results of experimental parasiti sm of four common Florida species. ...............................45 3-1 Summary of means of mor phometric data taken from breeding female Brown-headed cowbirds collected in Florida and Texas, 2005 2007 and results of statistical comparisons. ......................................................................................................................75 3-2 Estimate of clutch size of 2005 Texas cowbirds. ...............................................................75 3-3 Estimate of clutch size of 2006 2007 Florida cowbirds. ..................................................76 4-1 Host species documented as breeders in Florida also known to have fledged Brownheaded Cowbirds. .............................................................................................................100 7

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LIST OF FIGURES Figure page 1-1 Breeding Bird Survey data of Brown-head ed Cowbirds detected per route in Florida from 1966 2006.................................................................................................................29 1-2 Estimated number of Brown-headed Cowbirds per Breeding Bird Survey route for northern and southern Florida, 1966 2006........................................................................29 3-1 Location of Brown-headed Cowbird co llection sites from April-August, 2005-2007. .....77 3-2 Number of female cowbirds collected per month at sites in north-central Florida in 2006 and 2007. ...................................................................................................................77 3-3 Mean number of female cowbirds collected per trap day at sites in north-central Florida from April to July, 2006 2007..............................................................................78 3-4 Number of female cowbirds collected pe r month in Florida by trap and by pellet gun from April July 2007. .......................................................................................................78 3-5 Mean weekly mass of ovaries of all Fl orida and Texas female cowbirds collected between 15 April and 15 July, 2005 2007........................................................................79 3-6 Mean weekly mass of oviducts of all Fl orida and Texas female cowbirds collected between 15 April and 15 July, 2005 2007........................................................................79 3-7 Mean weekly mass of ovaries of bree ding Brown-headed Cowbirds collected in Florida, 2006 2007............................................................................................................80 3-8 Mean weekly mass of oviducts of bree ding Brown-headed Cowbirds collected in Florida, 2006 2007............................................................................................................80 3-9 Mean weekly mass of ovaries of bree ding Brown-headed Cowbirds collected in Texas, 2005. .......................................................................................................................81 3-10 Mean weekly mass of oviducts of bree ding Brown-headed Cowbirds collected in Texas, 2005. .......................................................................................................................81 3-11 Mass of female Brown-headed Cowbirds collected in Florida between 23 April and 12 July, 2006 2007............................................................................................................82 3-12 Mass of female Brown-headed Cowbirds collected in Texas between 19 April and 6 July, 2005. ..........................................................................................................................82 3-13 Mean area of post-ovulatory follicles (POFs) of Brown-headed Cowbirds collected in Texas and Florida from April to June, 2005 2007. .......................................................83 8

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3-14 Estimated laying rate (eggs produced per day) of breeding cowbirds collected in Florida and Texas during four peri ods of the cowbird breeding season. ...........................83 4-1 Distribution of Brown-headed Cowbirds in Florida. Redrawn from Breeding Bird Survey abundance map (1994 2003). ...............................................................................97 4-2 Proportion of total abundance composed of species of varying quality as cowbird hosts ...................................................................................................................................97 4-3 Total number of host species of thre e types at locations in three regions. ........................98 4-4 Mean abundance of three types of Brow n-headed Cowbird hosts in three regions. ..........98 4-5 Mean abundance of forest species that represent good quality cowbird hosts in Illinois and Florida.. ...........................................................................................................99 9

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Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy PATTERNS OF BROWN-HEADED COWBIRD PARASITISM IN A RECENTLY INVADED AREA AND POTENTIAL MECHANISMS LIMITING COWBIRD REPRODUCTION By Matthew J. Reetz May 2008 Chair: Kathryn E. Sieving Cochair: Scott K. Robinson Major: Wildlife Ecology and Conservation Despite conservation implications su rrounding recent Brown-headed Cowbird ( Molothrus ater ) expansion into the southeastern United States there are few ecological data on cowbirds in Florida. Objectives for this project were to (1 ) quantify parasitism rates to determine the extent of brood parasitism and identity of host specie s and (2) test hypothese s explaining patterns of parasitism and cowbird dist ribution in Florida. I surveyed parasitism of 35 host species breeding in Florida through nest monitoring and observations of family groups with fledglings. Although cowbirds are moderately abundant in the study area, only 1.6% of 1117 nests and 6.1% of 278 family gr oups with fledglings were parasitized. While rates of parasitism ar e extremely low across the community and for individual hosts, my results suggest that co wbirds may preferentially parasitize a few host species nesting in the midstory and canopy. To help explain observed low cowbird parasitism rates, I tested egg rejection behavior of four common Florida songbird species known to re ject parasitism elsewher e. I experimentally parasitized 78 nests using eggs obtained from a captive cowbird colony. Three of four species 10

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tested typically accepted experime ntal cowbird eggs, rejecting parasitism at significantly lower rates than those published in the literature for each species. Rejection behaviors do not appear to limit Brown-headed Cowbird reproduc tion in north-central Florida. I tested the hypothesis that reduced physiolo gical fecundity of female Brown-headed Cowbirds contributed to low observed parasiti sm rates. I compared ovarian reproductive condition of breeding female cowbirds in Florida with those from Texas, where high parasitism rates have been recorded. Based on reproductiv e assays, I estimated that average female cowbirds in Florida and Texas ha ve similarly high fecundity at 34 eggs per female per breeding season. Low observed cowbird parasitism frequencies combined with moderate abundance of cowbirds in north-central Florida presents a paradox that my data do not explain. Cowbird reproduction may be limited by high nest predation rates or limited diversity and abundance of host species. I present a summa ry of census and reproductive da ta that suggest that these mechanisms may contribute to observed patter ns of cowbird parasitism in Florida. 11

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CHAPTER 1 PATTERNS OF PARASITISM IN NORTH-CEN TRAL FLORIDA, A REGION OF RECENT COWBIRD EXPANSION Introduction The Brown-headed Cowbird ( Molothrus ater ) is one of three brood-parasitic cowbird species in North America. It was prehistorica lly mainly found in the gr asslands of the Great Plains region of central North America where it foraged with bison herds and parasitized locally breeding host populations. With the arrival of European settlers that caused habitat fragmentation, destruction of bison populations, and establishment of permanent feeding areas, cowbirds began expanding their range in response to habitat fragmentation and the replacement of nomadic bison with largely sedentary cattle Until the 1800s, Brown-headed Cowbirds were uncommon in the eastern United States, probably b ecause of a lack of suitable open feeding habitats among large tracts of forest (Mayfiel d 1965). With the incr ease in human population, however, cowbirds began to appear in states su ccessively farther east (Lowther 1993) and local records of cowbirds changed from rare or absent to common or abundant (Mayfield 1965). Presently, the breeding range of the Brown-h eaded Cowbird includes the entire contiguous United States and much of North America (Sauer et al. 2006). Because of its recent, widespread expansi on, the Brown-headed Cowbird has been of great conservation concern. It is a generalist obligate brood pa rasite that conti nually parasitizes new host species, and has a prodigious reproduc tive output with females laying 40 eggs per season (Scott and Ankney 1980) in the nests of over 220 host species (Friedmann and Kiff 1985). Prior to human alteration of the landsca pe, cowbirds were naturally limited to host species with which they were sympatric during the breeding season. The spread of the Brownheaded Cowbird brought it into contact with a new suite of host populations and species, most of which lacked recent historical exposure to brood parasitism. Significant songbird population 12

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declines during the 20 th century were thought to be due to the steady increase of cowbird populations continent-wide (Mayfield 1977, Grz ybowski et al. 1986, Robbins et al. 1989, Trail and Baptista 1993). Where Brown-headed Cowbird populations ar e well established, brood parasitism can significantly reduce reproductive success of hos t species and communities (Friedmann et al. 1977, Robinson et al. 1995b). For example, Brown-headed Cowbird parasitism contributes to negative net reproduction of some songbird populations, which are likely maintained only by immigration from other areas (Robinson et al. 19 95b). Cowbirds limit reproduction of hosts in a variety of ways, including reduced clutch sizes (Mayfield 1961, Burhans et al. 2000, Hoover 2003) through removal and predation of host eggs or young (Friedmann 1963, Sealy 1992, Arcese et al. 1996, Granfors et al. 2001, H oover and Robinson 2007), or reduced hatching (McMaster and Sealy 1998, Burhan s et al. 2000) and host fledging success (McMaster and Sealy 1998, Burhans et al. 2000). Costs can also be incurred beyond the ne st through increased juvenile and adult mortality (Pay ne 1965, Hoover and Reetz 2006). Overall costs of cowbird parasiti sm are particularly evident at local scales in contact zones where cowbirds begin parasitizing novel hosts that are highly suscepti ble to parasitism. Cowbirds spread rapidly throughout California beginning in the early 1900s, for example, and brood parasitism has contributed to the extirpatio n of local populations of Least Bells Vireo ( Vireo bellii ) in much of its California range (U SFWS 1998). Furthermore, parasitism by Brown-headed Cowbirds has threatened the extin ction of species with limited ranges such as Kirtlands Warbler ( Dendroica kirtlandii ) in northern Michigan (Mayfield 1992). As a result, Brown-headed Cowbirds became a regional management problem for particular songbird species and were often aggressively controlled to improve reproductive success of hosts (Kelly and 13

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DeCapita 1982, Beezley and Rieger 1987, Eckrich et al. 1999). Identifying situations where management of cowbird populations is necessary to prevent such declines is of importance in conservation of some populations of host species (Rothstein and Cook 2000), particularly since the cowbird has spread over most of North Amer ica, coming into contact with new host species and populations. There are many reasons to monitor the re productive success (and thereby, cowbird parasitism rates) of songbirds for conservati on and management of av ian habitats, populations, and species. Potential impacts of cowbird parasi tism can be identified us ing rates of parasitism as an initial cue, with further detailed work leading to conservation of affected host species. Research to characterize Shiny Cowbird ( Molothrus bonariensis ) expansion to Puerto Rico (since the 1940s), for example, identified extremely high rates of parasitism (75 100%) on multiple native species (Post and Wiley 1977, Wile y 1985). This work ultimately led to a conservation program for the endangered, endemic Yellow-shouldered Blackbird ( Agelaius xanthomus ), which had suffered population declines of >80% as a result of intense parasitism (Wiley et al. 1991). A large-scale cowbird tr apping program at Fort Hood, Texas reduced parasitism of the federally endangered Black-capped Vireo ( Vireo atricapilla ) from 91% in 1987 to 8.6% ten years later (Eckrich et al. 1999). Cowbird management is also an ongoing and critical component in the conservation of the threatened and endangered populations of Southwestern Willow Flycatcher ( Empidonax traillii extimus ), Least Bells Vireo, Black-capped Vireo, California Gnatcatcher ( Polioptila californica ), Kirtlands Warbler, and Golden-cheeked Warbler ( Dendroica chrysoparia ). Where cowbirds have clear negative effects on host populations and communities, rates of parasitism (percentage of nest s/broods with cowbird eggs/c hicks) can be as high as 80 100 % 14

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(Norris 1947, Hatch 1983, Grzybowski et al. 1986, Robinson 1992) and often above 50% (Elliott 1978, Burgham and Picman 1989, Hahn and Hatfield 1995, Trine et al. 1998). Therefore, monitoring songbird reproductive suc cess and rates of cowbird parasi tism can help determine the potential for serious (or unimporta nt) consequences of cowbird act ivity in conservation planning. This is particularly important in areas wher e cowbirds have most recently expanded their breeding range, such as the southeastern United States. Patterns of Cowbird Expansion in the Southeast The first records of Brown-headed Cowbirds breeding in the south eastern United States were in far western North Carolina in 1933 (Pears on et al. 1959) and far western South Carolina in 1934 (Sprunt and Chamberlain 1970). Breeding co wbirds reached northern Georgia in the late 1940s (Parks 1950). By the mid-1960s, cowbirds were parasitizing nests in many parts of the Carolinas but were still scarce in the eastern Piedmont and in ner coastal plain regions, although they were first reported during the breeding seas on near the coastal cities of Wilmington, NC and Charleston, SC in 1967 and 1965, respectiv ely (Potter and Whitehurst 1981). Brown-headed Cowbirds were first document ed breeding in Florida during the mid-1950s, when a fledgling was sighted in Escambia Coun ty in the far western panhandle (Monroe 1957). Cowbirds were subsequently confirmed breeding in Jacksonville in 1965 (Ogden 1965) and south to Alachua County in 1980 (Edscorn 1980). By the end of the 1980s, the breeding range of the Brown-headed Cowbird had expanded to incorporate all of Florida (Hoffman and Woolfenden 1986, Paul 1989) and thus the entire s outheastern portion of the United States. Brown-headed Cowbirds are presently confirmed br eeding in at least 50 of Floridas 67 counties (FFWCC 2007). Since the mid-1960s, the populati on of Brown-headed Cowbirds in Florida grew slowly but steadily (Figure 1-1) and also increased in Breeding Bird Survey stratum three, which includes northern Florida (Figure 1-2; Sauer et al. 2006) Cowbirds are roughly as 15

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abundant in the northern portion of Florida as they are in much of th e rest of the continent (Sauer et al. 2006), so there is good reason to expect that they may represen t a threat to breeding songbird populations. Objectives Most of the approximately 50 songbird speci es breeding in Florida are known to be parasitized by cowbirds in other parts of their ranges, and some are known as suitable hosts (i.e., have reared cowbird young) in ot her areas (Ortega 1998). Some of these species are locally rare in north-central Flor ida (e.g., Hooded Warbler Wilsonia citrina Yellow-breasted Chat Ictera virens ; FFWCC 2007) or are experiencing significant declines in the southeast (e.g., Red-eyed Vireo Vireo olivaceus Northern Parula Parula americana ; Sauer et al. 2006) and would, therefore, be particularly susceptible to any ne gative effects of brood parasitism they may incur. Isolated records of parasitism of breeding songbir ds in Florida suggest that many species may be at risk to Brown-headed Cowbird parasitism (see review in Cruz et al. 1998). Despite widespread concern over potentia l impacts of cowbird parasitism for native breeding songbirds, only scattered data on parasitism were collected in Florida during the 60 years since cowbirds began breeding in the state. To determine potential impacts of Brown-h eaded Cowbird expansion into Florida, I conducted a comprehensive survey of cowbird parasitism in songbird communities in northcentral Florida, including (1) monitoring understory bird nests for rates of occurrence of cowbird eggs and nestlings in host nests and (2) survey ing post-fledgling family groups (adults with fledglings) of both understory and canopy songbird species to determine the rates of successful brood parasitism by cowbirds among the species sampled. More than 30 species were included in my study; each is known to have been parasi tized elsewhere in their breeding range by Brown16

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headed Cowbirds. This represents the first community-wide survey documenting cowbird parasitism rates in both nests and post-fledging families of host species breeding in Florida. Methods I surveyed nests from a diversity of potential hosts to determine natural parasitism rates in north-central Florida. Nest searches were conducted daily between 0700 and 1900 EST at sites within 25 km of Gainesville, FL (ca. 29 N, 82W) that included many different types of habitats (Table A-1). Sites ranged from urban ( downtown Gainesville) to rural (no or low nearby housing density), from semi-natural (e.g., city pa rks) to natural (e.g., managed state preserves), and from disturbed (e.g., regenerati ng rangeland) to quasi-natural (large natural communities in state preserves). Nests were monitored every 1 to 3 days. Data were supplemented from nest records from an avian monitoring project (Sieving & Contrera s, unpubl. data) conducted in Myakka River State Park near the southwest Flor ida gulf coast in Sarasota and Manatee Counties (ca. 27 N, 82 W) from April to July, 2001 2005. Cowbird abundance in this area is approximately one-quarter of th at in northern Florida (Sauer et al. 2006) but large mixed-sex roosting flocks with 1500+ cowbirds have been seen during the breedi ng season in nearby St. Petersburg, FL (D. Margeson, pers. comm.). Data on nest success for canopy species such as Red-eyed Vireo, Northern Parula, and Pine Warbler ( Dendroica pinus) are rare or non-existent because th eir nests are gene rally high in the canopy (>10m), and therefore difficult to monitor re liably using current te chnologies. Nests of cryptic species (e.g., Carolina Wren Thryothorus ludovicianus ) are also under-represented in typical nest searching/monitoring samples due to low detection rate s. Therefore, to account for these biases in the data set, I conducted in tensive surveys for family groups (adults and fledglings) for all species that nest thr oughout the understory and canopy (Verner and Ritter 17

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1983). Monitoring of adults feeding young outside of the nest is the main component of reproductive indices used to estimate reproductiv e success (Vickery et al 1992) and are useful when nest monitoring is logistical ly infeasible (Bonifait et al. 2006). Family groups were found by watching foraging adults and following adults carrying food or by tracking fledgling begging calls. Fledglings of some species gi ve more audible begging calls, but I listened for any call that might repr esent fledgling feeding or presence. Family groups of the study species that I encountered were followed for at least ten minutes to determine if a cowbird fledgling was present. Multiple species have rarely been observed feeding a single cowbird fledgling (Klein and Rosenberg 1986), so I assumed cowbird fledglings were being fed by the species that raised them. I typically observed repeated feedings of cowbirds by the same species and never witnessed a feed ing of a given cowbird by more than one host species. Lower nesting (understory) species tend to be better represented in standa rdized samples of forest bird communities. To overcome any potential bias this may represent, I spent approximately 75% of time conducting visual scans for adult birds and families in the midstory and canopy and 25% in lower strata at forested sites. Many times, family groups for lower nesting species were located opportunistically when searching for and monitoring nests. Results From March August, 2004 2006, I monitored the parasitism status of 35 potential host species breeding in Florida (Tables 1-1, A-2). Nest surveys in north-central Florida included monitoring of 994 nests of 29 potential cowbird host species. Nesting data from Myakka River State Park added 123 nest records of nine host species and included 26 re cords for three species not monitored in this study (32 species total) Only 18 nests (1.6%) of eight potential host species contained a cowbird egg or nestling. The most records of parasitism (N=6) were from 18

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Northern Cardinal ( Cardinalis cardinalis ), but constituted only 2.5% of cardinal nests monitored. Other species parasitized with at least five nest records were Blue-gray Gnatcatcher ( Polioptila caerulea ; 20%), Hooded Warbler (2 0%), Blue Grosbeak ( Guiraca caerulea ; 14.3%), White-eyed Vireo ( Vireo griseus ; 10.5%), Bachmans Sparrow ( Aimophila aestivalis ; 9.1%), Eastern Towhee ( Pipilo erythrophthalmus alleni ; 6.1%), and Northern Mockingbird ( Mimus polyglottos; 0.2%). One of three (33%) Common Yellowthroat nests was parasitized. The only Yellow-throated Vireo ( Vireo flavifrons ) nest contained a lone cowbird ne stling. Of nests of the 10 species parasitized, 2.4% (18 of 750) cont ained cowbird eggs or nestlings. Nest records of Northern Mockingbird constituted the larg est proportion of the sample of parasitized birds (N=397) and may have biased average parasitism rate over multiple species. However, even after excluding these records, the parasitism frequency of nest s was still low at 5.7% (17 of 299) of nests. Cowbirds fledged successfully from five nests (one each of Yellow-throated Vireo, Blue-gray Gnatcatcher, Common Yellowthroat, Bachmans Sp arrow, Northern Cardinal). Bachmans Sparrow nests are known to be parasitized (Ortega 1998), but this represen ts the first report of this species raising a cowbird to fledging and the first record of cowbird parasitism on the Bachmans Sparrow in Florida (Reetz et al. 200 8). Among the few nests of midstory and canopy species that I was able to mon itor, 1.7% (2 of 116) were para sitized compared to 1.6% (16 of 1001) of nests of the remaining species. Excludi ng Northern Mockingbird nests from the sample of lower nesting species, 2.5% (15 of 592) of nests were parasitized. I monitored 278 family groups of 20 species, including three species (Carolina Chickadee Poecile carolinensis Red-eyed Vireo, and Northern Paru la) for which no nest data were available. Seventeen (6.1%) of these family gr oups contained a cowbird fl edgling (Table 1-1). Of the six species with at least one parasitized family group, 10.6% (17 of 160) of families 19

