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Symbiosis between Long Legged Wading Birds (Ciconiiformes) and Alligators (Alligator mississippiensis)? Testing the 'Ne...

Permanent Link: http://ufdc.ufl.edu/UFE0043815/00001

Material Information

Title: Symbiosis between Long Legged Wading Birds (Ciconiiformes) and Alligators (Alligator mississippiensis)? Testing the 'Nest Protector' Hypothesis
Physical Description: 1 online resource (67 p.)
Language: english
Creator: Burtner, Brittany F
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: alligator -- everglades -- predation -- waterbird
Interdisciplinary Ecology -- Dissertations, Academic -- UF
Genre: Interdisciplinary Ecology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Wading birds (Ciconiiformes) appear to preferentially nest above alligators and alligator habitat. Alligators could benefit nesting birds by deterring mammalian predators. Chicks or food dropped from the bird nests could provide alligators with food. I tested selected predictions of this hypothesis using small willow-dominated colonies of little blue herons (Egretta caerulea), tricolored herons (Egretta tricolor), and snowy egrets (Egretta thula) in the central Everglades as experimental units. I experimentally manipulated apparent densities of alligators and conspecific birds using alligator and white bird decoys to determine if wading birds were attracted to alligators via visual cues. Egretta herons showed a strong preference for sites with both alligator and bird decoys in 2010 (?2(3,N=45)=17.133, p=0.001) and 2011 (?2(3,N=261)=72.452, p=0.0001). Using helicopter surveys, I found that alligators and wading birds associated significantly more often than expected by chance (p=0.007667, Fisher's Exact Test, N=40). I deployed predator tracking stations and found that raccoon presence in the Everglades was strongly dependent upon low water levels. Utilizing throughfall traps, I estimated that a colony of 50 pairs has the potential to drop 102 grams of food over a 60-day nesting cycle. This may be nutritionally important to alligators, particularly during the dry season when movements may be limited and food is harder to find. My evidence suggests that there is a mutualism between Egretta herons and alligators; herons are attracted to nest near alligators, and alligators receive nontrivial food benefits from nesting birds.
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.
Statement of Responsibility: by Brittany F Burtner.
Thesis: Thesis (M.S.)--University of Florida, 2011.
Local: Adviser: Frederick, Peter C.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043815:00001

Permanent Link: http://ufdc.ufl.edu/UFE0043815/00001

Material Information

Title: Symbiosis between Long Legged Wading Birds (Ciconiiformes) and Alligators (Alligator mississippiensis)? Testing the 'Nest Protector' Hypothesis
Physical Description: 1 online resource (67 p.)
Language: english
Creator: Burtner, Brittany F
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: alligator -- everglades -- predation -- waterbird
Interdisciplinary Ecology -- Dissertations, Academic -- UF
Genre: Interdisciplinary Ecology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Wading birds (Ciconiiformes) appear to preferentially nest above alligators and alligator habitat. Alligators could benefit nesting birds by deterring mammalian predators. Chicks or food dropped from the bird nests could provide alligators with food. I tested selected predictions of this hypothesis using small willow-dominated colonies of little blue herons (Egretta caerulea), tricolored herons (Egretta tricolor), and snowy egrets (Egretta thula) in the central Everglades as experimental units. I experimentally manipulated apparent densities of alligators and conspecific birds using alligator and white bird decoys to determine if wading birds were attracted to alligators via visual cues. Egretta herons showed a strong preference for sites with both alligator and bird decoys in 2010 (?2(3,N=45)=17.133, p=0.001) and 2011 (?2(3,N=261)=72.452, p=0.0001). Using helicopter surveys, I found that alligators and wading birds associated significantly more often than expected by chance (p=0.007667, Fisher's Exact Test, N=40). I deployed predator tracking stations and found that raccoon presence in the Everglades was strongly dependent upon low water levels. Utilizing throughfall traps, I estimated that a colony of 50 pairs has the potential to drop 102 grams of food over a 60-day nesting cycle. This may be nutritionally important to alligators, particularly during the dry season when movements may be limited and food is harder to find. My evidence suggests that there is a mutualism between Egretta herons and alligators; herons are attracted to nest near alligators, and alligators receive nontrivial food benefits from nesting birds.
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.
Statement of Responsibility: by Brittany F Burtner.
Thesis: Thesis (M.S.)--University of Florida, 2011.
Local: Adviser: Frederick, Peter C.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043815:00001


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1 SYMBIOSIS BETWEEN LONG LEGGED WADING BIRDS (Ciconiiformes) AND ALLIGATORS ( Alligator mississippiensis )? TESTING THE NEST PROTECTOR HYPOTHESIS By BRITTANY F. BURTNER A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVE RSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2011

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2 2011 Brittany F. Burtner

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3 To my parents, for encouraging my imagination

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4 ACKNOWLEDGMENTS I would like to thank my advisor and committee chair, Peter Fredrick for his invaluable guidance and support throughout this project. I am indebted to my committee members Rob Fletcher and Kent Vliet for their thoughtful input to this manuscript and to my project along the way. I owe Frank Mazzotti and Brian Jeffery great thanks for providing much needed equipment and support. I owe great thanks to many people who helped me in the field including: Eric Fishel, Lindsey Garner, Erin Posthumous, Melissa Schlothan, John Simon, Nick Vi tale and Chris Winchester. Furthermore, I thank my lab mates Jason Fidorra and Louise Venne for their assistance in the field, muchwelcomed suggestions, and commiseration. Greg Smith not only provided assistance in the field, he produced a great number of the alligator decoys and modified flamingo bird decoys for this project. Without his assistance, I would probably still be making decoys. This work was supported by the United States Army Corps of Engineers and an assistantship from the School of Natur al Resources and the Environment. Finally, I thank my family and friends for unflagging love and support.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES ............................................................................................................ 7 LIST OF FIGURES .......................................................................................................... 8 ABSTRACT ..................................................................................................................... 9 CHAPTER 1 D O HERONS CHOOSE NESTING SITE S BASED ON ATTRACTION TO ALLIGATORS ? ....................................................................................................... 11 Introduction ............................................................................................................. 11 Methods .................................................................................................................. 15 Study Area ........................................................................................................ 15 Wading Bird Alligator Association Survey ...................................................... 16 Decoy Experiment Design ................................................................................ 16 Detecting Responses to Decoys ...................................................................... 18 Statistical Analysis ............................................................................................ 19 Results .................................................................................................................... 19 Wading Bird Alligator Association .................................................................... 19 Bird Response to Decoys ................................................................................. 20 Discussion .............................................................................................................. 21 2 DO WADING BIRDS BENEFIT FROM NESTING NEAR ALLIGATORS? .............. 36 Introduction ............................................................................................................. 36 Methods .................................................................................................................. 39 Predator Distribution ......................................................................................... 39 Survival rates of nestlings and fledgling herons ............................................... 41 Results .................................................................................................................... 41 Predator Distribution ......................................................................................... 41 Water Depth ..................................................................................................... 42 Color Banded Birds .......................................................................................... 43 Discussion .............................................................................................................. 43 3 DO ALLIGATORS BENEFIT FROM ASSOCIATING WITH N ESTING WADING BIRDS? ................................................................................................................... 50 Introduction ............................................................................................................. 50 Methods .................................................................................................................. 51 Results .................................................................................................................... 53 Discussion .............................................................................................................. 54

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6 LIST OF REFERENCES ............................................................................................... 61 BIOGRAPHICAL SKETCH ............................................................................................ 67

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7 LIST OF TABLES T able page 1 1. Pairwise comparisons of numbers of birds responding to decoy treatments in 2010. .................................................................................................................. 33 1 2. Pairwise comparisons of numbers of birds responding to decoy treatments in 2011. .................................................................................................................. 33 3 1. Locations of colonies in WCA 3A where throughfall traps were placed in 2010 and 2011. ........................................................................................................... 57 3 2. The fate of nests with throughfall traps in 2010 and 2011. .................................... 59 3 3. The number, total mass, grams per nest, and percent of total mass of all food it ems recovered from throughfall traps beneath 20 Egretta heron nests ........... 59 3 4. The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall tra ps beneath 37 Egretta heron nests. ........... 59 3 5. The potential fresh food mass input from a 50nest Egretta heron colony in 2010 and 2011 after 20, 40, and 60 days. .......................................................... 60

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8 LIST OF FIGURES Figure page 1 1. Map of s tudy area in south Florida, USA .............................................................. 24 1 2. Aerial photograph of an alli gator hole and small willow dominated tree island in WCA 3A ............................................................................................................ 25 1 3. Photograph of an alligator decoy in the field. ........................................................ 26 1 4. Ph otograph of a great egret decoy ....................................................................... 27 1 5. Photograph of an altered flamingo decoy in the field. ........................................... 28 1 6. Photograph of a bird decoy array at midcanopy height in willow trees surrounding an alligator hole. ............................................................................. 29 1 7. Comparison of alligator and Egretta heron distribution on small willow dominated tree islands in the Ever glades of Florida. .......................................... 30 1 8. Totals of maximum number of Egretta herons per treatment site that responded to each decoy treatment; 2010. ........................................................ 31 1 9. Totals of maximum number of Egretta herons per treatment site that responded to each decoy treatment; 2011. ........................................................ 32 1 10. Numbers of islands per treatment group that Egretta herons nested on; 2010. ... 34 1 11. Numbers of islands per treatment group that Egretta herons nested on; 2011. ... 35 2 1. Satellite image (adapted from Google Earth v.6.1; 2011) of predator tracking station locations in 2010 (diamonds) and 2011 (circles) .................................... 46 2 2. Stages at 3 4 gauge in WCA 3A from January 2010 until July 2011. .................... 47 2 3. Cumulative percentage per day of color banded Egretta herons resighted, dead, or not seen. ............................................................................................... 48 2 4. Raccoon scat contai ning feathers of young herons.. ............................................. 49 3 1. Throughfall trap set up below an Egretta heron nest with three eggs. .................. 58

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SYMBIOSIS BETWEEN LONG LEGGED WADING BIRDS (Ciconiifo rmes) AND ALLIGATORS ( Alligator mississippiensis )? TESTING THE NEST PROTECTOR HYPOTHESIS By Brittany F. Burtner December 2011 Chair: Peter Frederick Major: Interdisciplinary Ecology Wading birds (Ciconiiformes) appear to preferentially nest above all igators and alligator habitat. Alligators could benefit nesting birds by deterring mammalian predators. Chicks or food dropped from the bird nests could provide alligators with food. I tested selected predictions of this hypothesis using small willow dominated colonies of little blue herons ( Egretta caerulea), tricolored herons ( Egretta tricolor ), and snowy egrets ( Egretta thula) in the central Everglades as experimental units. I experimentally manipulated apparent densities of alligators and conspecif ic birds using alligator and white bird decoys to determine if wading birds were attracted to alligators via visual cues. Egretta herons showed a strong preference for sites with both alligator and bird decoys in 2(3, N=45)=17.133, p=0.001) and 2011 ( 2(3, N=261)=72.452, p=0.0001) Using helicopter surveys, I found that alligators and wading birds associated significantly more often than expected by chance (p=0.007667, Fishers Exact Test, N=40). I deployed predator tracking stations and found that raccoon presence in the Everglades was strongly dependent upon low water levels. Utilizing throughfall traps I

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10 estimated that a colony of 50 pairs has the potential to drop 102 grams of food over a 6 0 day nesting cycle. This may be nutritionally import ant to alligators, particularly during the dry season when movements may be limited and food is harder to find. My evidence suggests that there is a mutualism between Egretta herons and alligators; herons are attracted to nest near alligators, and alligat ors receive nontrivial fo od benefits from nesting birds.