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contained a cowbird fledgling. Family groups of midstory or canopy nesting species were more likely to be found feeding a cowbird young than lower nesting species (Log Odds Ratio=3.04, CI=4.65 93.61). Indeed, of the 17 parasitized family groups monitored, 15 (88.2%) were of midstory or canopy nesting species, and 17.9% (15 of 84) of midstory or canopy nesting family groups observed were parasitized. For ex ample, 3 of 5 (60%) Summer Tanager ( Piranga rubra ), 7 of 18 (38.9%) Northern Parula, 3 of 9 (33%) Bl ue-gray Gnatcatcher, 1 of 4 (25%) Red-eyed Vireo, and 1 of 14 (7.1%) Pine Warbler family groups monitored were observed feeding a cowbird fledgling. Northern Cardinals were the only lower nesting species that was observed feeding a fledgling cowbird (2 of 93 family groups or 2.2%). Discussion Cowbird Parasitism in North-Central Florida Despite the apparent abundance of cowbirds and their feeding habitat in northern Florida, I detected very low rates of Brown-headed Cowbird parasitism across the community of potential host species breeding in north-centr al Florida. Less than 2% of nests were found with cowbird eggs and approximately 6% of family groups contained a cowbird fledgling. While this represents the first community-wide survey of Brown-headed Cowbird parasitism in Florida, some more narrowly focused reproductive stud ies present similarly low rates of cowbird parasitism. For example, Prather and Cruz (1995) found no parasitism cases among 42 Prairie Warbler ( Dendroica discolor paludicola) and 20 Yellow Warbler ( Dendroica petechia gundlachi ) nests in south Florida, and only two cases in 108 nests of ten species on the southwestern gulf coast (2002) During the six years (1986 1991) of the Florida Breeding Bird Atlas, a collaborative project de signed to record breeding distri butions of all av ian species in Florida, volunteers and researchers monitored more than 2400 nests of the same species surveyed 20

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in this study. Less than 0.5% of nests were found with cowbird eggs or nestlings (FFWCC 2007). Species seen feeding fledgling Brown-h eaded Cowbirds during the Atlas project were similar to those in this survey and included Bl ue-gray Gnatcatcher, Red-eyed Vireo, Northern Parula, Pine Warbler, and Summer Tanager, among others. Taken together, available monitoring data of Florida host species show very limited Brown-headed Cowbird nest parasitism. Results from parasitism surveys in this projec t are nonetheless surprising given the relative abundance of Brown-headed Cowbirds in north-central Florida, ranging from 3 10 birds per Breeding Bird Survey route (1994 2003; Sauer et al. 2006). Such abundances are comparable to those in most of the eastern and far western United States, where reproductive studies have recorded much higher community-w ide parasitism rates than reported here. For example, nest parasitism rates of 36 host species in northeas tern New Mexico are 20.8% of 850 nests (Goguen and Mathews 1998) and of 32 host species in so utheastern New York are 16.53% of 301 nests (Hahn and Hatfield 1995). Parasitism frequency of common host specie s in recently-invaded South Carolina and Georgia is lower at 8.2% of nests (Kilgo and Moorman 2003), but still significantly higher than the frequency reported here. Even species with high rates of parasitism elsewhere in their ranges seem to elude it in Florida. Northern Cardinals had parasitism rate s of 2% of nests and family groups in this study, but suffer parasitism rates of 25 100% of nests in other parts of their breeding range (reviewed in Ortega 1998). Rates of para sitism are also very high (40 83%) for Eastern Towhee and White-eyed Vireo in other regions (Norri s 1947, Young 1963, Goertz 1977, Hopp et al. 1995), yet were only 5.3% and 10.5%, respec tively, in north-central Florida. Finally, fewer than 2% of nests of understory nesting species contained cowbird eggs, yet 13% of nests in a similar suite of understory species were parasitized elsewhere in the southeastern U.S. (Whitehead et al. 2002). 21

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Various procedural and ecological-evolutionary mechanisms may underlie the low rates of parasitism detected in this study. Potential Limits on Parasitism Ra tes or Their Accurate Detection First, rates may have been affected by inclusion of certain species that are likely incidental or inappropriate hosts. For example, rates of nest parasitism in sout heastern Ontario, where cowbirds are abundant, are only 6.7% across all 87 species for which parasitism has been recorded at least once (Peck and James 1987). Ma ny of these species, however, are accidental or unsuitable cowbird hosts (e.g., Virginia Rail Rallus limicola ) not known to fledge cowbirds (Ortega 1998). Nine of the ten most parasiti zed species in Ontario, however, are appropriate hosts and some averaged as high as 48% of ne sts parasitized (Peck and James 1997, 1998a, b). Therefore, parasitism rates of at least marginal ly suitable hosts are like ly significantly higher than the 7% reported for the entire community of breeding birds. I surveyed a total of 35 host species, all of which have been recorded as hosts of Brown-h eaded Cowbirds (Ortega 1998). but some hosts included in th e survey, (e.g., Mourning Dove Zenaida macroura, Blue Jay Cyanocitta cristata Northern Mockingbird, Brown Thrasher Toxostoma rufum and House Finch Carpodacus mexicanus ) are inappropriate or poor hosts due to life-history tr aits (e.g., nestling diet, defense behaviors) incompatible with co wbird parasitism. When all nests of the 14 unsuitable host species surveyed are removed fr om the sample, the parasitism rate of the remaining 21 species is still onl y 3.6% of nests (18 of 486) and 7.5% of family groups (17 of 238; Table 1-1). Therefore, surveys of parasitism frequency do not appear to be biased by selective inclusion of host species. Second, many hosts have nests th at are difficult to find (e .g., Common Yellowthroat) or extremely difficult to monitor (e.g., Northern Paru la and other canopy nesting species), and thus parasitism may go undetected. For example, ve ry low rates of nest parasitism (1%) were 22

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detected across a host community in the Sierra Nevada of California (Verner and Ritter 1983), a survey that included species heavily parasitized el sewhere in the region. Observations of family groups still showed low parasitism rates (4%), but the authors anal yses suggested that parasitism was biased toward species with nests that are diffi cult to monitor. Results from my surveys also suggest that nest parasitism may go undetected due to preference for particular species infeasible to survey. I was able to monitor few midstory and canopy nest, and parasitism rate (1.9%) in this limited sample was similar to that for lower nest ing species (1.5%). However, I note that the vast majority (88%) of parasitized family groups I observed was of species nesting in the higher strata, presenting an overall para sitism rate of approximately 20% for the nesting guild with the lowest nest detectabilit y in nest monitoring surveys. As a result, it appears that low observed frequencies across the community of hosts is at least partially due to difficulties in obtaining adequate samples for all potential species. Third, estimates of cowbird parasitism that are based on the percent of parasitized nests can underestimate actual parasitism levels if nests are abandone d, nest contents are readily depredated (Robinson et al. 1995a), or cowbird eggs are ejected. Many species nesting in northcentral Florida practice egg reje ction behaviors (i.e., egg ejecti on, egg burial, and abandonment of nests) in some parts of their ranges. Many of the most abundant songbird species that breed in Florida are known to readily ej ect cowbird eggs or abandon para sitized nests. For example, Northern Mockingbirds and Brown Thrashers eject up to 69% (Peer et al. 2002) and 100% (Rothstein 1975b, Haas and Haas 1998) of experiment al cowbird eggs placed in their nests. If anti-parasitism behaviors are prevalent in many of the host species breeding in a region, a large portion of actual nest parasitism will go undetect ed leading to lower observed parasitism rates 23

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overall. Egg rejection behavior s do not appear to be prevalent in common hosts in Florida (Chapter 2). Fourth, though Brown-headed Cowbird fecundity is usually very high with an average female producing 40 eggs in a breeding season (Scott and Ankney 1980), there is significant variation in fecundity over the species range (Payne 1965, 1973a, 1976, Scott and Ankney 1980). It is possible, therefore, that while cowbirds are abundant in north-central Florida, host communities may show reduced parasitism of ne sts simply because breeding female cowbirds lay fewer eggs. Cowbird fecundity has been m easured using a variety of methods, and limited reproduction would provide a parsimon ious explanation for low rates of parasitism in Florida. Females breeding in north-central Florida show hi gh fecundity (Chapter 3), with a rate of egg production similar to the highest rate estimated for the species. Finally, low parasitism rates may be related to the limitation of suitable hosts. Cowbirds have been identified as generalist parasi tes because they are know n to parasitize over 220 species (Ortega 1998). Because parasitism often affects many or sometimes most potential host species in a community, parasiti sm may not be host-specific because eggs are spread among hosts according to their abundance (shotgun hypothe sis; Rothstein 1975b). In this case, good hosts (those able to successfully fledge cowbird young) are parasi tized as frequently as poor hosts. Host-specific characteristic s can affect the survival probability of cowbird young so that selection may favor parasitism of hosts successful at raising cowbirds. As a result, parasitism of nests may be biased toward a few select spec ies (host selection hypot hesis; Rothstein and Robinson 2002). Host selection is supported by data showing nonr andom parasitism frequencies of a suite of potential hosts (Woolfenden et al. 2003, Ellison et al. 2006). In areas where 24

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cowbirds are very abundant, individuals are likely to employ both host selection and shotgun tactics because competition fo r resources (nests of good hosts) is more intense. For successful expansion and population growth of cowbirds in new regions such as Florida, the availability of at least some suitable hosts that accep t cowbird eggs and appropriately nourish juveniles is critical. Re sultant parasitism frequencies ar e often higher than those that reject eggs (Sealy and Bazin 1995) or feed young a diet inappropriate to cowbirds (Middleton 1991), but patterns of parasitism acr oss the host community have not typically been addressed in areas where cowbirds have recently expanded. In a newly colonized area such as Florida, low initial cowbird densities should favor host selection because of low co mpetition for nests. In this case, the availability (abundance/diversity) of quality hosts may represent a limiting factor to reproduction. Data presented in this chapter are consistent with host selection because certain species, particularly those that nest in the midstory or canopy, appear to have been preferentially parasitized. This result probabl y does not reflect a selection bias by cowbirds toward higher nests, per se, but rather toward suitable host species. Lower nesting species such as Northern Mockingbird, Northern Cardinal, and Carolina Wren are probably poor cowbird host species because of anti-parasitism behaviors, nest site choice, or other life-hist ory traits (Rothstein 1975b, Haggerty and Morton 1995, Halkin and Linvill e 1999). In north-Florida, these and similar species constitute a rela tively higher proportion of lower nesting spec ies abundance than in other regions with a more di verse and abundant community of quality understory nesters (e.g., emberizids, parulids, etc.). Sp ecies such as Indigo Bunting (Passerina cyanea), Yellow-breasted Chat and Orchard Oriole are quality understory ho sts and are moderately parasitized in other southeastern states where th ey are common (Whitehead et al. 2002, Kilgo and Moorman 2003). 25

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However, north Florida represents the southern extent of their breeding range where they are found in very low densities (Sauer et al. 2006). As a result, a subset of midstory and canopy nesting species that is preferenti ally parasitized may be bearing the brunt of cowbird parasitism in north-central Florida because it represents the highest abundance of quality hosts but has relatively low species diversity. Conclusions and Management Implications Expansion of breeding Brown-headed Cowbirds into Florida was greeted with widespread concern, particularly with the a dditional arrival of the Shiny Cowb ird in south Florida. Atherton and Atherton (1988:63) stated it is conceivable that the br eeding ranges of the two cowbird species will meet within the next few years a nd that both Prairie Warb ler and Black-whiskered Vireo will be in grave danger of being extirpated from the state. The observation of a flock of 1000+ Brown-headed Cowbirds with six Shiny Co wbirds in south Florida in July 1988 (Paul 1988:1281) prompted the dramatic pr ediction that it represented ... the end of birding as we know it in south Florida. To date, there has been only one published breeding observation (Paul 1988), and one breeding specimen collected (Chapter 3) that confirm reproduction of Shiny Cowbirds in Florida. Furthermore, through monitoring of nests and family groups with fledglings, I detected very low rates of Brown-headed Cowbird parasitism on the community of potential host species breeding in north-central Florida. Therefore, brood parasitism by cowbirds does not appear to currently repres ent an urgent threat to most of Floridas breeding songbirds. My nest parasitism surveys suggest, however, th at a subset of host species, particularly midstory and canopy nesting species, accounts for a disproportionate amount of recruitment of juvenile cowbirds in north-central Florid a; these species could be suffering negative consequences of parasitism. Despite the logistical difficulties in locating and monitoring nests of midstory and canopy nesting species, I stress a criti cal need nest data to determine parasitism 26

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rates. In tandem, monitoring of populations of th ese species is needed to determine if declines are occurring from negative effects of brood para sitism. In addition, the relationship between cowbirds and their hosts should continue to be monitored across different host communities and in areas with different abundances of cowbirds. Brown-headed Cowbird abundance is highest in northwestern Florida, where diversity and abundance of good quality hosts al so are the highest in the state (Sauer et al. 2006). Fi nally, nest parasitism frequenc ies of species that are locally abundant but have restricted dist ributions should be determine d. In particular, the breeding distribution of the Painted Bunting ( Passerina ciris) in Florida is concentrated in the northeastern corner of the state with smaller populations scattered along th e Atlantic coast (FFWCC 2007). At a site in South Carolina, Painted Buntings had 35.3% of nests parasitized by Brown-headed Cowbirds (Whitehead et al. 2002). Such a ra te could threaten sma ll, isolated populations breeding in Florida. Perhaps the most useful conclusion of this study to management is that monitoring of family groups may provide a more comprehensive technique for assessment of the true incidence of successful cowbird parasitism in any area a ffected by increasing co wbird populations. Of individual species with reasonable sample sizes, the highest parasitism rates recorded were 39% (Northern Parula) and 33% (Blue-gray Gnatcatch er) of family groups. While these rates are considered moderate among those nest parasitism rates published in the literature (Ortega 1998), they do not consider cowbird eggs that do not hatch or are lost from nests, or increased nest predation of parasitized nests (Burhans et al. 2002). On the other hand, these rates may also overestimate relative parasitism due to samp ling biases inherent in the methodology. For example, cowbird fledglings are notoriously an d relentlessly boisterous when begging for food from host adults, which may make fledged fam ilies more conspicuous. I attempted to avoid 27

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over-sampling of families with noisy cowbird fl edges by finding and following family groups by scanning for adults with food, or listening for begging of fledglings of all sp ecies. Fledglings of some species have relatively soft calls and cr yptic behaviors, making them more difficult to detect from a distance than cowbird young, especi ally high in the canopy Therefore, this method is at least representative of parasitism in species nests that su ccessfully fledge young. In any case, these data suggest that parasitism rate s of selected species may be much higher than community-wide nest surveys alon e would detect. Therefore, in addition to cowbird population monitoring and nest monitoring, family group survey s may be a particularly valuable means of assessing the overall intensity of cowbird parasitis m affecting songbird communities. It is also, perhaps, the most efficient technique for iden tifying the host species most likely to recruit young cowbirds into the breeding population. 28

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29 Figure 1-2. Estimated number of Brown-headed Cowbirds per Breeding Bird Survey route for northern and southern Florida, 1966 2006. BBS Routes considered Southern Florida include all routes south of Marion County (ca. 28 N), the county immediately south of Alachua, where this study was c onducted. Data presented represent raw counts and only routes recording Brown-h eaded Cowbird at least once were included. Figure 1-1. Breeding Bird Survey data of Brown-h eaded Cowbirds detected per route in Florida from 1966 2006. 0 5 10 15 20 25 196619711976198119861991Birds/Route 199620012006 All Routes Northern Florida Southern Florida

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Table 1-1 Summary of nests and family groups for 32 species monito red in the north-central Flor ida from March to August, 2004 2006. U: species nesting in the midstory to canopy, L: species nesting low or on ground. Nesting category based on field observations and Ehrlich et al. ( 1988). Species totals for each nesting cat egory are given for nests/family groups monitored. Number of supplemental nests (number pa rasitized) from Contreras & Sieving (unpubl. data, 200 0 2004) are: MODO = 19, GCFL = 12, BLJA = 8, EABL = 7, NOMO = 18, COYE = 2 (1), EATO = 43 (3), BACS = 11 (1), EAME = 3. Species with an asterisk are c onsidered incidental or poor hosts. Common Name Latin Name Category Unparasitized Nests Parasitized Nests Unparasitized Families Parasitized Families Eurasian Collared Dove* Streptopelia decaocto U 3 Mourning Dove* Zenaida macroura U 52 Great-crested Flycatcher* Myiarchus crinitus U 12 Acadian Flycatcher Empidonax virescens U 14 9 Loggerhead Shrike* Lanius ludovicianus U 5 1 White-eyed Vireo Vireo griseus L 17 2 25 Yellow-throated Vireo Vireo flavifrons U 1 5 Red-eyed Vireo Vireo olivaceus U 4 1 Blue Jay* Cyanocitta cristata U 9 Carolina Chickadee* Poecile carolinensis U 1 Eastern Tufted Titmouse* Baeolophus bicolor U 2 Carolina Wren Thryothorus ludovicianus L 10 12 Blue-gray Gnatcatcher Polioptila caerulea U 4 1 9 3 Eastern Bluebird* Sialia sialis L 51 3 Wood Thrush Hylocichla mustelina U 3 Northern Mockingbird* Mimus polyglottos L 397 1 28 Brown Thrasher* Toxostoma rufum L 80 7 Northern Parula Parula americana U 18 7 Pine Warbler Dendroica pinus U 3 14 1 30

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31 Table 1-1. Continued. Common Name Latin Name Category Unparasitized Nests Parasitized Nests Unparasitized Families Parasitized Families Prothonotary Warbler Protonotaria citrea L 1 Common Yellowthroat Geothylpis trichas L 2 1 2 Hooded Warbler Wilsonia citrina L 4 1 7 Yellow-breasted Chat Icteria virens L 1 Summer Tanager Piranga rubra U 1 5 3 Bachmans Sparrow Aimophila aestivalis L 10 1 Eastern Towhee Pipilo erythrophthalmus alleni L 54 3 12 Northern Cardinal Cardinalis cardinalis L 237 6 93 2 Blue Grosbeak Guiraca caerulea L 6 1 Indigo Bunting Passerina cyanea L 2 3 Eastern Meadowlark* Sturnella magna L 3 Red-winged Blackbird Agelaius phoeniceus L 81 Common Grackle* Quiscalus quiscala U 2 Boat-tailed Grackle* Quiscalus major U 2 Orchard Oriole Icterus spurius U 2 3 House Finch* Carpodacus mexicanus L 29 TOTALS 35 1099 18 261 17 Midstory/Canopy Species 17/10 114 2 69 15 Lower Nesting Species 15/10 985 16 192 2 Quality Hosts 469 17 221 17