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11 CHAPTER 1 DO HERONS CHOOSE NES TING SITES BASED ON ATTRACTION TO ALLIGATORS? Introduction Animals are constantly faced with situations where they need to make decisions without complete information. Other individuals also encounter similar circumstances, allowing one individual to observe the decision made and the consequences of the decision for another individual. This interaction, termed social information, can be conveyed intentionally or inadv ertently by the behavior or presence of individuals. Historically, bird habitat selection theory has focused mainly on physical and environmental factors including habitat structure, patchiness, and food availability or biotic factors such as predation and competition (see Jones 2001 for a review). Recent research has begun to explore habitat select ion based on social information ( Danchin et al. 2004). Social information use by an individual is a process consisting of (1) an observable state or event, (2) the observation of that state or event, (3) a decision manifested as an altered action, and (4) the consequences of that action (Seppanen et al. 2007). Initial research about social information focused on conspecific attraction and was shown to occur across a wide variety of taxa such as Norway rats ( Rattus norvegicus ) (Laland & Plotkin 1992 ) spotless starlings ( Sturnus unicolor ) ( Parejo et al. 2008) and numerous species of marine invertebrate larvae (Burke 1986) In recent years, use of information between heterospecific s has garnered increased attention in the literature (see Seppanen et al. 2007 for a review). Although heterospecific information has been shown to occur within and between a number of taxa, many studies have focused on interactions b etween avian species Monkkonen et al. (1990) suggest that migrant birds

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12 in Finland use resident birds as indicators of safe and/ or productive breeding sites in unpredictable circumstances. Forsman et al. (1998) found that migrant birds use resident tit species ( Parus spp.) as indicators of a profitable breeding patch. Furthermore, Forsman et al. (2002) demonstrated that pied flycatchers ( Ficedula hypoleuca) were attracted to and accrued fitness benefits from resident titmice ( Parus spp.). Least flycat chers ( Empidonax minimus ) have been shown to use the vocal cues of a competitor, the American redstart ( Setophaga ruticilla), as well as those of conspecifics when making settlement decisions (Fletcher 2007). Additionally, Fletcher (2008) found that the a ddition of least flycatcher vocal cues, regardless of actual presence of the bird, reduces small bodied migratory bird species richness by up to 30% through the lack of local colonization. Forsman et al. (2008) found that collared flycatchers ( Ficedula al bicollis ) used the density of Parus spp. to assess overall habitat quality and make offspring investment decisions. Nest protection association, an interaction involving the transmission of social information, has been described between bird species and between birds and other taxa such as insects and reptiles. Protective nesting associations occur when one species places its nest by active choice near that of another, more formidable species that drives away predators of the first species simply by defending its own territory. Descriptive studies of protective nesting associations are common (e.g. Myers 1929; Moreau 1936; Durango 1949; Chisholm 1952; Grimes 1973 ; Uchida 1986; Richardson & Bolen 1999); however most have not examined whether the associati on occurs by active choice or if there is an actual benefit to association (Haemig 2001 ).

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13 Crocodilians are good potential bird nest protectors because they prey on bird nest predators such as snakes and mammals (Bondavalli & Ulanowicz 1999) and aggressivel y defend their own nests (Kushlan & Kushlan 1980). American alligators ( Alligator mississippiensis ) have been anecdotally reported to make islands in South Carolina predator secure for nesting boat tailed grackles ( Quiscalus major ) (Post & Seals 1991; P ost 1998a), least bitterns ( Ixobrychus exilis) (Post 1998b), and common moorhens ( Gallinula chloropus ) (Post & Seals 2000) by deterring mammalian predators such as raccoons. Robinson (1985) suggested that black caimans ( Melanosuchus niger ) and yellow rump ed caciques ( Cacicus cela ) may have a similar association. Biologists at the St. Augustine Alligator Farm ( St. Johns Co F lorida USA) have noted that wading birds began to nest in the trees overhanging the water of their Alligator Swamp exhibit after its creation and that the birds are apparently protected from mammalian predators (K. Vliet, personal communication). While these studies noted or implied an association, they provided no formal test of association. In Ghana, Hudgens (1997) found that bluebilled malimbes ( Malimbus nitens ) nested much closer to African dwarf crocodile ( Osteolamus tetraspis ) nest sites than would be expected if they nested at random. Hudgens demonstrated that nesting was nonrandom with respect to crocodiles, although the cos ts and benefits to the birds and the reptiles were not quantified. Long legged w ading birds ( herons, eg rets, ibises, storks, spoonbills; Ciconiiformes) nesting near alligators or alligator habitat in the Everglades of Florida (USA) may present a good oppor tunity to study the potential symbiosis between the two taxa. Anecdotal observations suggest that wading birds preferentially nest above

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14 alligators and alligator habitat Birds, mammals, and snakes commonly prey on wading bird nests (Frederick & Collopy 1989, Coulter & Bryan 1995) Although Ciconiiformes are colonial nesters, there is almost no group or individual nest protection behavior (Rodgers 1987). Access to the colony by only a few predators can cause significant destruction to nest contents and facilitate nest abandonment (Rodgers 1987). Wading birds thus appear to avoid nest predation by selecting inaccessible nesting sites, such as islands surrounded by water and inhabited by crocodilians (Frederick & C o llopy 1989). I report here on an experim ental examination of the relationship between the presence of alligators and nesting colonies of three species of wading birds: little blue herons ( Egretta caerulea), tricolored herons ( Egretta tricolor ), and snowy egrets ( Egretta thula) (collectively her eafter, Egretta herons ) in the Everglades. The central Everglades is a shallowly inundated (0 3 m) freshwater marsh with slightly elevated sites called tree islands (Loveless 1959). Here, wading birds tend to nest in aggregations of two to 100 individuals on islands dominated by willow ( Salix caroliniana) ( Frederick 1995). These willow heads often are associated with an alligator hole; a small pond created and maintained by American alligators (Mazzotti & Brandt 1994). In fact, many small willow hea ds have probably formed as a result of the action of alligators that dig holes during the dry season, creating slightly elevated areas surrounding the hole by displacing sediment (Palmer & Mazzotti 2004) I hypothesized that Egretta herons choose to asso ciate with alligators Not all willow heads are occupied by herons, and not all willow heads have alligators associated with them. I tested the prediction that alligators and wading birds would

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15 associate more often than expected by chance. In addition, I experimentally manipulated apparent densities of alligators and conspecific birds using alligator and bird decoys to determine if wading birds were attracted to alligators via visual cues. I used maximum numbers of Egretta herons nesting as a response v ariable to four differe nt types of decoy treatments. Methods Study Area Study sites were located in Water Conservation Area3A (WCA 3A) of Dade and Broward Counties, Florida. WCA 3A is a large ( cf 400 km2) impounded area of seasonally flooded sawgrass and wet prairie dotted with small tree islands (Figure 1 1) Although there are numerous types of tree islands in the Everglades (Loveless 1959) Egretta herons have been shown to nest almost exclusively on willow dominated tree islands (Frederick & Collopy 1988, 1989). I selected 40 small willowhead tree islands, all of which had an alligator hole ( Figure 12 ). Based on systematic annual survey information, all sites had been unoccupied by nesting wading birds for at least 16 consecutive years (P. Frederick unpublished database). Islands ranged from approximately 450 square meters to 5 000 square meters (mean = 1 200 square meters). This range of island sizes is typical of Egretta heron nesting habitat in Water Conservation Area 3A (P. Frederick unpubl ished database). All sites were examined on the ground to confirm willow tree structure appropriate for wading bird nesting and alligator hole presence. Only sites with alligator holes were chosen because the holes are a visible indication of regular all igator presence.

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16 Wading Bird Alligator Association Survey I surveyed 40 willow dominated tree islands using a helicopter ( Bell 206B JetRanger III ) on April 27, 2011 from 0715 until 0900. The survey was timed to coincide with the point in the small wading bird nesting cycle where most eggs had hatched, but no young had fledged. This increased our chances of at least one adult being present at the nest to brood young. The helicopter hovered at approximately 125 meters after approaching the island from the east. Two observers sat on the left side of the aircraft. One observer counted, identified, and recorded all small wading birds flushed from the island. The other observer took photographs through the window using a Canon EOS 50D with a 28 135mm ima ge stabilizing lens. Both observers searched for alligators or recent tracks in the mud. Due to low water conditions in WCA 3A at the time, alligator holes were the only areas in the marsh with standing water. Alligator tracks to and from their holes were readily visible in the surrounding mud. I accepted the count of birds flushed from the island plus birds seen roosting in tree tops in the photos as the maximum count for each island and species. Alligators were deemed present when an alligator or fr esh tracks leading to the alligator hole were visible from the air or in the photographs. I used Fishers Exact Test with an alpha of 0.05 to test for an association between presenceabsence of alligators and wading birds. Decoy Experiment Design I manipulated a pparent densities of alligators and wading birds using decoys with tree islands as the unit of treatment. I used four different treatments: 10 alligator decoys, 20 bird decoys, 10 alligator plus 20 bird decoys, and no decoys. Each treatment was replicated 10 times one per tree island. Decoy treatments were

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17 randomly assigned to tree island sites Live alligators were present and free to move throughout the study area for the duration of the study, therefore the treatments with alligator decoys were intended to create a supernormal stimulus. Alligator decoys were cast using polyurethane spray insulation (Foam It Green, Guardian Energy Technologies Inc ., Riverwoods, Illinois ) from a silicone mold of a 2.3 meter harvested alligator. The mold w as for the top half of the alligator, making the decoys look as if they were floating at the surface of the water. The alligator decoys were larger than 1.8 meters, the minimum size at which alligators begin to breed and defend territories. The size of d ecoy used ensured that the decoys would represent a dominant individual that could both occupy a gator hole and be a significant predation threat to a raccoon. Alligator d ecoys were painted in realistic colors using latex paint applied via a pneumatic spray gun ( Figure 13). Bird decoy arrays presented at each island were a mixture of 3 commercially available great egret decoys ( Flambeau, Inc ., Middlefield, Ohio Figure 14 ) and 15 modified lawn flamingo decoys (Garden Plast, Inc ., Accra, Ghana ) Flamin go decoys were painted white with a pneumatic spray gun, and their heads replaced by a polyurethanecast piece similar to a small, white wading birds head and bill structure (Figure 1 5). Modified flamingo decoys were used due to their cost effectiveness and proven ability to initially attract small wading birds (Dusi 1985, Crozier & Gawlik 2003). Bird d ecoys were placed at midcanopy height in willow trees surrounding the alligator hole at each treatment site in early March 2010, 1 3 weeks prior to the initiation of breeding by Egretta herons (Figure 16). Egretta herons were not observed in WCA 3A before this time, so it is unlikely that the birds chose breeding locations prior

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18 to decoy installation. Alligator decoys were positioned in the water at t he islandopenwater interface surrounding tree islands. Islands without decoys were entered and explored in the same fashion as other treatments. All decoys were removed from the Everglades by July 2010, after the wading bird breeding season. I repeated the study in 2011, installing the decoys in midFebruary 2011 prior to Egretta heron arrival to WCA 3A In anticipation of impending low water levels, I replaced the 20 northernmost treatment sites with new ones farther south, in deeper water areas of W CA 3A in an attempt to ensure that all sites would retain surface water during the entire breeding season. All 40 sites were randomly reassigned decoy treatments. No site in 2011 had the same decoy treatment as it did in 2010. Detecting Responses to Decoys Treatment sites were surveyed using airboats, which provided access to all wetted areas of WCA 3A. We approached each site as closely as possible with the airboat to flush any nesting birds. All ciconiiform birds were identified and counted (Frederic k et al. 1996). Nests visible from the exterior of the island were also noted. Survey frequency varied slightly from 2010 to 2011 due to manpower availability and environmental conditions. 2010 was an abnormally wet year in the Everglades and all sites were accessible for the entire study period. Three survey periods were conducted: early breeding (late March early April), mid breeding (late April early May), and late breeding (late May early June). Maximum counts of each species at each colony d uring these surveys were taken as the response variable. Drought and low water conditions in 2011 limited accessibility by airboat as the season progressed. Surveys were conducted bi weekly beginning the first week of March and continued until the last w eek of April, when only two of the 40 sites were accessible via airboat. The helicopter

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19 survey (above) was used as a final survey of avian response to decoy treatments in 2011. Statistical Analysis I used a Chi squared test of equal proportions to detect departures from an even distribution of the maximum number of birds nesting at each site by treatment group for both 2010 and 2011. Years were compared separately due to the difference in survey timing and regularity in 2010 versus 2011. Alpha was equal to 0.05 for each analysis To further examine bird response, I performed post hoc pairwise comparisons between all treatments. A Bonferroni adjustment was applied to maintain the familywise error rate. I divided alpha (0.05) by the number of pairwise comparisons (6), to obtain the adjusted alpha, 0.008333. Additionally, I also used presence rather than number of birds as a response variable to the experimental treatments. I compared the number of islands per treatment group that birds nest ed on using a Chisquared test of equal proportions to detect departures from an even distribution of bird presence. Results Wading BirdAlligator Association Alligators and wading birds were found in association more often than expected by chance (p=0.002176, N=40, Fi shers Exact Test) (Figure 17). The two groups were found together at 60% of islands surveyed. Alligators were found alone at 20% of the sites. Birds were found without alligators at 2.5% of the islands. Seven sites were occupied by neither birds nor alligators (17.5%). It is important to note that Fishers Exact Test assumes that 50% of alligator holes will be unoccupied by alligators. Campbell & Mazzotti (2004) found that in WCA 3, 44 % of alligator holes ( similar to