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CHAPTER 2 RESPONSES OF FOUR SONGBIRD SPECIES TO EXPERIMENTAL COWBIRD PARASITISM IN A RECE NTLY INVADED AREA Introduction Multiple direct and indirect costs are associ ated with brood parasitism by Brown-headed Cowbirds that reduce survival and fecundity of ho st species (see Chapter 1). As a result, nest parasitism can impose strong selection on hosts for behaviors to reduce the number of cowbird eggs that host females incubate (Rothstein 1990). Two predominant behaviors commonly expressed by hosts that reject parasitism are rem oval of cowbird eggs from the nest and desertion of parasitized clutches. Ind eed, quantification of the freque ncy of egg ejection and clutch desertion behaviors, via experime ntal emulation of cowbird nest parasitism (egg additions), is used to determine the prevalence of rejection responses within a speci es or host community (reviewed in Peer and Sealy 2004a ). In this way, host species can be categorized as either ejecters (species that physically remove parasitic eggs from their nest) or accepters. Species that initially accept eggs into their nests may s till reject parasitism by abandoning the parasitized clutch; either by deserting the nest (Roths tein 1975b, reviewed in Ortega 1998:192) or by burying the eggs under a new nest lining (Sealy 1995). Despite its apparent benefit, only a relatively small subset of species (ca. 29 of >220) that are parasitized exhib it at least intermediate egg ejection frequencies (Table 2.1 in Ortega 1998). Furthermore, many species that do not eject cowbird eggs from their nests do not subseque ntly abandon nests or bury cowbird eggs. Two major explanations are proposed for why pote ntial host species accept nest parasitism. Under the evolutionary equilibr ium hypothesis, nest parasitism is tolerated as a result of conflicting selection pressures (Zahavi 1979, Rohwer and Spaw 1988) Costs of ejection errors or abandonment of nests, representing losses of host eggs and energy expended in replacement nesting, may outweigh the costs of raisi ng parasite young; thus acceptance could be 32

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evolutionarily favored (Lotem and Nakamura 199 8). Under conditions of coexistence between cowbirds and host species that accept parasitism under this scenario, selection simultaneously favors cowbird traits that maximize efficiency of host use and host traits that minimize costs of parasitism (Robertson and Norman 1976, Dawkins and Krebs 1979). Under the evolutionary lag hypothesis (Rothstein 1975b, Dawkins and Krebs 1979, Davies and Brooke 1989), acceptance of parasitic eggs occurs because adaptations th at eliminate or reduce the frequency of brood parasitism have not yet developed, spread, or become fixed in the hos t population (Rothstein 1975a). As a result, host populations are not able to recognize parasitic eggs (Rothstein 1982) or females (Smith et al. 1984, Bazin and Sealy 1993), or lack appropriate responses to foreign eggs. In host populations in recent contact with expanding populations of Brown-headed Cowbirds, evolutionary lag is the most reasona ble explanation for a general lack of adaptive response to novel brood parasitism (Rothstein 1975b, Burgham and Picman 1989). In such situations, naive host populations may be at increased risk of decline due to high rates of nest failure (Cruz et al. 1989) if they are unable to recognize or respond to nest parasitism (Rothstein et al. 1980). Despite having only come into cont act with Shiny Cowbirds in the 1990s, however, Gray Kingbirds ( Tyrannus dominicensis) in the Bahamas are known to eject artificial cowbird eggs (Baltz and Burhans 1998), a behavior that is also expres sed where the two species have been sympatric for longer periods (Cruz et al. 1985, Post et al. 1990). In populations no longer sympatric with cowbirds, multiple species have s hown similar retention of cowbird egg rejection behaviors displayed in populations still exposed to parasitism (Rothstein 2001, Peer et al. 2007). Therefore, retention of behavioral defenses against brood parasites in some species and populations may buffer them from immediat e negative consequences of recent cowbird expansion. 33

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Colonization of Florida by Brown-headed Cowbirds has occurred over the last 50 60 years (see Cruz et al. 2000) and was fi rst thought to represent a signif icant threat to many breeding songbirds (Cruz et al. 1998). However, observed pa rasitism frequencies fo r individual hosts and across an entire host co mmunity are unusually low despite moderate Brown-headed Cowbird abundance (Chapter 1). Elsewhere in the sout heastern United States, many host species are known to have displayed anti-parasitism behavi ors during the recent southward expansion of cowbirds (Whitehead et al. 2002). These behavi ors are likely maintained by gene flow from regions where cowbirds have consistently been breeding over the last tw o centuries (Rothstein 1990). In addition, fossil evidence suggests th at Brown-headed Cowbirds (Hamon 1964, Ligon 1966, Emslie 1998) and another extinct cowbird species ( Pandanerus floridana ; Brodkorb 1957) were common in Florida in the late Pleistocene. During this time rejection behaviors may have evolved in some species in res ponse to heavy brood parasitism a nd may be retained in current populations in the absence of cowb irds. Indeed, at least one sp ecies endemic to Florida, the Florida Scrub Jay (Aphelocoma coerulescens ), is known to readily ejec t artificial cowbird eggs (Fleischer and Woolfenden 2004). Species kno wn as ejecters (e.g., Northern Mockingbird, Brown Thrasher) and deserters (e.g., Northern Cardinal) elsewhere within their breeding distribution are among the most abundant songbird species currently breeding in Florida. I tested the hypothesis that th e prevalence of egg rejection behaviors in Florida hosts contributes to limiting the reproduct ion of Brown-headed Cowbirds. I experimentally parasitized four common host species that are known to vary in the degree of their rejection response, but that commonly respond adaptively to parasitism in some part of their range. I predicted that parasitism rejection behaviors in populations of Florida songbird species would be comparable in frequency to populations exhibiting moderate to high rejection freque ncies in other parts of their 34

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range. If rejection behaviors ar e generally less prevalent or ab sent in Florida study species, reflecting a typical evolutionary lag in adaptive responses to cowbird expansion, factors other than effective host rejection of parasitism are more likely to be contributing to limited Brownheaded Cowbird reproduction in Florida. While most studies of egg ejecti on have used artificial cowbird eggs to examine host responses (e.g., Rothstein 1975b, Baltz and Burhans 1998, Robert and Sorci 1999), nests in this experiment were parasitized with actual cowbird eggs obtained from a captive colony raised for this purpose. Th e goal was to simulate parasitism as accurately as possible and avoid potential complications inhere nt in the use of artificial eggs (Rothstein 1976, 1977, Martin-Vivaldi et al. 2002). Methods Study Design In September 2004, I captured 40 hatch-year cowbirds (20 male, 20 female) in Alachua County, FL using baited drop-dow n decoy traps. From March August 2005 and 2006, 10 pairs were separated for breeding in individual out door pens (4 x 2 x 2m) at the USDA National Wildlife Research Center, Florida Field Station in Gainesville, FL under an approved animal use protocol (IACUC D539). Birds were fed a specialty diet that me t caloric and dietary needs for reproduction (Holford and Roby 1993). Each pen contained two or more open-cup or domed artificial nests and an occasional inactive Northern Cardinal nest. Some ar tificial nests contained a single wooden artificial egg that was similar in size, appearance, and weight to the egg of a Red-eyed Vireo. To improve the probability th at a captive female cowbird would lay eggs, cowbird pairs were housed with Zebra Finches (Taeniopygia guttata ) and Society Finches ( Lonchura striata domestica ). These finches bred readily in captivity and provided stimuli associated with active bird nests (e.g., nest-bu ilding, incubation behavior, warm eggs). Pens were monitored daily for the presence of cowbird eggs. 35

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From 28 April to 14 July, 2004 and 2005, eggs obt ained from captive birds were used to experimentally parasitize four primary species that are abundan t during the breeding season in Florida. Nests were searched for daily in a variety of habitats at sites within 25 km of Gainesville, FL (Chapter 1). Sites included nature preserves (Ordway-Swisher, Bolen Bluff, Split Rock), wetlands (Newnans Lake) and numer ous Gainesville city parks and neighborhoods. A single experimental cowbird egg was placed in each host nest between the egg-laying stage and 2 3 days following initiation of incubation as determined by candling of eggs (Lokemoen and Koford 1996). In one case, a Northern Cardinal nest received an experimental egg before the female laid her first egg. Eggs were warmed slightly in the hand prior to being placed in the nest. To mimic the common removal of a host e gg by a female cowbird (Nolan 1978), a single host egg was often removed (75% of cases) from nests containing at leas t two host eggs. Nests were visited daily and host re sponses were categorized as r ejected or accepted based on Rothstein (1975b). Two types of rejection behavior were distinguished. Eggs were considered ejected if the cowbird egg or fragments were found outside the nest, or if the egg was missing with no apparent decrease in numbers of host eggs Nests were considered abandoned if eggs were cold and nests undisturbed with no sign of nest activity for three consecutive days. If no change occurred on subsequent days and the ne st remained active, the egg was considered accepted. To assure that cowbird eggs were missing due to ejection and not partial nest predation, I excluded experimental nests that showed partial or complete host egg loss within two days following apparent cowbird egg ejection. Selection of Study Species Study species chosen are known to respond to cowbird parasitism through ejection or nest abandonment behaviors in other regions: Brow n Thrasher, Northern Mockingbird, Northern 36

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Cardinal, and Red-winged Blackbird. Study species were categorized as either ejecters or accepters based on criteria in Peer et al. (2002) using ejection rate s of experimentally parasitized nests in populations with at le ast recent exposure to cowbird pa rasitism. Brown Thrashers were considered ejecters (75 100% ejection rate) whereas Nort hern Cardinal and Red-winged Blackbirds were categorized as accepters (0 20% ejection rate). Northern Mockingbirds were categorized as intermediate ejecters (21 74%) because ejection rates vary significantly among populations. Nests of these species were the focu s of search efforts in this experiment and represented an array of generally high to low pa rasite rejection rates al lowing determination of relative rejection rates in the study. Observed frequencies of rejection and acceptance behaviors for each study species were compared to expect ed frequencies based on artificial parasitism studies in other regions. Expected responses fr om multiple studies were compared to observed responses using two-tailed Fishers Exact Probabi lity Test. Expected responses (number of cases of rejection/acceptance) were 62/28 for Brown Th rasher (Rothstein 1975b, Haas and Haas 1998) 29/26 for Northern Mockingbird (Rothstein 197 5b, Friedmann and Kiff 1985, Peer et al. 2002). For Red-winged Blackbird and Northern Cardin al, I derived two expected frequencies, a conservative estimate and one incorporating the highest frequencies reported for each species (Graham 1988, Clotfelter and Yasukawa 1999). Cons ervative expected fr equencies were 16/143 for Red-winged Blackbird (Rothstein 1975b, Orte ga 1991, Prather and Cruz 2006), and 4/60 for Northern Cardinal (Rothstein 1975b, Eckerle an d Breitwisch 1997, Whitehead et al. 2002). High expected values were 97/284 for Red-winged Blac kbird and 33/85 for Northern Cardinal. If rejection behaviors contribute to limiting cowbird reproduction in Florida, I predicted that observed rejection frequencies for each species woul d be similar or higher in Florida populations than in representative populations from other regions. If observe d frequencies are significantly 37

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lower than expected, the hypothesis will be rejected indicating that rejection of parasitism does not explain low observed parasitism of hosts in Florida. Results Ten pairs of captive cowbirds produced 34 eggs in 2005 and 122 eggs in 2006 (mean mass=2.88g, range 1.16 3.54g). At least some experimental eggs were viable because three eggs that were accepted into nests subsequently hatched, but 47% (27 of 58) of accepted eggs did not hatch. All cowbirds from hatched eggs were de predated in the nest by natural predators. All other experimental eggs were ejected by hosts, ab andoned in nests, or depredated. For a limited number of Brown Thrasher nests (N=9), I made observations of behavi ors following artificial parasitism. Egg ejection always occurred within twenty minutes after the return of the female, and some females that eventually ejected the egg initially sat on the parasitized clutch. Eggs obtained from the cowbird colony were us ed to artificially para sitize 75 nests of the four selected cowbird hosts (T able 2-1). Brown Thrasher wa s the only species tested that consistently rejected cowbird pa rasitism of their nests while al l other species typically accepted cowbird eggs. Northern Mockingbirds, Northern Cardinals and Red-winged Blackbirds accepted cowbird eggs in 85%, 95% and 100% of cases, re spectively. Observed rejection frequencies were higher than expected based on the literatu re for Brown Thrasher, and lower for the other three study species. Northern Mockingbird show ed significantly different rejection frequencies than expected (P<0.01; Table 2-1). Rejecti on frequencies for Northern Cardinals and Redwinged Blackbirds were not signifi cantly different than conservative expected values, but were significantly lower than high expected values (P<0.05). 38

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Discussion Rejection Responses of Florida Hosts The hypothesis that egg rejection behavior s of Florida hosts explain low observed parasitism frequencies was not supported by the data. Results show a low prevalence of egg rejection in three of four comm on cowbird hosts that were experi mentally parasitized. Northern Mockingbird, Northern Cardinal and Red-winged Blackbird accepted cowbird eggs placed in their nests in the overwhelming majority (93%) of cases. Results are consistent with similarly high acceptance rates of artificial cowbird eggs found for Red-winged Blackbirds in southern Florida (Prather and Cruz 2006). Northern Cardinals breeding el sewhere in the southeast have been shown to abandon 50% of naturally parasiti zed nests (N=4; Whitehead et al. 2002). This pattern was not observed in north-central Florida where cardinals accepted eggs in 95% of nests experimentally parasitized, many of which were parasitized before completion of the clutch. In the one case in which a lone experimental egg was placed in a cardinal nest before host eggs were laid, the host female still laid an entire clutch in the nest. Surprisingly, Northern Mockingbirds ejected very few experimental eg gs, accepting parasitic eggs in most cases. Mockingbirds breeding elsewhere s how intermediate responses with frequencies of cowbird egg ejection ranging from 25 100% (Rothstein 1975b, Post et al. 1990, Peer et al. 2002). Frequency of egg ejection by Northern Mockingbirds in this experiment is the lowest recorded for this species. Therefore, while all four study species display rejection and ab andonment behaviors in other parts of their distributions three of four study species do not appear to have recently developed or retained these be haviors in Florida populations. Brown Thrashers have only recent exposure to cowbird parasitism in Florida but ejected nearly all experimental cowbird eggs and abando ned one parasitized nest. Other studies have shown a high prevalence of ejection behavi ors in populations no l onger exposed to brood 39

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parasitism (Rothstein 2001, Peer and Sealy 2004b, P eer et al. 2007). For ex ample, Gray Catbirds ( Dumetella carolinensis ) introduced to Bermuda retain ejecti on behaviors despite an absence of brood parasites there (Rothstein 2001). There are three possible explanations for observed high ejection frequency of experimental cowbird eggs by thrashers in this experiment. First, Rothstein (1975a) estimated that it would take 20 100 years for some parasitized populations to switch from primarily accepting to ejecting cowbird eggs. As the overall intensity of parasitism on the host community in north-central Florida is probably not significan t (Chapter 1), rapid evolution of rejection in Brow n Thrashers is unlikely. Secon d, Florida populations may be receiving genes for egg rejection from areas with longer exposure to cowbirds (Rothstein 1990). All four study species, however, have sedent ary populations and are year-long residents in Florida. Sedentary populations have been show n to have significantly lower gene flow than migratory populations (Arguedas and Parker 2000) suggesting gene flow of rejection alleles is unlikely for Florida populations. Finally, some species may retain rejection behaviors for extremely long periods of time because the reje ction response is select ively neutral in the absence of parasitism (Rothstein 1975a). For example, Boat-tailed Grackles ( Quiscalus major ) may have retained ejection behaviors for as long as 800,000 years without exposure to brood parasitism (Peer and Sealy 2004b). Fossil eviden ce suggests that all four study species were sympatric with cowbird species in central Florida at least 10,0 00 years ago (Hamon 1964, Emslie 1998). During this time rejection behaviors may have evolved in all of the study species but were retained only by Brown Thrashers when co wbird species went extinct or changed their geographic distribution. 40

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Acceptance of Cowbird Eggs Northern Mockingbirds, Northern Cardin als, and Red-winged Blackbirds typically accepted experimental parasitism of their nests in this experi ment. These species may have accepted cowbird eggs because they were unable to discriminate cowbird eggs from their own (Rothstein 1982). Northern Cardinals eggs are similar in size, color, and patterning to Brownheaded Cowbird eggs whereas the other study speci es have eggs that differ significantly in appearance. However, the degree of dissimilarity between host and parasite eggs would not be important in populations lacking th e ability to discriminate eggs In these populations, any egg recognition errors may result in hosts damaging or ejecting their own egg when attempting to remove cowbird eggs (Rothstein 1977). Theref ore, costs associated with host egg loss or abandonment of unparasitized nests would outweigh the costs associat ed with retain ing rejection behaviors, and would select for acceptance of eggs in populations with absent (Rskaft et al. 2006) or limited parasitism pressure, as in Florida. Many species may accept parasitism because they have ratios of bill length to egg-width too small to eject cowbird eggs by grasping them (Rothstein 1 975b). Some smaller species are known to eject cowbird eggs by puncturing (Roths tein 1976, 1977, Sealy 1996) but this behavior is not widespread. Perhaps the unusual strengt h of cowbird eggs (Picman 1989) makes them difficult to puncture, resulting in incidental damage to host eggs (Spaw and Rohwer 1987). The species experimentally parasitized in this experiment are all simila r in size or la rger than Brownheaded Cowbirds with bills large enough to grasp cowbird eggs. Therefore, study species did not accept eggs simply because of morphological constraints. Ind eed, Rothstein (1975b) artificially parasitized four accep ter species with miniature cowbir d eggs capable of being graspejected, but eggs were still accepted in 87% of nests. However, many of the species that constitute the host community in north-central Florida are smaller species (e.g., parulids, 41

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vireonids) that may accept parasitism due to inab ility to remove larger cowbird eggs, and may instead rely on other behavioral defenses to cowbird parasiti sm. For example, early nest attentiveness (Neudorf and Sealy 1994) and active nest defense (Gill and Sealy 1996) by Yellow Warblers may deter female cowbird intruders. Acceptance of cowbird eggs may have occurred in the study species because cues associated with a female cowbird intruder were not simulated. For example, Meadow Pipits ( Anthus pratensis) eject experimental European Cuckoo ( Cuculus canorus) eggs more frequently when a model cuckoo is placed at the nest (Moksn es et al. 1991). However, other studies have found no correlation between parasite presence a nd rejection behaviors (Moksnes and Roskaft 1988, Sealy 1995, Soler et al. 2000). In this experiment, lack of re sponse to cowbird eggs in the nest was likely not due to the host missing the cue of female cowbird presence at the nest. Responses occurring before cowbird eggs are pla ced in the nest, however may be important to reducing overall cowbird parasitism. Red-winged Blackbirds, for example, often mob cowbirds and chase them from nesting colonies (Robert son and Norman 1976). Density of nesting Redwinged Blackbirds has been negatively correlated with parasitism rate suggesting that group defense may be an important cowbird deterrent (Freeman et al. 1990). Northern Cardinals and Northern Mockingbirds are also known to practi ce aggressive behaviors toward many types of nest intruders (Breitwisch 1988, Nealen and Brei twisch 1997), which may limit the number of eggs that female cowbirds are able to place in their nests. The two species known to commonly abandon para sitized nests, Northern Cardinal and Red-winged Blackbird, did not do so in this experiment. Ne st abandonment was not observed perhaps because it is not strictly a response to cowbird parasitism in some populations. For example, adults often only abandon nests if too many host eggs are removed (Rothstein 1982, 42

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Rothstein 1986). Furthermore, Least Bells Vireos only abandone d nests when removal of a host egg took place one day before it was replaced with a cowbird egg (Kosciuch et al. 2006). These studies suggest that adults may choose to desert a nest in response to the selection pressure of nest predation, not necessarily brood parasitism. During experiment al parasitism of nests in this study, a single host egg was remove d and replaced by a cowbird egg only in cases when two or more host eggs were present. Therefore, it is not known if Florida bird s would abandon nests as a result of partial clutch lo ss instead of addition of cowbir d eggs. Individuals in other populations of Northern Cardinal and Red-winged Blackbird aba ndon naturally parasitized nests at significantly higher rates than unparasitized nests (Graham 1988, Ortega 1991, Clotfelter and Yasukawa 1999, Whitehead et al. 20 02). Florida populations in this study appear to lack the ability to abandon nests strictly in response to brood para sitism, but may employ abandonment behavior in response to other pressures. Conclusions In several species, loss of reject ion behavior has been demonstr ated in populations that are no longer exposed to parasitism. For example, American Robins ( Turdus migratorius ) eject cowbird eggs only in areas where Brown-headed Cowb irds have historically bred (Briskie et al. 1992). Cruz and Wiley (1989) showed that the prevalence of egg ejectio n in Village Weavers ( Ploceus cucullatus ) declined significantly in a population introduced to Hispaniola where the selective pressure of brood parasi tism was absent. Interestingly, ejection frequencies in the same population increased from 14% to 83% in 16 years, perhaps due to novel parasitism pressures exerted by colonizing Shiny Cowbirds (Robert an d Sorci 1999). In Florida, I observed low rejection frequencies for three of the four st udy species and yet also observed relatively low natural parasitism frequencies of each species and the entire host community, overall. Results 43

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44 reported here suggest that sel ective pressure currently imposed by Brown-headed Cowbirds on ejection behaviors of certain hosts in Florida is likely weak. I assumed that the prevalence of rejection be haviors in hosts commonl y known to practice them would be representative of the community of hosts in north-central Florida. However, cowbirds are known to show preferences for spec ific hosts (Woolfenden et al. 2003, Ellison et al. 2006) on which they may exert more intense se lective pressure for development of antiparasitism behaviors. Indeed, observations of family groups feeding fledglings indicate that species such as Northern Parula and Blue-gra y Gnatcatcher may be preferred hosts in northcentral Florida (Chapter 1). Small species such as these that are incapable of ejecting eggs may instead rely on abandonment behaviors or active de fense. Indeed, some species show a positive relationship between nest defense and degr ee of parasitism (Robertson and Norman 1976, Strausberger and Burhans 2001). While results of this study suggest that rejection responses of some species do not explain parasitism frequencies in north-central Florida, behavioral responses of preferred host species ma y be more important to limiting Brown-headed Cowbird reproduction in the region.

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45 Table 2-1. Results of experimental parasi tism of four common Florida species. All species are known to eject cowbird eggs or abandon parasitized nests. Brown Thrasher s consistently ejected cowbird eggs whereas the other species tested generally accepted cowbird eggs experimentally placed in their nests. P-values indicate results of Fishers Exact Tests comparing observed frequency of rejection behavior with expected values for each species derived from the literature. For Redwinged Blackbird and Northern Cardinal, pvalues represent comparisons with both co nservative and high expected values. Asterisks (*) indicate comparisons in whic h observed rejection frequencies were sign ificantly lower than expected for each species. Species N Accepted Ejected Abandoned % Rejected P-value Brown Thrasher 17 2 14 1 88.2 0.1433 Northern Mockingbird 20 17 3 0 15.0 0.0038* Red-winged Blackbird 19 19 0 0 0.0 1.00, 0.0058* Northern Cardinal 19 18 1 0 5.3 0.2433, 0.0427*

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CHAPTER 3 DOES VARIATION IN FEMALE COWBIRD FE CUNDITY EXPLAIN OBSERVED LOW LEVELS OF PARASITIZATION? Introduction Many ultimate morphological, physiological, and ecological factors are thought to determine reproductive life-histor y traits such as clutch si ze (Moreau 1944, Skutch 1949, Lack 1954, Ashmole 1963, Cody 1966). Intraspecific clutch size increases with latitude (Koenig 1984, Jarvinen 1989, Dunn et al. 2000) and long itude (Johnston 1954, Bell 1996), and decreases with elevation (Badyaev a nd Ghalambor 2001, Johnson et al. 2006). These gross geographic patterns are altered by regionally sp ecific proximate factors. For example, fecundity of Spotted Owls ( Strix occidentalis ) was negatively affected by weather patterns prior to and during the breeding season (LaHaye et al. 2004), perhaps b ecause decreased precipitation and temperature altered the abundance of food or water resources. Under experimental conditions, breeding birds given supplemental water (Coe and Rotenberry 2003) or food (Arcese and Smith 1988, Boutin 1990, Nager et al. 1997) will lay larger clutches than birds in control groups. Food availability and other nutritional factors, therefore, may underlie regi onal variation in reproductive parameters such as clutch size or fecundity. Because both additional energy and specific nutri ents are required for egg production, diet quality and availability of suitable foods may be equally or more important than food abundance in determining fecundity, clut ch size and overall reproductive output. Reproduction of captive Zebra Finches given food resources ad libitum for example, was constrained more by nutrient reserves, energy expenditure (D eerenberg et al. 1996), and diet quality (Semel and Sherman 1991) than by food abundance. Calcium, in particul ar, is not available in quantities sufficient for egg production in many environments (Barclay 1994, Graveland and van Gijzen 1994) and birds often supplement calcium intake during th e breeding season (Simkiss 1975, St. Louis and 46

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Breebaart 1991, Graveland 1996). Indeed, Great Tits ( Parus major) supplemented with calcium increased clutch sizes (Tilgar et al. 1999). Songbirds breeding in areas with limited calcium have reduced reproductive output (Graveland 19 90, Graveland et al. 1994, Graveland and Drent 1997). Because the abundance of exchangeable calcium in the soil shows extensive geographic variation, reproductive indices such as clutch size or fecundity may be correlated with calcium availability over large spa tial scales (Patten 2007). Brown-headed Cowbirds have the highest known fecundity of any passerine in North America (Smith and Rothstein 2000). Females can produce as many as 77 eggs in 89 days of breeding (Holford and Roby 1993). This high rate of egg production is made possible because the Brown-headed Cowbird is the only known s ongbird species that lacks ovarian regression following clutch completion (Lewis 1975) and can start new clutches in as lit tle as one day (Scott and Ankney 1980). Brown-headed Cowbird fec undity (number of eggs laid per breeding season), however, shows dramatic geographic variation. Female cowbirds, for example, are estimated to lay 2 8 eggs per breeding season in New York, 10 12 eggs in Michigan (Payne 1965, 1976), 24 30 eggs in Oklahoma and California (Payne 1965, 1973a), and 40 eggs in southeastern Ontario (Scott and Ankney 1980). This extreme intraspecific variability in fecundity suggests that the impact s of cowbird parasitism on hosts may also be highly variable. In this chapter, I explore the possibility that limited reproduction of cowbirds may underlie low observed parasitism rates. Limitations to Cowbird Reproduction Various nutritional factors have been impli cated in regional differences in reproductive values of Brown-headed Cowbirds over their breeding distribution. Cowbirds exhibit high fecundity in areas where grai n-based agriculture is widesp read (e.g., California) probably 47

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because food sources are accessible and abundant. Where food resources are not abundant or are sparsely distributed, food acquisition may increase reproductive costs. For example, egg-laying rate of Brown-headed Cowbirds was significantly lower in birds that commuted long rather than short distances between feeding and breeding areas (Curson and Math ews 2003). Differences between West Indian islands in abundance of Shi ny Cowbirds are attributed to availability of food rather than hosts (Post et al. 1990). Finally, as the most fecund North American songbird species, Brown-headed Cowbirds require large amounts of calcium for egg synthesis. As a result, cowbirds are known to supplement calcium through consumption of removed host eggs (Scott et al. 1992) and a variety of other ca lcareous sources (Ankney and Scott 1980). Any variation in supplemental calcium availability can thus explain regional di fferences in fecundity, as well as observed parasitism rates (Holford and Roby 1993). Furthermore, as nest parasites cowbirds may be uniquely limited by the abundance of nests of suitable hosts. Cowbirds in northern Michigan do not breed until approximately a month la ter than those in southern Michigan, because suitable hosts have not yet begun breeding (Payne 1965). Low Nest Parasitism in Flor ida: Low Cowbird Fecundity? Despite relatively high breeding season abunda nce of Brown-headed Cowbirds in northcentral Florida (3 10 birds per Breeding Bird Survey Route; Sauer et al. 2006), observed parasitism frequency is unusua lly low; only 1.6% of nests of 29 potential host species were parasitized (Chapter 1). Rates of parasitism on specific host species are also lower than in other regions, even those that cowbirds have recently invaded. Parasitism rates on Northern Cardinals ( Cardinalis cardinalis ), for example, are 20.1% in Onta rio (Peck and James 1998b), 7.5% in Texas (Barber and Martin 1997), 5.4% in South Carolina (Whitehead et al. 2000), and only 2.3% in north-central Florida. 48

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To help explain low observed parasitism frequencies, I tested the hypothesis that reproductive output of Brown-headed Cowbirds was limited in north-central Florida by low fecundity. To determine differences in reproduc tive parameters that may influence patterns of parasitism, I analyzed female reproductive organs to estimate fecundity and clutch size using a methodology that has been performed for a va riety of species (Hannon 1981, Kennedy et al. 1989, Arnold et al. 1997, Pearson and Rohwer 1998, Lindstrom et al. 2006), including Brownheaded Cowbirds (Payne 1965, 1973a, 1976, Scott and Ankney 1980, 1983). Reproductive condition of female cowbirds from Florida was co mpared with cowbirds from Fort Hood, Texas, an area with high rates of cowbird parasitism (e .g., Barber and Martin 19 97). I predicted that cowbird fecundity, in particular, would be lower in Florida than in Texas but did not specifically test any mechanism that would explain observed differences. Methods Cowbird Collection Brown-headed Cowbirds were collected at sites in both nor th-central Florida and eastcentral Texas (Figure 3-1). Sites were located at roughly similar latitude to control for potential biases. Cowbirds breeding in Florida are the subspecies M. ater ater which tend to be larger (Lowther 1993) than those breeding in Texas, wh ich are typically the dwarf cowbird subspecies M. a. obscurus (Summers et al. 2006). Texas specimens were trapped or shot at Fort Hood in Bell and Coryell Counties in central Texas (ca. 31 N, 97 W) between April and July, 2005 by the Nature Conservancy according to prot ocol used by Summers et al. (2006). Cowbird control has been conducted at Fort Hood since 1988 (Eckrich et al. 1999) due to negative impacts of parasitism on native br eeding birds. Cowbirds were either captured using a series of baited decoy traps typically placed near free-ranging cattle or shot from within 30 m using a 20gauge shotgun, full choke, number 9 lead shot an d 2-3/4 inch shotshells Birds were shot 49

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between 0800 1100 CST when foraging in groups on the ground. Potential local breeders were distinguished from migrant females based on morphometric, phenotypic and physiological characters (Summers et al. 2006). Females were captured in Florida from April July of 2006 and 2007 in baited drop-down decoy traps. Traps were baited daily with F-R-M Game Bird Starter (Flint River Mills, Bainbridge, GA) placed on a mesh tray below trap openings. In 2006, two trap sizes were used. Large traps (2.4 x 2.4 x 1.8 m) owned by the USDA National Wildlife Research Center, Florida Field Station were constructed of wood and wire mesh. Medium-sized traps (1.7 x 1.4 x 1.4 m) were based on the pick-up trap design of the Texas Parks and Wildlife Department (2008) and were primarily constructed of one-inch hardware cloth. Small traps (1.45 x 1.2 x 1.2 m) of similar design were used in 2007. Live cowbird decoys placed in traps typically included one female and one or more male cowbirds collect ed in September 2004 and housed at the USDA Wildlife Research Center in Gainesville, FL. Traps were checked daily from 0900 1100 or 1400 1600 EST and females were euthanized huma nely on site via cervical dislocation according to IACUC-approved protocol. Trapping e ffort ranged from one to eight open traps per day at up to three sites. In 2006, trapping was co nducted at the University of Florida (UF) Dairy Research Unit (DRU), Beef Research Unit (BRU ), Horse Teaching Unit, and Santa Fe Beef Research Unit in Alachua County, FL. In 2007, trapping was conducted at the same sites in addition to the UF Large Animal Clinic and Beef Teaching Unit (BTU). With the onset of the breeding season in May, the number of female cowbirds that can be trapped often drops significantly (Beezley and Rieger 1987, Summers et al. 2006). Shooting of females in feeding areas allows a steady collection of specimens throughout the breeding season (Summers et al. 2006). Trapping of cowbirds may also be biased toward individuals of poorer 50

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condition (Dufour and Weatherhead 1991). Therefore, trapping was supplemented in Florida in 2007 with collection of individual birds us ing pellets and a Crosman 664 Powermaster pneumatic air rifle. Birds were shot oppor tunistically between 1400 and 1800 EST at DRU, BRU, and BTU sites in or near fiel ds where they foraged with cattle. I used playback of female chatter calls in some cases to improve shooting efficiency as recommended by Rothstein et al. (1987). All collected birds were la beled and frozen until analyzed. Reproductive Assays All females collected were analyzed at the Florida Museum of Natural History in Gainesville, FL. Breeding condition of fema les was assessed using pre-ovulatory and postovulatory follicles (POFs; Scott and Ankney 1983). Pre-ovulatory follicles represent developing ova that are subsequently released into the a bdominal cavity and taken in to the oviduct for egg production. After ovulation, POFs remain attached to the ovary and are reabsorbed over time. The condition of the oviduct was examined for pr esence of an oviducal egg or signs that an egg had passed (straight vs. convoluted). Ovaries were examined with a dissection probe, #5 tweezers, and Bausch & Lomb Stereo Zoom 4 (7 30x magnification) dissecting microscope because macroscopic counts of follicles may be unreliable (Arnold et al. 1997). I recorded the number and size of pre ovulatory follicles (ones that could potentially produce the yolk of an egg) in each specimen. The widths of pre-ovulatory follicles were measured to the nearest 0.01mm using a Mitutoyo Absolute Digimatic caliper (model CD6:CS). Round, orange-yellow pre-ovulatory follicles were distinguished from degenerating or atretic follicles which are irregular in shape and cream-colored. Pre-ovulatory follicles generally grow quickly and sequences of developing follicles may represent eggs of the same clutch. K-means hierarchical cluster analysis (maximum 10 iterations) using SPSS (2003) was used to group 51

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follicles into categories A B or C based on size (sensu Payne 1965, Scott 1978) to identify distinguishable sequence breaks re presenting separate clutches. A follicles are those that would likely have ovulated on the ne xt day following collection, B follicles two days from collection and so on. Although smaller pre-ovulatory follicles (e.g., D follicles) were sometimes present in birds in the sample, some follicles are known to regress before ovulation (Payne 1976). However, Scott (1978) found only 3 cases of atresi a in 69 follicles >5mm indicating that larger follicles (i.e., A and B ) are unlikely to regress before releas ing the yolk. Therefore, analysis was limited to three clusters to improve the probability of including only follicles that would reach ovulation. I also recorded the number and size of all recognizable POFs which are flat and ovalshaped and have an open slit (stigma) through whic h ova are released into the abdominal cavity during ovulation. Only those follicle s with ruptured stigma were considered POFs. POFs were distinguished from burst atre tic follicles (Pearson and Rohw er 1998) by examining color and other differentiating characteristics (Scott and Ankne y 1983). Burst atretic follicles are ruptured but typically contain a cream-color ed residue that is lacking in POFs. The area of each flattened POF was estimated as the product of width a nd maximum lengths measured to the nearest 0.01mm. Ovaries, oviducts, and oviducal eggs were removed and weighed to the nearest 0.0001g using a Mettler Toledo Analytical Balance (model AB54-S). Average start and end dates of the cowbird breeding season were determined separately for Texas and Florida based on presence of enlarged oviducts, large pre-ovulatory follicles (i.e., A follicles) and oviducal eggs. Collected birds found in reproductive condition with en larged oviducts (>0.7g), oviducal eggs, or the presence of one or more large pre-ovulatory ( A follicles) or post-ovulatory follicles are 52

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breeding. All other bi rds are considered non-breeding birds. Breeding birds that were collected with a developing egg anywhere in the oviduct are called laying birds while all other breeding birds are called non-laying ( sensu Scott and Ankney 1980). Shapiro-Wilk tests of normality indicated that body mass, ovary mass, and oviduct mass variables from each collection location were norma lly distributed (P>0.15 for all). Therefore, measures of reproductive condition of breeding birds were compared between Florida and Texas, and laying and non-laying cowbir ds using two-tailed t-tests a ssuming unequal variances in SPSS (2003). Change in body mass over the course of the breeding season at ea ch collection location was analyzed using simple linear regression. Finally, as a resu lt of the change in size of reproductive organs with the beginning and e nd of the breeding season, I used regression analysis to test if mass of oviducts and ovaries of breeding birds differed between Florida and Texas. PROC GLM (SAS Institute Inc. 2003) with Type IV sum of squares was used to fit a general linear model to the data because it allows for both con tinuous and categorical predictor variables. The response variables in each of the two models were oviduct mass and ovary mass with collection week and location (Florida and Texas) as predictor variables. A quadratic regression function was applied to the week variable in both models due to the observed growth and regression curve of reproductive organs. Residuals plots were used to assure that there was a good fit between the model and the data. Clutch Size Estimates Average clutch size for Brown-headed Cowbirds has been estimated based on sequences of developing pre-ovulatory follicles and regressing post-ovulatory follicles (Payne 1965, Scott 1978). There are, however, some limitations to using this met hod. Some developing follicles may regress prior to ovulation (Payne 1976), rele ased ova may get reabsorbed before entering the oviduct (Meyer et al. 1947) and post-ovulatory follicles may be difficult to distinguish from 53

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burst atretic follicles (Payne 1965, Lindstrom et al. 2006). As a result, clutch size can be overestimated when relying on assumed follic le sequences (Davis 1942, Payne 1965, Scott 1978, Arnold et al. 1997). Clutch size can also be underestimated using post-ovulatory follicles if they are reabsorbed quickly (Arnold et al. 1997). For these reasons, I estimated clutch size by determining numbers of post-ovulatory follicles observed in breeding birds (Scott and Ankney 1983). The method used relies on two clutch size estimates based on numbers of post-ovulatory follicles in (1) laying birds and (2) non-laying birds. First, I estimated the total number of eggs laying birds would have laid in their current clutch. This number was derived by finding the total number of clearly-i dentifiable POFs in the sample population of laying birds, which should represent the averag e midpoint of laying sequences. That is, a sample of 10 laying cowbirds with an average clutch size of 5 would lay 50 eg gs and develop 50 POFs over the laying period. At the midpoint of their sequences they should have 30 POFs because th e third POF represents the midpoint of a 5-egg clutch. Thus, the midpoint number of POFs can be used to determine the total number of eggs the sample would have laid by multiplying it by 2, and subtracting the sample size (i.e., (30 x 2)=50). Because size breaks in regressing follicle sequences indicate distinct clutches (Payne 1976, Ha nnon 1981), I only included POFs that were clearly in the same laying sequence. Second, clutch size of non-laying birds was estimated. Non-laying birds do not have oviducal eggs by definition, so they are likely in between clutches. Therefore, recent clutch sizes were estimated based on total number of POFs di vided by the sample size. In some cases, nonlaying birds did not have clearly id entifiable POFs so were assumed to have laid a clutch earlier. This assumption is supported by the observation that all non-laying birds showed evidence of 54