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20 those in my study ) were un occupi ed by alligators during the spring. W ading birds and alligators we re clearly associating more often than expected by chance. Bird Response to Decoys The distribution of nesting herons relative to decoy treatment was significantly different from random in 2( 3, N=45)=17.133, p=0.001) (Figure 1 8) and 2011 ( 2(3, N=261)=72.452, p=0.0001) (Figure 1 9 ). To make these two experiments more directly comparable, I subsampled the 2011 bird response data to mimic the 3 period sampling regime used in 2010. I fo und that the distribution of Egretta herons relative to decoy treatment was still significantly different than random ( 2(3, N=2 30)= 58.383, p=0.0001). In 2010, pairwise comparisons (Table 11) showed that birds responded more positively to the alligator+bird treatment than to the birds only treatment 2(1, N= 31)= 7.258, p= 0.0070 6) or the no2(1, N= 28)= 11.571, p= 0.000670). There was no significant difference between the numbers of birds attracted to the alligator+bird treatment and the al ligator 2(1, N= 32)= 6.125, p= 0.0133). Furthermore, there was no significant difference between the numbers of birds attracted to the birds only treatment, the alligators only treatment, and the no decoy treatment. In 2011, pairwise compar isons (Table 12) showed the alligator+bird treatment response was higher than all other groups (p<.0001). There was no significant difference between birds attracted to the bird decoy treatment and the no decoy treatment ( 2(1, N= 167)= 1.012, p= 0.2636). Birds were significantly less attracted to the alligator decoy treatment than the bird decoy ( 2(1, N= 131)= 18.328, p=0.0001) or no decoy ( 2(1, N= 118)= 10.983, p=0.007) treatments in 2011

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21 The number of islands per treatment group that birds nested on was n ot 2(3, N=4 0 )= 0.200 p= 0.9776) (Figure 1 10) or 2011 ( 2(3, N= 40)= 2.000 p= 0. 5724) (Figure 1 11) Discussion My results from both the helicopter survey and the decoy experiment strongly suggest that birds and alligators occur together in a nonrandom fashion, and that birds are attracted to a visual display of both alligators and birds. I interpret the decoy experiment to mean that wading birds are attracted to the visual stimulus of alligators, but only when ot her wading birds are present. Wading birds were clearly not attracted to alligators nor wading birds when presented by themselves. The combination of the two decoy types appeared to be necessary to create an attractive environment for nesting in novel lo cations Wading birds are often temporarily attracted to white decoys of various kinds in feeding areas (Crozier & Gawlik 2003, Heath & Frederick 2003). Unlike many seabirds Fratercula arctica, Kress & Nettleship 1988; least terns Ste rna antillarum Massey 1981; Kotliar & Burger 1984; Burger 1989; Arctic terns Sterna paradisaea, Kress 1983; and common terns Sterna hirundo, Dunlop et al. 1991; Blokpoel et al. 1997), there is no evidence that wading birds can be induced to nest in novel locations through the use of conspecific decoys alone. Dusi (1985) attempted to attract little blue herons and cattle egrets ( Bubulcus ibis ) to a small island with homemade decoys and recordings of each species. Although seven different species of wading birds visited the island briefly, none nested during the 3year study. I found no difference in nesting response between the no decoy treatment and the birdonly

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22 treatment. Together with the results presented here, this suggests that white decoys alone are insufficient to stimulate Egretta herons to nest in novel locations The alligator decoys alone could have been non attractive to herons for several reasons. I hypothesize that herons may have simply not seen the alligator decoys because the decoys were camouflaged to the point that it was sometimes difficult for humans to detect them. Without the long range stimulus of the highly visible white bird decoys, it is possible that birds never visited the islands and therefore did not assess the alligat or decoys. It is also possible that the form of the alligator alone, without the presence of conspecifics, was not a strong enough signal to incite nesting. T he mechanisms by which protective nesting associations occur have rarely been experimentally addr essed (Haemig 2001). My study demonstrates the existence of a strong association between herons and alligators that probably extends to other crocodilians. The association may come from alligator attraction to birds, or bird attraction to alligators, or both. The fact that wading birds nested without live alligators only 2.5% of the time, and that they were attracted to novel nesting locati ons by the combination of decoy types suggests that the highly mobile birds are probably actively choosing alligator s or their recently occupied habitat to nest in. Furthermore, alligators are known to be highly territorial during the wading bird nesting season, and may not be able to aggregate on wading bird colonies. This suggests, but does not prove, that wading birds are the active choosers in this association. My analysis used numbers of birds as a response variable rather than bird presence on a decoy treatment site, and the result may be driven by a handful of sites that had relatively large avian response However, since the birds are highly social,

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23 aggregative nesters, presence alone may be a strenuous test of the prediction that birds are attracted to alligators This is especially true since the treatment site locations were all places birds had avoided for at least 16 years prior and therefore may have had other physical or biotic features that were less than attractive for nesting The decoy experiment suggested that Egretta herons choose to nest in areas based at least partly on social information f rom both conspecifics (other Egretta herons) and heterospecifics (alligators). This use of dual or multiple social information sources may be similar to the process by which other protective nesting associations between colonial species and nest protector s occur; such as redbreasted geese ( Branta ruficollis ) nesting near peregrine falcons ( Falco peregrinus ) or snowy owls ( Nyctea scandiaca) (Quinn et al. 2003), blue billed malimbes nesting near dwarf crocodiles (Hudgens 1997) and yellow rumped caciques nesting near black caiman (Robinson 1985) Regardless of the specific mechanism by which Egretta herons choose their nesting habitat, my experiment showed that the birds can be attracted to novel nesting sites with the use of a combination of alligator and white wading bird decoys. This combination of decoys shows strong potential as a management tool for moving birds from an undesirable to a desirable location. It is unclear what part of the recognition of social information in nesting birds comes about v ia genetically programmed sign stimuli, or as part of a learning process through experience. In either case, I suggest that recognition of nest habitat quality through inter and intraspecific social cues may have wide applicability to birds.

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24 Figure 1 1. Map of study area in south Florida USA with Water Conservation Area 3A (WCA 3A), highlighted in green. Map courtesy of the U.S. Geological Survey.

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25 Figure 12. Aerial photograph of an alligator hole and small willow dominated tree island in WCA 3A. The alligator hole is the darkest part of the image in the center of the photograph. Well defined alligator tracks leading to the hole are visible in the right hand portion of the photograph. Photo courtesy of author.

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26 Figure 13. Photograph of an alligator decoy in the field. Photo courtesy of author.

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27 Figure 1 4. Photograph of a great egret decoy overlooking an alligator hole, mounted on a willow tree in the field. Photo courtesy of author.

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28 Figure 15. Photograph of an altered flamingo decoy in the field. Photo courtesy of author.

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29 Figure 16. Photograph of a bird decoy array at midcanopy height in willow trees surrounding an alligator hole. Photo courtesy of author.

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30 0 5 10 15 20 25 Gators + / Birds + Gators +/ Birds Gators / Birds + Gators / Birds Number of Sites Group presence (+)/ Absence ( ) Figure 1 7. Comparison of alligator and Egretta heron distribution on small willow dominated tree islands in the Everglades of Florida. Alligators and Egretta herons were found at the same sites more than expected (p=0.002176, N=40, Fishers Exact Test).

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31 Figure 18. Totals of maximum n umber of Egretta herons per treatment site that responded to each decoy treatment ; 2010. Bars with different letters were significantly different Bonferroni test

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32 Figure 19. Totals of maximum number of Egretta herons per treatment site that responded to each decoy treatment; 2011. Bars with different letters were significantly different Bonferroni test

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33 Table 11. Pairwise comparisons of numbers of birds responding to decoy treatments in 2010. Alpha = 0.008333. Decoy treatment 1 Decoy treatment 2 N= Chi square value p= Significant Gator + Bird Control 28 11.571 0.000670 yes Gator + Bird Bird 31 7.258 0.00706 yes Gator + Bird Gator 32 6.125 0.0133 no Control Gator 14 1.143 0.285 no Bird Gator 17 0.059 0.808 no Bird Control 13 0.692 0.405 no Table 12. Pairwise comparisons of numbers of birds responding to decoy treatments in 2011. Alpha = 0.008333. Decoy treatment 1 Decoy treatment 2 N= Chi square value p= Significant Gator + Bird Control 230 25.113 5.40E 07 yes Gator + Bird Bird 243 1 6.333 0.0000531 yes Gator + Bird Gator 194 64.66 << 0 .00001 yes Control Gator 118 10.983 0.0009 20 yes Bird Gator 131 18.328 0.0000186 yes Bird Control 167 1.012 0.314 no

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34 Figure 110. Numbers of islands per treatment group that Egretta herons nest ed on; 2010.

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35 Figure 111. Numbers of islands per treatment group that Egretta herons nested on; 2011.

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36 CHAPTER 2 DO WADING BIRDS BENEFIT FROM NESTING NEAR ALLIGATORS? Introduction Nest predation is a strong selective force in the evolution of avian nesting behavior and life history strategies (Martin 1993), and in the evolution of coloniality (Brown et al. 1990). Early warning, group defense, and predator swamping through synchronous breeding have often been listed as potential benefits of coloniali ty (Danchin & Wagner 1997). However, ciconiform s exhibit little group or individual nest protection behavior (Rogers 1987). In addition, mammalian predators such as raccoons ( Procyon lotor ) are capable of destroying a large number of nests in wading bird and seabird colon ies in a short amount of time often leading to abandonment of the entire colony (Rodgers 1987 ; Kelly et al. 1993), suggesting that swamping through synchronous breeding may be an unlikely benefit for many kinds of colonially breeding bir ds In the central Everglades wetlands of Florida, USA herons in the genus Egretta tend to nest on small slightly elevated islands dominated by willow trees (Frederick 1995) These islands are often associated with an alligator hole; an area where an American alligator ( Alligator mississippiensis ) excavates and maintains a small pond which it uses for reproduction and survival (Mazzotti & Brandt 1994). Water levels in the central Everglades sometimes fall below the ground surface during the annual dry season and alligator holes tend to hold water during that period (Davis et al. 2005). M any birds nest in inaccessible island locations to avoid mammalian predators and Egretta herons may nest on tree islands in the Everglades to do so as well For exampl e, Strong et al. (1991) and Erwin et al. (1995) found that whitecrowned pigeons and great white herons, respectively, avoided nesting on islands that could harbor

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37 mammalian predators in Florida Bay (Florida, USA) Others have inferred that inaccessible l ocations offer decreased predation risk by comparing predation rates at sites accessible and not accessible to mammalian predators (Robinson 1985; Post 1990; Kelly et al. 1993). Finally, some studies have reported an increase in nest predation when water levels in wetlands decrease significantly, apparently allowing access to colonies by semiaquatic mammals (Rodgers 1987 ; Frederick & Collopy 1989; Post & Seals 1991; Kelly 1993; C oulter & Bryan 1995). It is unclear why nest predation by semiaquatic mammal s should be responsive to changes in water levels in shallowly inundated wetlands. Raccoons have been known to make openwater crossings of up to 950 meters (Hartman & Eastman 1999), and readily move among widely separated offshore islands (200 meters) to prey on nests and eggs of waterbirds (Ellis et al. 2007). The water in many wetlands is generally shallow (03 meters) ( eg, Everglades, Loveless 1959, Mitsch & Gosselink 1993) and vegetation within wetlands often provides resting substrate for swimming m ammals. This evidence suggests that even large expanses of open water are not likely to function as a deterrent to raccoon use of wetlands and island archipelagoes. It therefore seems likely that there are other deterrents to small mammals traversing mars hes like the Everglades. The threat of predation by crocodilians may be a strong force. For example, r esearchers have long noted the occurrence of raccoons as prey items of large (>1.8 meters total length) alligators ( Giles & Childs 1949, Barr 1997, Shoop & Ruckdeschel 1990, Rice 2004). In the southeast United States, alligators are commonly found in freshwater lakes and marshes where many kinds of waterbirds nest, and it has been suggested that the presence of alligators prevents