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egg production (e.g., enlarged and convoluted oviducts ). Because it is assumed that these birds had the same frequency distributi on of clutch sizes, the contribu tion of these birds was estimated as the same proportion of POFs recorded in all other non-laying birds. For example, if a sample of 50 non-laying birds included 40 birds with 60 POFs (1.5 POFs per bird) the remaining 10 individuals without identifiable POFs would be estimated to have 15 POFs (10 birds x 1.5). To estimate average clutch size, the sum of (1) total number of eggs that la ying birds would have produced and (2) the estimated number of POFs of non-laying birds, we re divided by the total number of birds in the sample. Estimates we re performed separately for Florida and Texas samples. Inter-clutch interval (number of days between distinct clutches) was estimated using the proportion of non-laying birds in a particular portion of the n on-laying period (Scott and Ankney 1979). For example, some birds without an oviducal egg may have finished laying their clutch on the morning before collection. These birds would have large POFs that are less than a day old indicating ovulation occurred on the day prior to collection. This w ould represent the first day of the non-laying period for these females. Similarly, large pre-ovulatory follicles ( A follicles) in a cowbird without an egg in the oviduct would re present the end of the non-laying interval since ovulation was likely to occur on the following da y. The reciprocal of the proportion of nonlaying birds in each of these stages should represen t the inter-clutch interval. For example, in a sample of non-laying birds with an inter-clutch interval of two days, roughly one-half of the birds should be in one of the two non-laying days. These two non-laying states were used to estimate the non-laying interval in Fl orida and Texas female cowbirds. Fecundity Estimates Fecundity in this study is defined as the numbe r of eggs that an average female cowbird lays over a breeding season of average length. Average annual cowbird fecundity in each study 55

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area was determined as the product of daily fec undity (laying rate) and duration of the cowbird breeding season (Scott and Ankney 1980). Daily f ecundity was calculated using the proportions of breeding birds collect ed that had an egg in the oviduct (Payne 1973b). Average dates of the breeding season in Texas and Florida were es timated as the first and last days when approximately half of birds had enlarged oviducts, large pre-ovulatory follicles or oviducal eggs. Only breeding birds that were collected between respective breeding season dates in Texas and Florida were included in fecundity estimates. I assumed that all bi rds had an equal probability of being collected regardless of whether or not they were carrying a developing egg. Scott & Ankney (1979) found that the proportion of collecte d birds carrying an egg was not affected by variables such as collec tion location, method, or ti me of day, among others. Results Cowbird Collection A total 142 female cowbirds were collected from Texas between 22 March and 6 July, 2005. The majority (N=94) of these specimens we re collected by shooting. A total of 71 and 63 female cowbirds were collected in Flor ida between 23 April and 12 July, 2006 and 2007, respectively (Figure 3-2). Total effort (traps open x days open) was 146 trap days in 2006 and 463 in 2007. In general, more females were caugh t per trap day in large traps (0.415) than in small and medium traps combined (0.061; Figure 3-3). All 2006 specimens were collected in traps, the majority (59%) of which was trapped in July when a higher proportion of individuals was no longer in breeding condition. Trapping succe ss was low from April to May in both years but remained low in June only in 2007 (Figure 3-3). Trapping success increased from May to June in 2006 but the majority of these birds (74%) was collected after June 21 when some birds were in declining or non-reprodu ctive condition (see below). Shooting of individual female cowbirds in 2007 greatly increased sample si zes during May and June (Figure 3-4). 56

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Body mass of breeding Florida cowbirds was significantly lower in trapped than shot birds (35.2 vs. 38.5g, t=-4.581, P<0.001) but there was no difference when mass of oviducal eggs was removed (34.3 vs. 31.6g, t=1.115, P=0.272). Ov aries (0.3250 vs. 0.5217g; t=-0.3838, P<0.001) and oviducts (1.0253 vs. 1.4126g, t=-4.495, P<0.001) were also significantly lighter in trapped birds. However, wing chord (t=0.013, P0.990) and tail length (t=-0.767, P=0.448) were similar in trapped and shot birds in Florida. Among 20 Florida cowbir ds collected by shooting during the breeding season, 17 had an oviducal egg. Furt hermore, laying birds th at were shot were more likely to have an oviducal egg th an trapped birds (Log Odds Ratio=2.76, CI=3.63 68.41). Reproductive Assays Reproductive assays were conducted on all fema le cowbird specimens collected. Of 142 Texas females collected, 79 had enlarged oviducts POFs, large pre-ovulatory follicles, or oviducal eggs and were categorized as breeding birds. Of breedi ng birds, 40 were found with an egg in the oviduct and were called laying birds, and the remaining 39 were categorized as nonlaying birds. 134 specimens were collected in Florida and 56 were calle d breeding birds. Of Florida breeding birds, 29 were laying birds and 27 were non-laying birds. It was not possible to test for any effect of year on reproductive measures due to small sample sizes for part of the breeding season in Florida in 2006. However, there were no differences (independent samples t-test, P>0.20 for all) in the number of preand post-ovulatory follicles, body mass, or mass of oviducts and ovaries of samples collected during 1 June to 21 June in 2006 (N=8) or 2007 (N=16). These variab les were also not signif icantly different when samples from 2006 were paired with randomly-chos en samples from similar collection dates ( 2 days) in 2007 (paired samples t-test, P>0.30 for a ll, N=8). Therefore, all data presented for Florida birds represent pooled data over both collection years. 57

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Ovary measurements ( SE) of Texas cowbirds collected between 22 March and 6 April showed no signs of breeding season development (26.89 1.21mm 2 0.027 0.002g, N=40). Oviducts were similarly small a nd undeveloped during this time (0.043 0.009g, N=7). For both Florida and Texas females, growth and maturati on of ovarian follicles led to a dramatic (ca. 14-fold in Texas) increase in mean ovary mass in midto late-April (F igure 3-5). Similarly, oviducts of collected specimens in creased significantly (ca. 28-fol d in Texas) in midto late April (Figure 3-6). Mean ovary and oviduct mass re mained fairly consistent from May to early June, although there was much individual variatio n due to the dynamic nature of ovarian follicle growth and yolk release during ovula tion (Figures 3-7 to 3-10). On average, laying birds in Florida had significantly heavier ovaries (t=2.480, P=0.017) and oviducts (t=3.429, P=0.001) than non-laying bird s (Figures 3-7, 3-8). However, breeding birds without oviducal eggs still had large ov iducts (i.e., mean mass >1.0g), probably because they recently completed clutches or were prep aring to ovulate. Laying birds in Texas had significantly larger ovaries (t=2.091, P=0.041), but not oviducts (t=1.263, P=0.212) than nonlaying birds (Figures 3-9, 3-10). Ovaries and oviducts began to regress in the second week of June as resource allocation to reproduction was likely waning with the end of the cowbird breeding season. Breeding females from Texa s and Florida had similar ovary mass (t=0.805, P=0.422) over the course of the breeding season (Table 3-1) but oviducts were significantly heavier in Texas than Florida females (t=2.304, P=0.023). Because growth and regression of reproductive organs roughly followed a bell-shaped curve, I applied a gene ral linear model to the data to determine if oviduct and ovary mass di ffered between Florida a nd Texas cowbirds as a function of week of the breeding season. The regr ession model indicated th at collection location was not a significant predictor of ovary mass (parameter estimate=0.037, F=0.88, P=0.350; 58

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Figure 3-7). However, the model indicated that collection site was a significant predictor of oviduct mass (parameter estimate=-0.129, F=5.17, P=0.025) with Texas females having heavier oviducts than Florida females. Body mass of Florida females declined si gnificantly from 23 April until 12 July (R 2 =0.161, P<0.001, N=110; Figure 3-11). Mass of breeding bird s in Florida tended to decrease over time but not significantly (R 2 =0.024, P=0.263, N=53) especially when the mass of the oviducal egg was removed from body mass (R 2 =0.006, P=0.585). Mean mass of breeding females (R 2 <0.001, P=0.990, N=58) and all cowbirds collected in Texas between 19 April and 6 July (R 2 =0.001, P=0.826, N=79) did not change significantly wi th time (Figure 3-12). Similarly, no body mass change was observed over time when oviducal egg mass was removed from body mass of laying birds (R 2 =0.012, P=0.429). Overall body mass decline from April to July in Florida birds was likely due to mass differences between pre-breed ing season birds prepared for reproduction and post-breeding birds with regressi ng reproductive organs. That mass declined over the entire collection period in Florida but not Texas is due to a significant di fference in sample size of birds collected after 25 June (Florida=81, Texas=6) when most birds are no longer in breeding condition. Although the eastern su bspecies of cowbird is gene rally larger than the dwarf cowbird of Texas (Lowther 1993, Pyle 1997), mass breeding Florida and Texas females was not different (t=-1.113, P=0.268). Similarly, wing chords of Florida breeders also were not significantly different from those of Texas cowbir ds (Table 3-1) collected at Fort Hood during previous breeding seasons (Summers et al. 2006). Increase in ovarian mass was associated with the development of pre-ovulatory follicles in late April. Oocytes (immature ova) were gene rally ca. 1.0mm in diameter in March and early April whereas breeding birds had one or more large orange-yellow yolky follicles representing 59

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potential eggs. A follicles (those likely to ovulate the next day) were as large as 10.50mm in diameter in Texas and 9.89mm in Florida. Sequences of developing follicles were determined using cluster analysis. For Florida birds, A B and C follicles were those that were larger than 8.57, 5.85, and 3.39mm diameter, respectively. Simila rly, Texas birds cluster cutoff values for A C follicles were 8.72, 6.38 and 3.37mm diameter, respectively. These values are similar to those reported by Scott (1978) who found that th e three largest pre-ovulat ory follicles clustered at >8mm, and ca. 6 and 4mm. Earliest and latest A follicles were found in cowbirds collected on 26 April and 28 June in Texas and 26 April and 27 June in Florida. All but four breeding individuals had at least one post-ovul atory follicle (POFs), although birds without POFs were still clearly in reproductive condition. All laying birds had one large POF that was associated with the egg in their oviduct and which released the yolk one day prior to collection (Day -1). Laying birds had as many as four POFs that showed a graded series in size. Mean areas ( SE) of the sequence of POFs from larg est (Day -1) to smallest (Day -4) in laying birds in Florida were 22.89 0.72, 13.01 0.47, 7.80 0.44, and 6.57 1.50mm 2 respectively. In Texas cowbirds, mean areas of POFs in this sequence were 25.89 0.85, 25.42 0.73, 12.01 0.75, and 10.43 1.38mm 2 Day -2 POFs that were identified as likely being part of the same sequence showed ca. 40% decrease in size from the most recent follicle. This indicated that POFs regressed rapidly and that follicles older than four days would likely be difficult to identify. Furthermore, the three mo st recent POFs (Days -1, -2, and -3) of laying birds were significantly larger in Texas than in Florida (t-tes ts, P<0.01 for all; Figure 3-13). POFs from Days -1 and -2 were ca. 13% and 18. 5% smaller in laying Florida specimens, a result which may be caused by differences in the timi ng of specimen collection between the two study areas. The sample of Florida cowbirds included individuals collected throughout daylight hours 60

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whereas Texas cowbirds were all captured be fore 1100 hours. Ovula tion in Brown-headed Cowbirds occurs around 0700 hours (Scott and Ankney 1983) after which the follicle begins regressing. I was not able to speci fically test effect of time of co llection on size of POFs because appropriate data were not collect ed. However, all birds that we re shot were collected in the afternoon. Among laying birds collect ed in Florida, the largest POF was significantly smaller (t=2.653, P=0.017) in birds collected by shooting (N=18) versus trapping (N=10). This may explain observed differences betw een Florida and Texas in POF si ze. However, the total number of POFs counted in laying birds in Florida was the same regardless of collection method (t=3.760, P=0752) suggesting that all POFs that were present following ovula tion in the morning were recognizable in birds co llected later in the day. Clutch Size Estimates Clutch size and inter-clutch interval estimat es were determined using the sample of breeding birds from each collection location. In Texas, 40 laying females had between one and four readily identifiable POFs and most commonly three (Table 3-2). The mid-point of the laying sequence for these birds was estimated as 108 total POFs. Two non-laying birds were excluded from counts due to the poor condition of their ovaries. A total of 29 non-laying females had 66 recognizable POFs (2.3 POFs/b ird). The remaining eight non-laying birds without POFs were therefore estimated to cont ribute 18 POFs (8 birds x 2.3). Based on these figures, average clutch size of breeding birds in Texas is estimated as 3.38 eggs per female (Table 3-2). In Florida, one laying bird was excluded from clutch size estimates because the ovary was damaged when collected. A total of 28 laying females also had between one and four POFs, with a mode of three (Table 3-3). These fe males produced 83 POFs. Twenty-three non-laying birds had 46 distinct POFs (2.0 POFs/bird) so th at the remaining four birds without POFs were 61

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estimated to have eight POFs. Average clutch size of breeding birds in Florida is estimated as 3.49 eggs per female (Table 3-3). Inter-clutch intervals were estimated using two methods that use the proportion of nonlaying birds with (1) size A pre-ovulatory follicles and (2) POFs large enough to have ovulated an egg on the day before collection. In Texas, 13 of 37 non-laying birds had size A pre-ovulatory follicles that would likely have ovulated on the next day. Of 27 non-laying birds in Florida, 12 had size A ovulatory follicles. The reciprocals of thes e proportions results in estimates of interclutch interval of 2.85 and 2.25 for Texas and Florida, respectively. For method 2, I determined the size of the smalle st POF in all laying birds that could have released the egg found in the oviduc t. I assumed any non-laying bird with a POF larger than this size ovulated on the previous day. Since POFs re gress approximately 50% in 12 hours following ovulation (Scott and Ankney 1983), I used the smalle st POF of a laying bird collected in the afternoon to establish a cutoff size for POFs in non-laying birds. These values are 18.82mm 2 and 15.01mm 2 for Texas and Florida, resp ectively. Twenty of the 37 non-laying birds in Texas had POFs that indicated that the last day of ovulation was one day prior to collection. Of 27 nonlaying Florida birds, 14 had large POFs. The reci procals of these two pr oportions result in interclutch interval estimat es of 1.85 and 1.92 days for Texas a nd Florida, respec tively. Based on both estimation methods, the average non-laying pe riod of female Brown-headed Cowbirds is approximately 2 3 days in Texas and 2 days in Florida. In five captive Florida Brown-headed Cowbird females used to generate eggs for experimental parasitism research (Chapter 2), sequences of actu al egg-laying in artificial nests resulted in a mean estimate ( SE) of clutch size (3.68 0.57 eggs) and inter-clutch interval (1.80 0.23 days) similar to those estimated above for wild-breeding populations. 62

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Fecundity Estimates Average fecundity of cowbir ds was calculated based on th e method of Scott & Ankney (1980) using length of the breeding season and daily laying rate. Average start and end dates to the laying season were estimated using the pres ence of enlarged oviducts, large pre-ovulatory follicles and oviducal eggs. Of the eight Florid a birds collected from April 23 to 29, three had oviducal eggs and three of the five non-laying birds had large POFs and mid-sized ( B ) preovulatory follicles and were ther efore in breeding condition. Th e earliest bird that had an oviducal egg was collected on 26 April but had th ree distinct POFs i ndicating the ovulation probably began as early as 23 April. I was not able to collect an adequate sample of birds prior to this date. The earliest specimen was collect ed 23 April and had no POFs but some developing follicles. Therefore, I assume that the average female begins breeding during the final week of April. Therefore I estimate an av erage start date to the cowbird breeding season in Florida to be 26 April. Females were collected in Florid a as late as 17 July but the last female with an oviducal egg was collected on 6 July. This female is likely an outlier because only one of the remaining 17 females collected in the first week of July ha d post-ovulatory follicles that indicated laying of an egg could have occurred in early July. One of 46 birds collected after this date had a single small post-ovulatory follicle, indica ting that laying had likely ceased af ter the first week of July. To assign an average end date to the cowbir d breeding season in Flor ida, I estimated the last date when ca. 50% of the birds were in br eeding condition. From 25 June to 1 July, only one of 18 birds collected had an oviduc al egg. Of the remaining 17, six were likely still in breeding condition due to the presence of pre-ovulatory follicles larger than 5mm in ova ries with little or no atresia. Therefore, 38% of birds we re still in laying condition. From 18 24 June, four of 63

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seven birds (57%) were in breeding condition (one with an oviducal egg). Central dates for each of these weeks are 21 June and 28 June respectively. If it is assumed that the percentage of birds in breeding condition decreas es steadily between thes e dates, the first date when less than 50% of birds were in laying condition is estimate d to be 24 June (48.9% of birds). In Texas, birds were collected beginning in March and the first bird with an oviducal egg was collected on 26 April. The presence of two large POFs indicated th e ovulation could have begun as early as 24 April. However, of the other six birds collected during 23 29 April none had ovaries with developing follicles so were not in breeding condition. Because the sample from Texas does not include sufficient sample sizes of identified breeders from mid-late April to early May to deduce an average start date, I estima te a similar start date for Texas birds as those from Florida. Reproductive behaviors of Brownheaded Cowbirds begin as early as late March but few cowbirds are collected with an oviducal egg until mid-April (S. Summers, pers. comm.). Since the start date estimates when an averag e bird is laying (i.e., 50% of the population), 26 April represents at least a conservative estimate of the beginning of the breeding season in Texas. The latest oviducal egg-bearing female captu red at Fort Hood was collected 12 July, but for most years the latest is t ypically around 9 July (S. Summers pe rs. comm.). Consistent with these observations, the last Texas specimen analy zed that had an oviducal egg was collected on 6 July. From 25 June to 1 July, four of nine bird s collected had an oviducal egg. Of the remaining five, none were likely still in breeding conditi on because they lacked developing follicles. Therefore 44% of birds were sti ll in laying condition. From 18 24 June, 10 of 16 birds were in breeding condition, seven of which had an oviducal egg. Using ce ntral dates and the assumption of a steady decrease in breeding condition, the first date when le ss than 50% of birds were in laying condition is estimated to be 26 June (49.6% of birds). 64

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In sum, I estimate the average breeding season length as 60 days from 26 April to 24 June in Florida and 62 days from 26 April to 26 June in Texas. Both starting and ending dates are earlier than those reported for Brown-headed Co wbirds breeding in southern Ontario (Scott and Ankney 1980) but estimated breeding s easons for Florida and Texas are 4 6 days longer. Nine birds from Florida and seven birds from Texas that were in reproductive condition were excluded from estimates of daily laying rate because they were collected after the average breeding season end dates. Of excluded bird s, two Florida and fi ve Texas females had developing eggs in their oviducts Daily laying rate was estima ted as the proportion of breeding birds with eggs in the oviduct (Payne 1973b). In Texas, 40 of 71 br eeding birds collected between 26 April and 26 June had oviducal eggs resulting in an average daily laying rate of 0.56 eggs per female per day. In Florida, 27 of 47 breeding birds collected between 26 April and 24 June resulting in an average daily laying rate of 0.57 eggs. Small sample sizes precluded analysis, but daily laying rate showed some minor variation during the breeding season (Figure 3-14). Using average laying ra tes and the respectiv e lengths of the breeding season, it is estimated that each female Brown-headed Cowbird in both Texas and Florida lays an average of 34-35 eggs in a breeding season (TX: 62 days x 0.56 eggs/day = 34.93, FL: 60 days x 0.57 eggs/day = 34.47). Because these estimates only include breeding individuals, they only represent average fecundity for birds in reproductive condition a nd not fecundity of the entire population of birds present in the breeding area. Additional Trapping Observations During collection of Brown-headed Cowbirds in Florida, we collected four specimens of Shiny Cowbirds, three female and one male. B ecause there are few diagnostic features that distinguish females of Shiny and Brown-headed Cowbird (see Pyle 1997), tissues from each 65