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38 raccoons and other predators from reaching island nesting sites (Jenni 1969; Post 1998). Ruckdeschel & Shoop (1987) found that wood stork ( Mycteria americana) nesting success was related to water depth beneath the heronry and dropped sharply as the water level decreased. They attributed the decrease in nest success to an increase in raccoon access and predation within the colony Ruckdeschel & Shoop posited that alligator presence in the swamp below the colony was a deterrent to raccoons when water levels were high, but not when they were low. Coulter & Bryan (1995) also reported that raccoons were able to access and pre y upon nestlings in a wood stork colony in east central Georgia when the swamp under the nest trees became dry. Frederick & Collopy (1989) demonstrated that ot her common Egretta nest predator groups, snakes and birds, were minor to nonexistent threats in the central Everglades. They also showed that in years where the marsh stays flooded, nest predation is relatively low. Furthermore, Frederick & Collopy (1989) found that raccoons were very sparsely distributed throughout the deeper marshes of the central Everglades and that their presence on tree islands was limited to areas of the marsh that were nearly dry. After there is no surface water in the marsh, the t hreat alligators pose to raccoons and other mammals probably decreases due to decreased mobility of the alligator and increased mobility of the raccoon. Fleming et al. (1976) found a marked difference between raccoon predation on alligator nests in a Loui siana marsh ranging from no predation during a year with high water levels to 45 percent during a subsequent drought year. Similarly, Hunt and Ogden (1991) found that high predation rates on alligator nests by bears in a Louisiana marsh were associated wi th periods of low water.

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39 They attributed the increased predation rates to the alligators diminished ability to attack bears in low water. Thus there is reasonable evidence that alligators may present a strong deterrent to raccoons and other mammalian nest predators during flooded conditions. Wading birds apparently cue in on alligator presence when deciding where to nest, and may take advantage of the predator protection that alligators offer. I found that Egretta herons preferred to nest on islands w ith alligator and wading bird decoys and that Egretta herons and alligators were more likely to be found at the same tree island than alone ( C hapter 1 ) I suggest that when there is standing water in marshes, the presence of a lligators limit s the access o f raccoons to the marsh and bird colonies Conversely, I predicted that if surface water is very low or nonexistent, the deterrent effect of alligators diminishes considerably, and that raccoons would therefore have access to the marsh and to wading bird colonies. To understand the relationship between water, nesting wading birds and alligators, I examined the distribution of potential mammalian predators in relation to water levels and nesting distributions. I also documented the effect that raccoons ca n have on a wading bird colony. Methods Predator D istribution I estimated the local relative abundance of potential mammalian nest predators by attracting them to baited tracking stations during peak of the Egretta heron nesting period from midApril through June in 2010 and 2011. Tracking stations were 1 m by 1 m panel board finished with a smooth acrylic coating. Each station was placed on level ground and the surrounding area was cleared of branches and debris. Stations were

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40 baited with an open can of sardines packed in oil placed within a horizontal 10cm section of aluminum drain pipe, open at each end. The sardines and drain pipe were fastened to the panel board to prevent animals from removing the bait. When checked, the panel board was dusted w ith blue chalk dust to make any tracks visible. Beginning in April 2011 f ive tracking stations were placed in 500meter increments on levees on each side of the L67C canal bordering WCA 3A (Figure 21). Four stations were placed between 300 and 800 meters into the marsh away from the L 67C levee on dry land on three willow dominated tree islands typical of Egretta heron nesting habitat but which were unoccupied by nesting birds at the time. Eight tracking stations were also placed between 7.0 km and 7. 4 km away from levees on three large tree islands (cf 40,000 m2) with permanent dry ground, and on five small willow dominated tree islands (cf 600 m2) within 500 meters of the large islands. From 14 April until 7 June 2010, I monitored these 23 stations for a total of 140 trap nights. I included infared game cameras (Cuddeback, Green Bay, WI) aimed at 6 of the stations for 26 trap nights from 1 June until 7 June. All st ations were moved after a predator was detected or one week had elapsed. I also moni tored 14 tracking stations for 186 trap nights from 21 April until 17 May 2011. I simplified the tracking station design in 2011, suspending a sardine can with holes punched in it 1 m above the ground from a string tied to a branch. I used Cuddeback Capt ure IR game cameras to monitor all the sites. These t racking stations were again placed on six large tree islands with permanent dry ground and on eight nearby (within 500 meters) willow head tree islands unoccupied by, but typical nesting

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41 habitat of Egr etta herons within WCA 3 A I did not place tracking stations on levees in 2011. Survival r ates of n estlings and fledgling h erons To assess the impact of predation on fates of young herons in colonies, we individually marked 9 little blue herons and 17 tr icolored herons from a single nesting colony on a 0.09 ha willow head island ( N25 52.614, W80 45.318 ). Unique combinations of numbered red, green an d ye llow coiled plastic leg bands (N ational Band and Tag C o Newport, KY) were placed on the tarsometatars us. All banded birds were handcaught and banded at approximately 14 days old, after they had left the nest but before they were able to fly (Frederick et al. 1993). To minimize disturbance to the colony, all birds were captured, banded, and released within one hour on 29 April, 2011. I visited the island three subsequent times, ending on 17 May, 2011. I spent one hour during each visit with two observers attempting to re sight the banded birds. I ended the re sighting efforts when the birds became adept at flying and could leave the island during the survey attempt. Results Predator Distribution In 2010, I did not detect any mid sized or large mammals at the WCA 3A impoundment stations during 136 trap nights (Figure 21). Raccoon presence was detected on one night at 2 out of 6 stations on the mainland levee bordering the L67C canal that bounds the southeast edge of WCA 3A. I detected opossum s ( Didelphis virginiana) (n=4) on the L67C levee located between the marsh and the canal. The only potential nest predator that I detected in WCA 3A was an unidentified rat species (n=2) on two small willow tree islands. Additionally, an alligator (n=1), a Florida softshell

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42 turtle ( Apalone ferox ) (n=1), turkey vultures ( Cathartes aura) (n=4), and ninebanded arm adillos ( Dasypus novemcinctus ) (n=3) visited the tracking stations within WCA 3A in 2010. None of the latter are considered potential nest predators of herons. Mammalian nest predator distribution in WCA 3A was very different in 2011. Raccoons were found at 8 different sites over 186 trap nights (Figure 21). Raccoons were detected on large islands with permanent dry ground and on small willow dominated islands. I also detected rice rats ( Oryzomys palustris) (n=4) and a black rat ( Rattus rattus ) (n=1) on both large and small tree islands. River otters ( L u tra canadensis ) were detected at two small willow island sites but are not arboreal and therefore considered implausible as nest predators An alligator (n=1), turkey vultures (n=4), a limpkin ( Aramus guarauna) (n=1), a yellow crowned night heron ( Nyctanassa violacea) (n=1), a black crowned night heron ( Nycticorax nycticorax) (n=1), boat tailed grackles ( Quiscalus major ) (n=2) and a common grackle ( Quiscalus quiscula ) (n=1) also visited the tracking stat ions. Water Depth Water depth was greater than normal during 2010 and lower than normal during 2011. Water stages from the 34 gauge in central Water Conservation Area3A were greater than historical monthly maximum averages from January 2010 until late O ctober 2010 and exceeded one standard deviation higher than historical monthly maximum averages from late April 2010 until mid October 2010 (Figure 2 2 ). Water stages were less than historical monthly minimum averages from November 2011 through June 2011 and exceeded one standard deviation lower than historical monthly minimum averages from mid November 2011 through June 2011.