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female specimen were sent to the Bell Museum in Minneapolis for geneti c analyses to confirm species identification. DNA was extracted from the samples and PCR was used to amplify two different loci. The first locus was a 4 86-base-pair fragment of the cytochrome b gene from the mitochondrial genome. The second locus was a 565-base-pair fragment from the third intron of the MUSK gene on the Z chromosome. Sequences were compared to known M. ater and M. bonariensis sequences deposited in GenBank a nd vouchered tissue museum samples to determine species identity. All three specimens were confirmed as being Shiny Cowbirds. These contribute to only a handful of reported cases of Shiny Cowb irds in north-central Florida, all occurring since 2000. Furthermore, one of the females collected (UF 46077) also had a developed ovary with three la rge pre-ovulatory follicles and a single potential post-ovulatory follicle. The oviduct was large (~1g) and convoluted indicating that it recently passed an egg. These observations represent the best evidence to date of Shiny Cowbirds breeding in Florida. Discussion Based on assays of ovarian and oviducal condition, an average female Brown-headed Cowbird breeding in north-central Florida and Texa s lays approximately 35 eggs over the course of an 8 9 week breeding season. This estimate of f ecundity is slightly lower than the 40 eggs determined for birds breeding in Manitoba (S cott and Ankney 1980) and higher than the 30 eggs derived for cowbirds in Califor nia (Payne 1965, Fleischer et al. 1987). Estimated fecundity and clutch size of Florida cowbirds were nearly iden tical to values derived for females breeding in east-central Texas, where nest parasitism has led to critical declines of some species of native songbirds (Eckrich et al. 1999). Similarly, clutch sizes and inte r-laying intervals were similar between female cowbirds breeding in Florida an d Texas. Therefore, the hypothesis that low fecundity of cowbirds in Florida explains their observed low rates of parasitism is not supported. 66

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Factors that have been shown to affect reproductive indices in other studies do not appear to significantly affect Brown-headed Cowbirds in Florida. For example, calcium availability can limit reproduction in some populati ons. While some of the soils in north-central Florida are underlain with sand or clay and are depauperate in calcium (Abrahamson and Hartnett 1990), availability of calcium for egg production is probably not limited. The physiographic regions of north and central Florida are characterized by many areas with limestone at or near the surface (Brown et al. 1990), providing a rich source of calcium carbonate. Furthermore, the Floridan Aquifer is one of the most productive carbona te aquifers in the world (USGS 1990) and dissolved limestone delivers large concentrations of calcium into a water system that feeds the entire state (USGS 1990). Theref ore, calcium is at least seconda rily available through calcareous stone, or through bone and mollusc shells often eaten by cowbirds (Ankney and Scott 1980). In addition, Schoech and Bowman (2003) found no difference in plasma calcium levels between suburban and natural populations of Florida Scrub Jays (Aphelocoma coerulescens ) breeding in central Florida. The authors assumed that natural calcium sources would be highly limited in natural scrub habitats due to low pH levels, yet jays were able to find ample calcium for production of eggs. Individual female Brown-headed Cowbirds breeding in north-central Florida and Texas also appear to be able to get ample food resources for production of eggs. In 2005, Florida ranked seventeenth in the United States in th e total number of cattle (USDA 2002b) and Alachua County currently has nearly 49,000 head of beef and dairy cows (USDA 2007) on approximately 86,000 acres of agricultural land (USDA 2002a). Therefore, there are numerous feeding locations in the study area wher e female cowbirds can acquire resources for egg production. Cowbirds were observed foraging at all trapping locations as well as on road sides, at city parks, 67

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and private bird feeders. I did not specifically measure abundance of cowbirds at these locations. However, abundance at feeding areas where collec tion was performed varied by site and time of the breeding season. At collec tion sites with the la rgest numbers of dairy and beef cows (200 800), relatively few females were captured or s een foraging with cattle. Two sites located near the largest urban area in the county (Gaine sville) had large mixed-sex flocks with as many as 100 150 individuals during peak fo raging hours. Therefore, ma jor nutritional factors thought to determine cowbird fecundity or clutch size in other regions or in othe r species, do not appear to be limiting fecundity in Florida cowbirds. Clutch Size Estimates Clutch sizes in this study were estimated us ing the number of read ily recognizable postovulatory follicles that were in the same laying sequence. Clutch size estimates for Florida (3.68 eggs) and Texas (3.38 eggs) cowbirds were smaller than those (3.91 4.60 eggs) derived using a similar methodology for cowbirds in various locations in the United States (Payne 1976, Scott and Ankney 1983). Clutch sizes are known to increase with latitude (Koenig 1984, Jarvinen 1989, Dunn et al. 2000) which may part ially explain lower observed clutch sizes than those from Michigan and Ontario (Payne 1976, Scott and Ankney 1983). Lower clutch size estimates for the cowbirds in this study may also have been due to subtle differences in methodology or study populations. Post-ovulatory follicles in Brown-headed Cowbirds regress rapidly with 50% decrease in size every 12 hours in the two to three days following ovulation (Scott and Ankney 1983). Fu rthermore, the opening of ruptured postovulatory follicles closes in 3 4 days post-ovulation (Scott and A nkney 1983). As a result, it is often difficult to categorize follicles older than 3 5 days as having released an ovum. On the other hand, Payne (1965) was able to count post-ovulatory follicles of Michigan cowbirds for as 68

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many as twelve days past ovulation. In the pres ent study, the highest number of flaccid follicles of any kind (post-ovulatory, atreti c, burst prior to ovulation) found in a single specimen was six. Assuming some of these follicles were misclassified and indeed all post-ovulatory, it still represents only half the maximum number that Pa yne counted. Therefor e, the clutch sizes I estimated may be lower than those in the l iterature because I conservatively included only follicles that could reliably be distinguished as post-ovulatory, and cowbirds I collected may have absorbed post-ovulatory foll icles relatively more quickly. While an average wild female cowbird lays for 3 4 days in a row, captive females have laid eggs for up to 13 consecutive days in Florid a (Chapter 2), and are ca pable of laying 67 eggs in as many days (Holford and Roby 1993). Ther efore, the method used in this study probably generally underestimates clutch size because so me birds will lay for many days in a row but older post-ovulatory follicles will be difficult or im possible to identify. For birds such as Brownheaded Cowbirds that are indeterminate layers and not constrained by energetic or time demands of incubating eggs and caring for young, clutch size may be of little significance. That is, length of the laying sequences is probably less importa nt than the daily laying rate, for example, because a female cowbird can have a small clutch with short inter-clutch interval and lay as many eggs in the same time span as a female with a large clutch. Theref ore, overall fecundity is likely a more appropriate and consistent measur e of Brown-headed Cowb ird reproduction in an area. Fecundity Estimates Estimates of daily laying rate in this study were only sligh tly lower than those in other published studies of cowbird reproduction. Based on proportions of birds with oviducal eggs, I estimated that breeding female cowbirds in bot h Florida and Texas laid approximately 0.56 to 69

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0.57 eggs per day. Other studies have estimated average daily laying rates from 0.64 to 0.73 eggs per day using similar methodologies (Scott and Ankney 1980, Fleisc her et al. 1987). Differences in daily laying rate, however, may be related to the variation in the length of the cowbird breeding season geographically. Other st udies have shown a decreasing avian breeding season length with increasing latitude (Baker 1939, Johnston 1954). Average cowbird breeding season length is longer in Florida (60 days), Texas (62 days), and Oklahoma (8 10 weeks; Payne 1976) than in Ontario (56 days; Sc ott and Ankney 1980) and Michigan (5 6 weeks; Payne 1976). Cowbirds breeding in areas with short seasons, whether due to abiotic (e.g., climate, photoperiod) or biotic (e.g., host availability) fact ors, may have relatively higher average laying rates to maximize overall fecundity. I assumed all birds stayed on site throughout the breeding season and that daily laying rate was similar over the course of the breeding season. These assumptions are likely violated because cowbirds are known to leave study areas (Woolfenden et al. 2003) and because laying rate typically ch anges throughout the breeding season (Payne 1973a, Fleischer et al. 1987, W oolfenden et al. 2001, but see Scott and Ankney 1980). The estimate represents an average over the course of the breeding season and therefore does not consider temporal variation. Fecundity was defined in this study as the num ber of eggs produced by an average female cowbird during the breeding season. Some females that were in the study area likely laid fewer than 35 eggs over the course of the 8-week season, and may not have bred at all. Of female Brown-headed Cowbirds trapped in Ontario duri ng the height of the cowbird breeding season, only 2% of individuals were not in breeding condition (Scott 1978) However, Summers et al. (2006) detected enlarged ovaries in only 18% of female cowbirds categor ized as potential local breeder in Texas, although the sample included bi rds collected outside of the breeding season. In 70

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this study, 47 female cowbirds were collected in Florida between estimated beginning and end dates of the cowbird breeding season in 2006 and 2007. Only three (6.4%) of these females were not in reproductive condition as th ey lacked preor post-ovulator y follicles, or enlarged ovaries and oviducts. Therefore, the major ity of female cowbirds collected during the breeding season in this study had recently laid eggs, were ovulating, or preparing to lay oviducal eggs. It is also possible that the three indivi duals found in non-reproductive c ondition had already laid eggs prior to collection since they were all captured in midto late-June. Finally, many of the females collected in this study may have laid more eggs than the 35 estimated for an average bird. Therefore, it is likely that the fecundity value de rived for Florida Brown-headed Cowbirds in this study is at least a conservative es timate of the number of eggs a typical female lays during breeding. Cowbird Collection Trapping of Brown-headed Cowbirds has been used extensively to control populations where they have become problematic (Kelly and DeCapita 1982, Beezley and Rieger 1987, Eckrich et al. 1999). Trapping has been shown to more effective than shooting at removing large quantities of breeding female cowbirds over the course of a breeding season (Summers et al. 2006). However, it is less efficient overall th an shooting, and the effec tiveness of trapping and shooting has been shown to vary over the course of the breeding season (Summers et al. 2006). Cowbirds are easily trapped in March and April, but may become trap-shy after the onset of the breeding season (Dufour and Weatherhead 1991). As a result, birds were collected in Florida using a combination of trapping and shooting. Tr apping was most effective at capturing birds at the end of the breeding season when many bird s were in declining reproductive condition, although birds entered traps throughout the entire collection period. On the other hand, shooting was effective in collecting birds during May and June when trapping efficiency was low. 71

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Many factors may contribute to differences in the efficiency of each collection method. For example, trapping relies on a local enhanc ement of a food resource (Turner 1964) and conspecific attraction to decoys by females usi ng foraging areas. Breeding females are known to shift their diet during the breeding season from seeds to insects (Ankney and Scott 1980), presumably for production of eggs. Therefore, birds that are attracted to gr ains and seeds in traps may represent individuals that are food stressed and therefore generally in poorer condition (Dufour and Weatherhead 1991). Body mass, oviduc t mass, and ovary mass of Florida breeding cowbirds were significantly light er in trapped birds than shot birds. However, I detected no differences in morphological measurements such as wing chord and tail length between trapped and shot birds. Other research has also f ound similar body condition between trapped birds and those collected using other methods (Scott and Ankney 1979, Davis 2005). Furthermore, body mass of trapped and shot females in this study we re not significantly diffe rent when the mass of oviducal eggs was removed. Therefore, mass diffe rences of trapped versus shot birds may not reflect a condition bias (e .g., Weatherhead and Greenwood 1981, Dufour and Weatherhead 1991), per se, but rather a sampling bias. Indeed, shot birds were more likely to have an oviducal egg than trapped birds (Log Odds Ratio=2.76, CI=3.63 68.41) so that trapping may bias toward individuals in between clutches rather than toward birds in poorer condition. It is not clear why birds in between clutches would be more likely to be trapped than those that are ovulating. Egg production can be energetically costly (Carey 1996, Williams 2005) so that non-laying birds may be more attracted to easily-a ttainable, but nutritionally infe rior food resources than laying individuals. Some co wbird populations have been shown to be promiscuous (Strausberger and Ashley 2003), so female cowbirds preparing for another bout of egg-laying may also have been looking for extra pair copulations with male cowbird decoys in traps. 72

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While trapping conducted in this project may have been biased toward non-laying birds, it is also possible that the shooti ng protocol was biased toward individuals in laying condition. Birds were singled out and shot when foraging with in and near feed lots and pastures. Collecting females by shooting individual birds foraging in th ese areas is likely selective toward breeding birds because they are exhibiting breeding behavior within and near host ha bitat (Summers et al. 2006). Furthermore, I often collected females that were foraging alone or in small flocks of a few individuals. Scott & Ankney (1980) observed th at females foraging in larger flocks (groups of 10 or more) were less likely to be in laying condition. Inde ed, 17 of the 20 breeding-season cowbirds in reproductive condition that were co llected via shooting had oviducts containing developing eggs. If female cowbirds had been co llected solely using this method over the course of the field season, average female fecundity would be estimated as 51 eggs per female per breeding season. With trapping alone, the estimat e would be 22 eggs. Therefore, trapping or shooting methodology, when used alone, may bias f ecundity estimates that rely on the presence of oviducal eggs and should be used in tandem when sampling cowbird populations. Conclusions Studies of Brown-headed Cowbird reproducti on have distinguishe d potential fecundity from effective (Hahn et al. 1999) or realized fecundity (Alders on et al. 1999, Woolfenden et al. 2003) because some cowbird eggs are likely ejected by host adults, laid outside of nests, or placed in nests that cannot be monitored. For ex ample, Woolfenden (2003) used genetic analysis to assign eggs found in nests to individual female cowbirds. Realized fecundity was substantially lower than potenti al fecundity as measured by proportion of birds with oviducal eggs, a result partially due to female cowbirds laying eggs outside of the study area. Results from the present study indicate that potential cowb ird fecundity is high in both Florida and Texas populations. Therefore, fecundity is not physiologically limited in north-central Florida and does 73

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not help explain low observed parasitism rates ac ross the host community. However, fecundity may be ecologically limited (reduced realized fecundity) through multiple mechanisms such as egg rejection behaviors (Chapter 2), community interactions (e.g., nest predation), or the abundance and diversity of suitabl e host species (Chapter 4). 74

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Table 3-1. Summary of means (N, 95% confidence interval) of mo rphometric data taken from breeding female Brown-headed cowbirds collected in Florida and Texas, 2005 2007 and results of statistical co mparisons (independent sample s t-tests). Data for wing chord length of Texas birds are take n from Summers et al. (2006). Ovary Mass Oviduct Mass Body Mass (g) W ) ing Chord (mm Florida 0.41 1.20 36.0 94.98 (54, 0.35 0.46) (55, 1.11 1.29) (54, 35.14 36.86) (62, 94.82 95.14) Texas 0.39 1.38 36.61 94.95 (74, 0.33 0.45) (73, 1.29 1.46) (59, 35.95 37.26) (959, 94.34 95.56) P-value 0.422 0.023 0.268 Table 3-2. Estimate of clutch size of 2005 Texa s cowbirds using number of post-ovulatory follicles (POFs) clearly identified to have a ruptured stigma. Laying birds are those with oviducts containing a developing egg while non-laying birds are individuals in breeding condition (presence of enlarged oviducts, pre-ovulatory follicles, or postovulatory follicles) but without an oviducal egg. Number of POFs for 10 non-laying birds without clearly identifiable POFs ( ) is estimated using the proportion of POFs in all other non-laying bird s (66/29=2.3 POFs/bird). Me thod derived from Scott & Ankney (1983). Laying Birds Non-laying Birds No. POFs Number of Birds Total POFs Est. Clutch Size Number of Birds Est. POFs 1 2 2 8 18 2 13 26 1 8 8 3 20 60 2 7 14 4 5 20 3 12 36 4 2 8 Total 40 108 37 84 Clutch size = [(108 x 2) 40] + 84 = 3.38 eggs 40 + 37 75

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Table 3-3. Estimate of clutch size of 2006 2007 Florida cowbirds usi ng number of clearly identifiable post-ovulatory follicles (POFs) Number of POFs for non-laying birds without clearly id entifiable POFs ( ) is based on the proportion of 2.0 (26/23) POFs/bird in all other non-laying birds. Laying Birds Non-laying Birds Certain POFs Number of Birds Total POFs Est. Clutch Size Number of Birds Est. POFs 1 1 1 4 8 2 4 8 1 9 9 3 18 54 2 6 12 4 5 20 3 7 21 4 1 4 Total 28 83 27 54 Clutch size = [(83 x 2) 28] + 54 = 3.49 eggs 28 + 27 76

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Figure 3-1. Location of Brown-headed Cowbird collection sites from April August, 2005 2007. Site 1 is Fort Hood, Texas, and Site 2 is north-central Florida. Site 1 included collection of female cowbirds in 2005 a nd Site 2 included cowbird collection in 2006 and 2007. 11 27 42 6 17 19 210 5 10 15 20 25 30 35 40 45 AprilMayJuneJulyFemales Collected 2006 2007 Figure 3-2. Number of female cowb irds collected per month at site s in north-central Florida in 2006 and 2007. Sample sizes are given above each bar. 77

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 AprilMayJuneJulyFemales/trap/da y 2006 2007 Small & Medium Traps Large Traps Figure 3-3. Mean number of female cowbirds colle cted per trap day at sites in north-central Florida from April to July, 2006 2007. Total trap days were 146 and 463 in 2006 and 2007, respectively. Total trap days were 392 and 217 for small/medium and large traps, respectively. Trap efficiency data presented by trap size includes both 2006 and 2007 samples. 6 7 9 20 10 10 1 0 5 10 15 20 25 AprilMayJuneJulyNumber of Females Collected Shot Trapped Figure 3-4. Number of female cowbirds collecte d per month in Florida by trap and by pellet gun from April July 2007. Sample sizes are given for each collection method. 78

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21 34 18 5 8 5 7 6 3 2 8 2 11 19 18 3 7 8 7 3 1 12 20.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 4/154/295/135/276/106/247/8 Week BeginningMass (g) Florida Texas Figure 3-5. Mean weekly mass ( SE) of ovaries of all Florida and Texas female cowbirds collected between 15 April and 15 July, 2005 2007. Samples sizes for each week are given next to each point. 21 34 18 5 8 5 (FL) 7 6 3 2 8 2 1 1 3 6 9 6 4 19 18 11 20.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 4/154/295/135/276/106/247/8 Week BeginningMass (g) Florida Texas Figure 3-6. Mean weekly mass ( SE) of oviducts of all Florida and Texas female cowbirds collected between 15 April and 15 July, 2005 2007. Sample sizes for each week are given next to each point. 79

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1 1 4 3 5 6 3 2 3 1 5 3 4 2 2 50.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 4/154/295/135/276/106/247/8 Week BeginningMass (g) Laying Non-laying Figure 3-7. Mean weekly mass ( SE) of ovaries of breeding Brown-headed Cowbirds collected in Florida, 2006 2007. Laying birds are those with an oviducal egg while non-laying birds are in reproductive condi tion but lack an oviducal egg. Sample sizes for each collection week are given next to each point. 1 1 4 3 5 63 2 3 1 5 3 4 2 2 50.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 4/154/295/135/276/106/247/8 Week BeginningMass (g) Laying Non-laying Figure 3-8. Mean weekly mass ( SE) of oviducts of breeding Brown-headed Cowbirds collected in Florida, 2006 2007. Laying birds are those with an oviducal egg while non-laying birds are in reproductive condi tion but lack an oviducal egg. Sample sizes for each collection week are given next to each point. 80

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1 3 8 8 3 3 6 4 2 1 6 11 8 4 2 2 2 110.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 4/154/295/135/276/106/247/8 Week BeginningMass (g) Laying Non-laying Figure 3-9. Mean weekly mass ( SE) of ovaries of breeding Brown-headed Cowbirds collected in Texas, 2005. Laying birds are those w ith an oviducal egg while non-laying birds are in reproductive condition but lack an oviducal egg. Sample sizes for each collection week are given next to each point. 1 3 8 10 3 3 4 6 1 1 6 10 8 1 3 3 2 20.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 4/154/295/135/276/106/247/8 Week BeginningMass (g) Laying Non-laying Figure 3-10. Mean weekly mass ( SE) of oviducts of breeding Brown-headed Cowbirds collected in Texas, 2005. Laying birds ar e those with an oviducal egg while nonlaying birds are in reproductive condition but lack an oviduc al egg. Sample sizes for each week are given next to each point. 81

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25.0 30.0 35.0 40.0 45.0 4/154/295/135/276/106/247/87/22Female Mass (g ) All Birds Breeding Birds Figure 3-11. Mass of female Brown-headed Cowbir ds collected in Florida between 23 April and 12 July, 2006 2007. Mass of all birds decreased si gnificantly over time (hashed line, R 2 =0.16, P<0.001, N=110). Mass of breeding bird s did not significantly change over the course of the breeding season (solid line, R 2 =0.024, P=0.263, N=53). 25.0 30.0 35.0 40.0 45.0 50.0 4/154/295/135/276/106/247/8Female Mass (g ) All Birds Breeding Birds Figure 3-12. Mass of female Brown-headed Cowbir ds collected in Texas between 19 April and 6 July, 2005. Mass of all birds (hashed line, R 2 =0.001, P=0.826, N=79) and of breeding birds (solid line, R 2 <0.001, P=0.990, N=58) did not change significantly over time. 82

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3 18 34 40 4 22 25 260.0 5.0 10.0 15.0 20.0 25.0 30.0 -1 -2 -3 -4 Collection DayPOF Area (mm2) Texas Florida Figure 3-13. Mean area ( SE) of post-ovulatory follicles (POFs) of Brown-headed Cowbirds collected in Texas and Flor ida from April to June, 2005 2007. Sample presented includes only birds with ovidu cal eggs. Day -1 represents POFs associated with oviducal eggs and which released yolk one day prior to being co llected. Asterisks indicate comparisons between Florida and Texa s that are statistically different (t-tests, P<0.01). Sample sizes are given with each bar. 7/14 10/20 5/5 5/8 20/39 9/13 9/14 2/50 0.2 0.4 0.6 0.8 1 4/26-5/105/11-5/255/26-6/96/10-End*Laying Rate Florida Texas Figure 3-14. Estimated laying rate (eggs produced per day) of breeding cowbirds collected in Florida and Texas during four periods of th e cowbird breeding season. Laying rate is based on the proportion of re productively active birds th at have oviducal eggs. Proportions are next to each bar. *End of cowbird breeding season for Florida and Texas are 24 June and 26 June, respectively. 83

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CHAPTER 4 ALTERNATIVE HYPOTHESES FOR THE S LOW COWBIRD INVAS ION OF FLORIDA Introduction Brood parasitism by Brown-headed Cowbirds is thought to be a significant threat to particular songbird populations and communities, es pecially in areas where cowbirds have only recently invaded. Research on cowbird expansion to new areas has typically focused on effects of cowbirds on breeding avian species and comm unities (e.g., Mayfield 1965, Braden et al. 1997, Strausberger and Ashley 1997, Payne and Payne 1998) but not much attention has been paid to the reverse. That is, cowbirds have been depi cted as agents of extermination (Mayfield 1977) that steamroll into new areas wreaking havoc on nativ e bird populations. Ye t, regional and local dynamics of the host community ha ve the potential to significantly influence patterns of cowbird range expansion through their effect on reproducti ve success. Cowbird fecundity, for example, has been estimated to be extremely high in some regions where an average female lays 40 eggs per breeding season (Scott and Ankney 1980). Potent ial fecundity (total number of eggs that are laid), however, may not reflect actual fecund ity (total number of eggs incubated or young surviving; Alderson et al. 1999, Hahn et al. 1999 Woolfenden et al. 2003). Multiple local factors may reduce effective fecundity and the ov erall reproductive success of cowbirds. Host adults, for example, may eject cowbird eggs fr om their nests or abandon parasitized nests, thereby reducing the percent of parasitism det ected and the number of cowbird young that are produced. The arrival of two cowbird spec ies to Florida was thought to herald a conservation crisis for many species of potential hosts. Shiny Cowbir ds may have reached Florida as early as 1971 (Ogden 1971), but were officially recorded in so uth Florida in the mid-1980s (Smith and Sprunt 1987). By the mid 1990s Shiny Cowbirds were r ecorded in numerous localities (primarily 84

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coastal) throughout Florida (Cruz et al. 2000). Arrival of the Sh iny Cowbird to Florida was of particular concern because of impacts on songbi rds on recently colonized Caribbean islands and because Shiny Cowbirds are known to colonize cavity nesters that are rarely if ever parasitized by the Brown-headed Cowbird (Wiley 1985, Wiley et al. 1991). Shiny Cowbirds also often prefer only one host (Post and Wiley 1977) so th at populations of Black-whiskered Vireos ( Vireo altiloquus), Cuban Yellow Warblers and Florida Prairie Warblers conf ined to south Florida were thought to be at risk to extinction through brood parasitism (Atherton and Atherton 1988). So far, however, these dire predictions have not come to pass and there are only two cases suggesting breeding by Shiny Cowbirds in Florid a (Smith and Sprunt 1987, see Chapter 3). Brown-headed Cowbirds were first documented breeding in Florida in the mid-1950s (Monroe 1957) and have since spread through most of the state where they ar e found at moderate levels of abundance relative to other regions of North Am erica (Sauer et al. 200 6). Surveys of Brownheaded Cowbird parasitism of host nests and fa mily groups feeding fledglings show unusually low frequencies both for individu al species and for the entire community of potential hosts in north-central Florida (Chapter 1). In previous chapters of this dissertation, I explored hypotheses that might explain the seemingly slow invasion of Florida by cowbirds First, I tested the hypothesis that the low frequency of Brown-headed Cowbird parasitism observed in north-central Florida resulted from host defenses, i.e., cowbird eggs being removed by adults or nests being abandoned before the active nest could be monitore d (Chapter 2). I monitored over 1100 nests of 32 potential host species and noted no cases of natu ral cowbird egg ejection. Furthe rmore, parasitism experiments revealed that parasitism reje ction behaviors do not appear to be prevalent in many common Florida hosts and are certainly no more prevalent in Florida than in areas where cowbird invasion 85

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proceeded more rapidly (Rothstein 1994). Second, reproductive assays of female Brown-headed Cowbirds breeding in north-central Florida revealed that potential fecundity is very high, with an average female laying 35 eggs per breeding seas on (Chapter 3). Theref ore, these hypothesized limitations to cowbird reproduction do not appear to have influenced the observed rates of cowbird parasitism in the region. In this ch apter, I explore two mo re hypotheses that may explain either observed low rates of parasitism in north-central Florida or the seemingly slow invasion of Florida by cowbirds: host community composition and nest predation. The first hypothesis is that the community composition of hos ts in Florida slows invasion because they are dominated by poor cowbird hosts that either have defenses against parasitism or are poor at raising cowbird young. I test this hypothesis by comparing community composition of hosts in areas where cowbirds have been successful a nd in Florida where the invasion is ongoing. The second hypothesis is that host comm unities in Florida are themselves less productive than those elsewhere in the cowbirds range as a result of hi gh rates of nest predati on. I test these data using simple models for source-sink dynamics a nd data from two regions where data exist on cowbird productivity and host ne st predation rates. Host Community Composition The occurrence and abundance of Brown-headed Cowbirds over local landscape scales has been shown to be influenced by several fact ors, including the divers ity (Chace 2004, Purcell 2006), abundance, and population density of hosts (Johnson and Temple 1990, Robinson and Wilcove 1994, Tewksbury et al. 1999, Jensen and Cully 2005a, but see Evans and Gates 1997). Abundance of Brown-headed Cowbirds in thr ee vegetation types in western Montana, for example, was positively correlate d with the density of potenti al hosts (Tewksbury et al. 1999, Robinson et al. 2000). Levels of parasitism may also be influenced by the availability of suitable host species (Barber and Martin 1997, Brodhead et al. 2007, but see Jensen and Cully 2005b). 86

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Parasitism of Black-capped Vireos, for example, was lower in areas with lower cumulative density of four other potentia l host species (Barber and Mar tin 1997). Therefore, cowbird abundance and distribution may be affected by th e distribution and abundan ce of host species at local scales. Patterns of cowbird and host occurrence are more difficult to discern at larger (e.g., regional) scales because many f actors can affect species distributions. Research has shown a strong trend of diminishing faunal diversity (Means and Simberloff 1987) and density (Emlen 1978) from base to tip along Floridas peninsula. Several mechanisms have been proposed to explain this general pattern, in cluding changes in immigrati on and extinction rates (Cox 2006), past climate change and sea-level fluctuations (Robertson and Kushlan 1974), and differences in availability of different habitats (Emlen 1978, Wamer 1978) that would provide suitable resources. One likely limiting factor on cowbird reproductive success should be the availability of suitable hosts to raise cowbird young. Breeding Brown-headed Cowbirds are distributed throughout Florida but are primarily located in the northern third of the state (Figure 4-1). Of the ca. 50 host species known to breed in Florida that have raised cowbird young (Table 4-1), only 19 species have statewide distri butions whereas 28 of the remaining 31 species are either found only in north Florida (Cox 2006) or have distributions primarily in the northern half of the state (FFWCC 2007). Taken together, these data suggest that the spr ead of the cowbird into south Florida may be limited by the availability of quality hosts. To test this hypothesis, I categorized species breeding in Florida into one three categories base d on their quality as cowbird hosts (Table 4-1) based on criteria in Grzybowski and Pease (1999), Robinson et al (unpubl. data) and data from the literature. High quali ty host species are those that are of ten highly parasitized elsewhere and 87

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that raise cowbird young to fledging. Marginal quality species ar e those that accept cowbird eggs but are rarely parasiti zed and raise few cowbird young. Poor quality hosts represent cowbird egg sinks due to rejection of parasiti sm or life-history tra its (e.g., nestling diet) incompatible with brood parasiti sm. In Florida, the majority of species that are distributed statewide are abundant but are probably marginal hosts (e.g., Northern Cardinal), or poor hosts (e.g., Mourning Dove, Eastern Meadowlark) for a vari ety of reasons (Table 4-1). On the other hand, cowbird distribution roughly coincides with the highest diversity and abundance of good quality hosts (see Figure 1). These data are generally consistent w ith the hypothesis that cowbirds are most abundant in areas where good hosts are also found. If Brown-headed Cowbirds are located in areas in Florida with the highest diversity and abundance of suitable hosts, why ar e observed parasitism rates in north-central Florida so low? To qualitatively address whether the composition of the host community contributes to this pattern, I examined Breeding Bird Survey (BBS) data from 1997 2006 at routes within 75km of three study areas where parasitism rates have been recorded. Study ar eas included (1) northcentral Florida (city of Gainesville, N=10 rout es), (2) southwestern South Carolina (Savannah River Site near Aiken, N=8), and (3) southern I llinois (city of Anna, N=7) BBS routes in each region incorporated numerous habi tat types but primarily forest, pasture, row crops, grassland and wetland habitats (Sauer et al 2006). Routes for southern Il linois included more pasture and row crops (59%) on average than routes near Fl orida (14%) and South Ca rolina sites (37%). Illinois routes included more deciduous forest (21%) than South Carolina (12%) and Florida (0%), but routes in these latter areas al so included significant evergreen forest (24 38%). Documented rates of parasitism are lowest in Florida (Chapter 1), low to moderate in South Carolina (Whitehead et al. 2002, Kilgo and Moorman 2003), and high in Illinois (Robinson 88

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1992, Strausberger and Ashley 1997, Hoover 2003). Species breeding in each study area were categorized as good, marginal, or poor based on their suitability as cowbird hosts. Total abundance of species in each host category wa s compared among study areas using one-way ANOVA and Dunnetts post-hoc comp arisons assuming unequal variances. As in Gryzbowksi and Pease (1999), I excluded species that are known to be cowbird hosts, but have various traits that may limit parasitism of their nest s (e.g., parids, bluebirds, doves). In both Florida and South Carolina, the mean abundance of good quality hosts constitutes a relatively higher proportion of the ov erall bird abundance than in Il linois (Figure 4-2). This may suggest that Florida has a high availability of suitable hosts. However, the diversity of all potential hosts is 91 and 121% higher in South Carolina and Illinois, where there are approximately two times as many quality host species as in north-central Florida (Figure 4-3). Furthermore, there were signi ficant differences among study ar eas in the abundances of good (F=33.73, P<0.001), marginal (F=391.26, P<0.001), poor (F=160.25, P<0.001), and all (F=341.52, P<0.001) host species. The abundances of all three classes of hos ts as well as total host abundance are significantly lower in Florida than in both other regi ons (Dunnetts post-hoc comparisons, P<0.001; Figure 4-4). Kilgo and Moorman (2003) re ported low-to-moderate rates of nest parasitism for 24 species breeding in the southeastern coastal plain of South Carolina and Georgia. Among the species with the highest average parasitism rates were Prairie Warbler (13.6%), Hooded Warbler (14.4%), Yellow-breaste d Chat (18.6%), Blue Grosbeak (18.8%) and Indigo Bunting (16.5%). Relativ e abundances of these species are between 1.6 and 6.4 times higher in South Carolina than in Florida, wher e most of them reach th eir southern range limit (Sauer et al. 2006). 89

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The Breeding Bird Survey (BBS) data summ arized above are based on surveys conducted primarily along roadsides, which may not adequate ly sample forested habitat (Bart et al. 1995, Betts et al. 2007) that cowbirds may prefer (H ahn and Hatfield 1995). As a result, BBS surveys may not adequately represent forest -interior songbirds that may be preferred hosts of cowbirds. I compared census data obtained for forested habitats in Illinois (R obinson and Robinson 1999) and Florida (S.K. Robinson unpubl. data) to better estimate differences in abundance of quality forest hosts between regions. Illinois censuses we re conducted in the Trail of Tears State Forest from May July, 1989 1992 and Florida censuses were conducted at various locations primarily within 100km of Gainesville from May July in 2004 (Stracey a nd Robinson 2008). Abundance data obtained from 50-m radius point counts in Illinois were transformed to compare with 100-m radius count data obtained in Florida. Only da ta from species considered quality cowbird hosts were included. The abundance of quality forest hosts was much higher in Illinois than in Florida. Abundance of quality canopy and understory hosts was 5.2 and 3.2 times higher in Illinois than Florida, respectively (F igure 4-5) although the nu mber of species in each host type was similar between regions. Rates of parasitism in these same Illinois forests are very high (Robinson and Robinson 2001) and have more than 10 times as ma ny cowbirds as Florida forests (Figure 4-5). Cowbird numbers, however, may not be solely re lated to the overall abundance of the host community but may depend primarily on abundances of a small number of common species that act as good hosts (Rothstein and Robinson 2000). As a result, monitoring of high quality host species with abundant Florida popu lations such as Red-eyed Vire o, Northern Parula and Pine Warbler may be needed to determine how they ar e contributing to population growth of cowbirds in Florida. Thus, BBS data and forest census da ta, in particular, suggest that quality hosts are 90

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actually less abundant in Florid a than they are in areas wher e cowbirds invaded long ago and where cowbird populations have stabilized or even declined. Based on these crude and correlative analys es, Brown-headed Cowbirds breeding in Florida may suffer from a reduced availability of hosts. Lack of suitable hosts may in turn reduce effective fecundity of cowbirds. If cowbirds are unable to locate suitable hosts when eggs are ready to be laid, females ma y lay eggs outside of nests or in those of inappropriate hosts, which are often not monitored in st udies of brood parasitism. From 2004 2006, I monitored approximately 1100 nests of 35 potential host species breeding in Florida (Chapter 1), many of which are inappropriate hosts because of life-histor y traits incompatible with cowbird parasitism (e.g., Mourning Dove, Great-crested Flycatcher, Blue Jay). Of the limited cases of nest parasitism observed in this st udy, however, none included nests of these inappropriate hosts. If nests of preferred hosts are not available, fema les may also resorb the developing egg before it enters the oviduct (Payne 1998). Indeed, Payne (1965) suggested that atresia of developing ovarian follicles may be due to a shortage of suitable nests, although captive cowbirds do not appear to limit laying based on availability of artificial nests (Holford and Roby 1993). Finally, Hahn et al. (1999) suggested that a female cowbird unable to find an appropriate nest for her egg may consume it to retain important nutrients. Finally, host suitability is based on multiple factors such as nestling diet, breeding phenology, nest-site choice, nest defense and rejec tion behavior. While many of these traits may be retained among populations over their enti re distribution, regional differences in the expression of these traits may alter the suitabil ity of some populations as cowbird hosts. For example, some populations of Warbling Vireos ( Vireo gilvus ) accept cowbird parasitism of their nests whereas females in other populations punctu re-eject cowbird eggs (Sealy 1996). Data for 91

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Florida, however, are lacking for estimating reproductive parameters influencing cowbird population growth (e.g., cowbird fledging success, proportion of eggs accepted). Nest Predation Brood parasitism and nest predation are the two primary causes for reduced reproduction in many songbird populations (Mayfield 1977, Wilcove 1985, Martin 1992, Robinson et al. 1995, Arcese and Smith 1999). Nest predation, in particular, can account for >90% of nest losses in some populations also exposed to br ood parasitism (Mlle r 1988, Hoover et al. 1995, Martin 1998) and significantly limit annual reproductive succ ess of songbird populations (Robinson 1992, Pease and Grzybowski 1995). Schm idt & Whelan (1999) developed a model to examine the relative contributions of nest predation and parasitism to songbird seasonal fecundity. Results indicated that nest predation had a greater influence on reproductive success than parasitism. Furthermore, high rates of nest predation under some circumstances could lead to sink populations even with re-nesting and double-brooding. The au thors suggest that management practices that redu ce nest predation may be more important in some populations than management of brood parasitism through cowbird control. While effects of nest predation (and parasi tism) on host communities have been commonly studied, less attention has been paid to general effects of nest predation on reproduction of cowbirds. Winfree (2004) examined the possible effects of host community and habitat type on the population growth of Brown-headed Cowbirds breeding in southern Illinois. Cowbird offspring survival rates were 2 3 times lower in old fields than in forested habitats, the latter of which represents a more recently invaded habita t type and are characte rized by generally lower rates of nest predation. These results suggest that invasion success of Brown-headed Cowbirds may be facilitated by hab itat types that reduce nest predation pressures that could slow cowbird 92

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population growth. Winfree et al. (2006) even sp eculated that high nest predation rates in the scrubbier habitats that cover much of Florida may be slowing their rate of spread because such habitats tend to be associated with very high rates of nest predation. Nest failure rates of select understory nesti ng species breeding in nor th-central Florida are very high. For example, Mayfield daily surviv al rates for Northern Cardinal and Northern Mockingbird in north-central Florida are as low as 0.9187 (Reetz et al. unpubl. data) and 0.8932 (C.M. Stracey pers. comm.), respectively. Host populations may compen sate for high nest failure rates through re-nesting or multiple broods (Pease and Grzybowski 1995) over long breeding seasons. Indeed, both Northern Cardinal s and Northern Mockingbirds have very short re-nesting intervals following nest failure (Zaias and Breitwisch 1989, Filliater et al. 1994) and begin breeding as early as February (M. Reetz pers. obs), which may alleviate high predation pressures on these species. Because cowbirds only breed for a part of these long seasons, however, high rates of re-nesting may not actually increase the availability of hosts to parasitize (e.g., Morrison and Bolger 2002). By modifying a well-known source-sink equati on (Pulliam 1988: Equation 5a), Winfree (2004) calculated upper and lower values ( T crit ) for egg-to-fledgling transition success of cowbirds that would be requi red for a stable population. Values derived were based on published data for adult female cowbird survival (Woolfenden et al. 2001), juvenile survival (Ricklefs 1973, Greenberg 1980, Smith et al. 2002), and female cowbird fecundity (Scott and Ankney 1980, Woolfenden et al. 2003). Lower and upper bounds of cowbird egg-to-fledgling transition success needed to achieve co wbird replacement were calculated as 0.03 0.22. In north-central Florida, an average female cowb ird is estimated to produce 34 eggs per season (Chapter 3) resulting in modified T crit values of 0.04 0.11. To generate T values derived from 93

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field data, the following equati on can be used (Winfree 2004), T = U x V x W where U is the probability of egg acceptance by the host, V is the probabili ty of survival over the nesting interval, and W is the probability that the cowbird will su rvive to fledging in a successful nest. Based on this equation, Winfree determined th at certain habitats and species generate T values below that needed to sustain cowbird populations Using field data on cowbird egg acceptance (Chapter 2) and nest interval survival rates (Reetz et al. unpubl. data, C.M. Stracey pers. comm.) for three species breeding in Florida, values for U and V would be 0.85 and 0.20 for Northern Mockingbird, 0.12 and 0.16 for Brown Thrasher, and 0.95 and 0.15 for Northern Cardinal. Assuming an average V value of 0.79 based on thos e determined by Winfree, T estimates of cowbird egg-to-fledgling transition success for these three species in Florida are 0.13, 0.02, and 0.11. Two of these values are near the higher estimate of T crit (0.11) needed to sustain cowbird populations and one (Brown Thrasher) is below the lower estimate (0.04). Thus, even with liberal estimates of some of these values, th ese hosts could not sustain cowbird populations. These species, however, are probably not the most suitable hosts in the songbird community in north-central Florida. Indeed, a large sample of nests (N=826) of these three species was monitored with only six cases of parasitism record ed. Nevertheless, the rate of nest parasitism over the entire sample of species monitored was also low. It is possible that parasitism frequency was affected by nest predation occurring prior to monitoring of nests. This would require partial nest predation wh ere the cowbird egg(s) were rem oved and one or more host eggs remained to be found and monitored, an unlikely event as there were only a few cases of partial clutch loss in all unparas itized nests monitored. The most heavily parasitized species in norther n Florida are likely t hose that nest in the forest canopy. Family groups of these species were far more likely to contain a cowbird young 94

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than family groups of understory species. Pe rcent of family groups of canopy nesting species with cowbird young were 39% for Blue-gray Gnat catcher and 33% for No rthern Parula, which would be considered moderate as parasitism ra tes of nests. These estimates of parasitism frequency are based on the assumption of comple te survival of cowbird young in nests that fledge host young, which is very un likely. If we use the same V value (cowbird survival in a successful nest) of 0.79 for canopy species and assume cowbirds lay one egg per nest, as many as 49% and 42% of nests of these two species may be parasitized. Furthermore, cowbird parasitism may facilitat e or increase nest predation (Dearborn 1999, Burhans et al. 2002, Zanette et al. 2007) perhaps due to loud begging vocalizations of cowbird young. This would result in family group parasitism rates that also underestimat e actual parasitism of ne sts. Therefore, rates of nest parasitism on midstory and canopy nesting species may be higher than family group surveys indicate. If Brown-headed Cowbirds are preferentially la ying the majority of their eggs in nests of preferred host species th at are difficult to find or monitor (e .g., canopy nesting species), rates of nest failure combined with behaviors that re duce the number of eggs that are incubated (e.g., abandonment) in these species would be more im portant in affecting overall cowbird production. For example, Red-eyed Vireos are common in Florida (Sauer et al. 2006) and have been recorded to abandon 34% of parasi tized nests (Graber et al. 1985). If the probability of cowbird egg acceptance ( U ) is 0.66 and cowbird survival in a successful nest ( V ) is 0.81 (Winfree 2004), the interval survival rate would need to be 0.21 (daily survival of 0.937) to result in net positive cowbird reproduction in Florida ( T crti =0.11). This rate is lower than most data from the literature for this species (daily surv ival of 0.932-0.963: Burke and Nol 2000, Duguay et al. 2001, Winfree 2004), which suggests that they may be a sour ce host where they are found in sufficient 95

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numbers. Despite logistical difficulties in co llecting the data, comprehe nsive nest surveys of midstory and canopy nesting species are required to determine if both parasitism frequencies and rates of nest failure. Some evidence also sugge sts that variation in cowbird parasitism is negatively correlated with interspecific differe nces in nest predati on, indicating that the probability of nest failure can influence host choi ce by cowbirds (Avils et al. 2006). High nestloss rates in the limited subset of preferred Flor ida host species (as well as across the community of alternative lower quality hosts), however, would primarily limit their capacity to act as cowbird generators and would partially e xplain why Brown-headed Cowbirds have not invaded Florida as successfully as they have ot her parts of North America. Alternatively, the abundance of ideal source hosts, those that accept cowbird eggs and do not suffer high rates of nest predation, may also determine the rate at which cowbirds can spread. 96

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Figure 4-1. Distribution of Brown-headed Cowbirds in Florida. Redrawn from Breeding Bird Survey abundance map (1994 2003). Legend indicates count of birds per route. Figure 4-2. Proportion of total abundance composed of species of varyin g quality as cowbird hosts. Classifications roughly based on criteria in Grzybowski and Pease (1999). Data are summarized for Breeding Bird Surv ey routes within 75km of study sites in three regions. 97

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0 10 20 30 40 50 60 GoodMarginalPoor AllNo. Species Florida South Carolina Illinois Figure 4-3. Total number of host species of thre e types at locations in three regions. A A A A B B B B C B C C0 100 200 300 400 500 600 GoodMarginalPoorAllTotal Abundance Florida South Carolina Illinois Figure 4-4. Mean ( SE) abundance of three types of Br own-headed Cowbird hosts in three regions. Bars within each host category that do not share a letter are significantly different using average abundance values from 1997 2006 (One-way ANOVA, Dunnetts post-hoc test assu ming unequal variances, P<0.05). 98

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99 6 11 8 60 2 4 6 8 10 12 14 CanopyUnderstoryCowbirdBirds per count Illinois Florida Figure 4-5. Mean ( SD) abundance of forest species that represent good quality cowbird hosts in Illinois and Florida. Birds per count represents abundance of birds within 100-m radius point counts. Values above bars in dicate number of speci es recorded for each host type. Data are from Robinson a nd Robinson (2000) for Illinois, and from unpublished data (S.K. Robinson). Cowbir d abundance includes individuals of both sexes.

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Table 4-1. Host species documented as breed ers in Florida (FFWCC 2007) also known to have fledged Brown-headed Cowbirds. Species are arranged in descending order of relative abundance (Sauer et al. 2006). Florida dist ribution category based on Cox (2006) and Florida Breeding Bird Atlas (FFWCC 2007). Abbreviations indicate that species occur, SW: Statewide, N: only in northern half, S: only in southern half, PN: primarily in norther n half. Suitability is based on criteria in Grzybowsk i and Pease (1999) and Robinson et al. ( unpubl. data). G: good quality hosts; M: marginal hosts, P: poor hosts. Common Name Scientific Name Florida Distribution Suitability Relative Abundance Red-winged Blackbird Agelaius phoeniceus SW M 49.94 Northern Mockingbird Mimus polyglottos SW P 49.11 Northern Cardinal Cardinalis cardinalis SW M 44.03 Mourning Dove Zenaida macroura SW P 38.74 Eastern Towhee Pipilo erythrophthalmus alleni SW G 30.93 Carolina Wren Thryothorus ludovicianus SW P 25.49 Eastern Meadowlark Sturnella magna SW P 24.04 Common Yellowthroat Geothylpis trichas SW G 13.18 White-eyed Vireo Vireo griseus SW G 12.36 European Starling Sturnus vulgaris SW P 11.64 House Sparrow Passer domesticus SW P 9.74 Northern Parula Parula americana PN G 6.40 Pine Warbler Dendroica pinus SW G 6.37 Loggerhead Shrike Lanius ludovicianus SW P 4.60 Brown Thrasher Toxostoma rufum SW P 4.08 Eurasian Collared Dove Streptopelia decaocto SW P 4.07 Summer Tanager Piranga rubra PN G 3.92 Eastern Bluebird Sialia sialis SW P 3.76 Carolina Chickadee Poecile carolinensis N P 3.52 Indigo Bunting Passerina cyanea N G 3.42 Blue Grosbeak Guiraca caerulea N GM 3.26 Orchard Oriole Icterus spurious N G 2.99 Blue-gray Gnatcatcher Polioptila caerulea SW GM 2.91 Eastern Kingbird Tyrannus tyrannus SW P 2.79 Yellow-breasted Chat Icteria virens N G 2.35 100

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101 Table 4-1. Continued. Common Name Scientific Name Florida Distribution Suitability Relative Abundance Red-eyed Vireo Vireo olivaceus PN G 1.82 Bachmans Sparrow Aimophila aestivalis SW GM 1.62 Wood Thrush Hylocichla mustelina N G 1.39 Prothonotary Warbler Protonotaria citrea PN G 1.30 Yellow-throated Vireo Vireo flavifrons N G 1.19 Barn Swallow Hirundo rustica PN P 1.16 House Finch Carpodacus mexicanus PN P 0.91 Hooded Warbler Wilsonia citrina N G 0.85 Florida Prairie Warbler Dendroica discolor paludicola S GM 0.84 Acadian Flycatcher Empidonax virescens N G 0.73 Yellow-throated Warbler Dendroica dominica N G 0.48 Eastern Wood-Pewee Contopus virens N G 0.40 Field Sparrow Spizella pusilla N M 0.27 Painted Bunting Passerina ciris N G 0.17 Kentucky Warbler Oporornis formosus N G 0.13 Swainsons Warbler Limnothylpis swainsonii N G 0.10 Gray Catbird Dumetella carolinensis PN P 0.04 Yellow Warbler Dendroica petechia S GP 0.01 American Robin Turdus migratorius N P <0.01 Eastern Phoebe Sayornis phoebe N G <0.01 Louisiana Waterthrush Seiurus motacilla N G <0.01 Worm-eating Warbler Helmitheros vermivorus N G <0.01 American Redstart Setophaga ruticilla N G <0.01 Grasshopper Sparrow Ammodramus savannarum S G <0.01 Chipping Sparrow Spizella passerina N GM <0.01

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102 APPENDIX A STUDY SITE DESCRIPTION AN D HOST SPECIES SURVEYED These tables include general descriptions of study sites (Table A-1) and the number of nests and family groups of host species monitored at each site (Table A-2) during Brown-headed Cowbird parasitism surveys in north-cen tral Florida from March-August, 2004-2006. Approximate area (hectares) of each site includes only locations where field work was conducted. Coordinates represent the approxima te center of each field site. Minimum and maximum distances (meters) from nests at each site to cowbird feeding areas (livestock pastures) were determined using plant community and landcover GIS data (Kautz et al. 1993) and satellite imagery. Distance values in parentheses are to urban areas that provide cowbird feeding opportunities. Gainesville Public Areas include city parks, cemeteries, and parking lots and Gainesville Suburban Areas include private homes and neighborhoods in the city of Gainesville, Florida. General habitat categor ies where nests were located at each site were determined using GIS vegetation data. Habitat categories generally follow Kautz et al. (2007) and are 1 = Scrub, 2 = Sandhill, 3 = Dry Prairie, 4 = Upland Forest, 5 = Pineland, 6 = Freshw ater Marsh, 7 = Shrub Swamp, 8 = Forested Wetland, 9 = Shrub and Br ushland, 10 = Grassland or Pasture, 11 = Disturbed/Transitional, 12 = Urban/Developed.

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Table A-1. Description of study site s surveyed for cowbird parasitism in north-central Florida, 2004-2006. Site Area Latitude Longitude Minimum Distance Maximum Distance Habitats Bolen Bluff State Park 160 29N 82W 200 1000 4, 7, 10 Gainesville Public Areas 400 29 N 82W (0) (50-100) 4, 5, 9, 10, 12 Gainesville Suburban Areas 300 29 07N 82W (0) (50-100) 4, 5, 12 Gum Root Nature Park 124 29 40N 82W <100 650 4, 5, 6, 8 Loblolly Woods Nature Park 14 29N 82W <100 500 4, 5, 6, 8 Myakka River State Park 1880 27 37N 82 W 200 4500 1, 3, 4, 5, 6, 7, 8 Newnans Lake 44 29N 82W 900 1400 6, 7, 8 Ordway-Swisher Biological Station 1400 29N 81W 1500 4400 2, 4, 5, 6, 7, 8, 9, 10, 11 Paynes Prairie State Preserve 1000 29N 82W <100 1400 4, 5, 6, 7, 8, 10 San Felasco Hammock State Park 3800 29N 82W 200 2200 2, 4, 5, 6, 8, 9, 10 Split Rock/Sugarfoot Nature Preserves 122 29N 82W 800 1500 4, 5, 6, 7, 8 University of Florida Campus 670 2947N 82W (50) (200) 4, 5, 6, 8, 9, 10, 12 USDA National Wildlife Research Center 11 29N 82W (<50) (250) 4, 5, 9 103

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Table A-2. Number of nests and family groups of host species monitored at study sites in north -central Florida, 2004-2006. Study Site Species Total Nests Nests Parasitized Total Families Families Parasitized Bolen Bluff State Park White-eyed Vireo 1 2 Blue-Gray Gnatcatcher 1 Brown Thrasher 1 Northern Parula 5 3 Eastern Towhee 1 Northern Cardinal 26 1 14 Gainesville Public Areas Mourning Dove 2 Loggerhead Shrike 4 1 Carolina Wren 1 Eastern Bluebird 2 1 Northern Mockingbird 113 6 Brown Thrasher 13 5 Northern Cardinal 9 6 Gainesville Suburban Areas Mourning Dove 8 Eastern Tufted Titmouse 1 Carolina Wren 2 Northern Mockingbird 131 14 Brown Thrasher 8 1 Northern Parula 1 1 Summer Tanager 2 2 Northern Cardinal 5 12 1 Gum Root Nature Park White-eyed Vireo 1 Red-eyed Vireo 1 104

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Table A-2. Continued. Study Site Species Total Nests Nests Parasitized Total Families Families Parasitized Northern Mockingbird 1 Brown Thrasher 1 Northern Cardinal 5 Loblolly Woods Nature Park Carolina Wren 1 1 Northern Parula 1 Northern Cardinal 4 1 Myakka River State Park Mourning Dove 19 Great-crested Flycatcher 12 Blue Jay 8 Eastern Bluebird 7 Northern Mockingbird 18 Common Yellowthroat 2 1 Eastern Towhee 43 3 Bachmans Sparrow 11 1 Eastern Meadowlark 3 Newnans Lake Red-winged Blackbird 61 Ordway-Swisher Biological Station Mourning Dove 2 White-eyed Vireo 4 8 Yellow-throated Vireo 5 Blue Jay 1 Eastern Tufted Titmouse 1 Blue-Gray Gnatcatcher 2 5 1 Eastern Bluebird 42 1 105

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Table A-2. Continued. Study Site Species Total Nests Nests Parasitized Total Families Families Parasitized Northern Mockingbird 1 Brown Thrasher 6 Northern Parula 4 Pine Warbler 3 13 1 Common Yellowthroat 1 Summer Tanager 3 Eastern Towhee 6 1 Northern Cardinal 32 8 Paynes Prairie State Preserve Mourning Dove 3 White-eyed Vireo 3 7 Carolina Chickadee 1 Carolina Wren 3 Blue-gray Gnatcatcher 2 1 3 1 Brown Thrasher 5 Northern Parula 8 3 Pine Warbler 1 Prothonotary Warbler 1 Common Yellowthroat 1 Summer Tanager 1 1 Eastern Towhee 5 2 Northern Cardinal 20 14 Blue Grosbeak 2 Indigo Bunting 1 106

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Table A-2. Continued. Study Site Species Total Nests Nests Parasitized Total Families Families Parasitized Red-winged Blackbird 9 Orchard Oriole 1 San Felasco Hammock State Park Mourning Dove 2 Acadian Flycatcher 14 9 White-eyed Vireo 4 2 1 Red-eyed Vireo 3 1 Carolina Wren 2 2 Blue-gray Gnatcatcher 1 Eastern Bluebird 1 Wood Thrush 3 Northern Parula 3 Pine Warbler 1 Hooded Warbler 5 1 7 Summer Tanager 2 1 Eastern Towhee 1 Northern Cardinal 27 11 Blue Grosbeak 2 Common Grackle 1 Split Rock/Sugarfoot Preserves Mourning Dove 1 White-eyed Vireo 5 7 Yellow-throated Vireo 1 1 Red-eyed Vireo 1 Carolina Wren 2 4 107

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Table A-2. Continued. Study Site Species Total Nests Nests Parasitized Total Families Families Parasitized Blue-gray Gnatcatcher 1 2 1 Northern Mockingbird 1 Northern Parula 3 Common Yellowthroat 1 Yellow-breasted Chat 1 Northern Cardinal 64 1 14 Blue Grosbeak 3 1 Indigo Bunting 1 3 University of Florida Campus Eurasian Collared Dove 3 Mourning Dove 14 Loggerhead Shrike 1 White-eyed Vireo 1 Carolina Wren 2 1 Northern Mockingbird 130 5 Brown Thrasher 41 Northern Cardinal 44 4 11 1 Red-winged Blackbird 11 Common Grackle 1 Boat-tailed Grackle 2 Orchard Oriole 2 2 USDA Wildlife Research Center Mourning Dove 1 Carolina Wren 1 Northern Mockingbird 3 1 3 108

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109 Table A-2. Continued. Study Site Species Total Nests Nests Parasitized Total Families Families Parasitized Brown Thrasher 5 1 Eastern Towhee 1 9 Northern Cardinal 7 4

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BIOGRAPHICAL SKETCH Matthew J. Reetz was born on July 30, 1973 in Willoughby Hills, Ohio. From 1991 to 1995, he attended the Univers ity of Illinois in Urbana Champaign, earning a bachelors degree in biology with concentration in ecology, ethol ogy and evolution. Repeated field seasons working in southern Illinois on projects dea ling with songbird nesting biology encouraged the pursuit of a higher degree. In the fall of 1997, he entered the University of Floridas graduate program to study ecology and ornithology and ea rned a Master of Science degree from the Department of Wildlife Ecology and Conservati on in August of 2000. Matthew earned an Alumni Fellowship which facilitated dissertati on research resulting in a doctorate in wildlife ecology and conservation in May 2008. In Augus t 2008, Matthew will be an Assistant Professor in the Natural Sciences Division at Franklin College where he will teach biology, mentor undergraduate projects, and conduct research in community ecology. 129