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43 Color Banded Birds In the 2011 nestling survival study, I resighted 18 individuals (70% of 26 banded birds) 6 days after their init ial capture (Figure 23 ). When I returned 7 days later, I found a large number of dead young mostly dismembered. The majority of the remains consisted only of the legs, allowing me to recover a number of color bands. Eight of the dead birds had been banded 13 days prior (23% of banded birds, 37.5% of birds resighted that day). Raccoon tracks and scat containing bird feathers (Figure 24 ) around and on the island indicated the major source of predation. To further confirm raccoon presence in the area, a predator tracking station was installed on the island; raccoons visited the station. I also resighted 10 live banded herons (38% of banded birds) during my second visit to the island. I visited the island a third time and found an additional preyed upon young bird (9 dead total, 35% of banded birds). I resighted 4 live birds (15% of banded birds). Most of the young birds present flew into the surrounding slough when observers entered the island on this last visit, suggesting that most birds had become flighted by this point and were less vulnerable to mammalian predation. Discussion Raccoon presence in WCA 3A appear ed to be directly related to water depth. 2010 was an exceptionally wet year in the Everglades and water stages were greater than historic al monthly maximum averages and exceeded one standard deviation higher than historical monthly maximum av erages from April until October Raccoons were present in low occurrences on the levees across the canal from WCA 3A but were never detected in the marsh, including on large islands that have year round dry ground. Distances across the canal surrounding WCA 3A and between tree islands or

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44 dry ground within WCA 3A (50 m 500 m) are significantly less than those which raccoons have been shown to swim acr oss in order to prey upon bird nests in alligator free environments (Hartman & Eastman 1999). Thus, it seems unlikely that surface water in the marsh by itself is sufficient to keep raccoo ns from heron nesting sites. I believe that the absence of raccoons in 2010 was due to the threat of predation posed by swimming alligators in WCA 3A This argument is further evidenced by the drastic shift in raccoon presence in WCA 3A during the extremely dry conditions of 2011. Unlike 2010, almost the entire marsh in WCA 3A was devoid of surface water in 2011 at the end of the Egretta heron nesting season (midMay until mid June). A helicopter survey at this time indicated that alligators were almost entirely confined to small pools and deeper canals ( Chapter 1) The lack of water therefore dramatically decreased the alligators mobility and distribution, and thus the predation threat that they posed to raccoons (Willey et al. 2004) Raccoons were found to be present in both WCA 3A on large islands with permanent dry ground and on small willow head islands that usually have no dry ground, up to 13 kilometers from the edge of WCA 3A. R accoons were detected in 2011 on a large island that was raccoon free 2010. T he island where I had a large proportion of our color banded birds preyed upon by raccoons was at least 11.5 kilometers from permanent dry ground at the edge of WCA 3A, the nearest location they had been detected in 2010. In mid May 2011, r accoons preyed upon 9 of the young herons that I color banded. Simil ar to patterns seen by other researchers (Ruckdeschel & Shoop 1987, Frederick & Collopy 1989, Coulter & Bryan 1995), the predation was associated with a low water period. These observations

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45 suggest that raccoons are highly attracted to wading bird colonies, and capable of locating widely dispersed colonies even at long distances. In addition to this study, the relationship between crocodilian movement, water depth and mammalian presence has been suggested by Ruckdeschel & Shoop (1987) and Frederick & Collopy (1989). It has also been noted that raccoons readily entered an inundated impounded marsh without alligators in North Carolina and preyed upon hundreds of muskrat nests (Wilson 1953). Furthermore, Urban (1970) found that 87 percent of the home ranges of raccoons in a managed wetland near Lake Erie were inundated marsh (including 25 percent open water). Together with these studies, m y results suggest that Egretta heron nests could be more vulnerable to raccoon predation in parts of their range where alligators are not present This results in the prediction that outside of the range of alligators, colonial birds are more reliant on large distance from mainland, nesting tree structure, or other potential mammalian predator s to reduce access by mammals Indeed, wading birds in the northern United States nest exclusively on islands in the middle of large bays (Parsons et al. 2001, Parsons 2003, Paton et al. 2005) rat her than in shallower wetlands.

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46 Figure 21. Satellite image (adapted from Google Ear th v.6.1; 2011) of p redator tracking station locations in 2010 (diamonds) and 2011 (circles) in the southeast corner of WCA 3A Dark green areas are tree islands. Raccoon presence is denoted with a yellow icon (diamond, 2010; circle 2011). Canals are hi ghlighted in red. Location of the satellite image highlighted in green on map of southern Florida (courtesy of the U.S. Geological Survey ) in lower right corner of figure.

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47 Figure 22. Stages at 34 gauge in WCA 3A from January 2010 until July 2011. Daily stage is shown as a solid line. Period of record monthly mean minimums (squares), maximums (xs), minimums minus 1 standard deviation (triangles), and maximums plus 1 standard deviation (asterisks) are shown for reference to long term trends.

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48 F igure 23 Cumulative percentage per day of color banded Egretta herons resighted, dead, or not seen over three 1hour observation periods at a single colony in the Florida Everglades.

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49 Figure 24 Raccoon scat containing feathers of young herons. Phot o courtesy of author.

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50 CHAPTER 3 DO ALLIGATORS BENEFI T FROM ASSOCIATING W ITH NESTING WADING B IRDS? Introduction Alligators ( Alligator mississippiensis ) are top predators in wetland systems throughout their range in the southeastern United States. As s uch, they consume a wide variety of prey items. Small alligators mainly consume molluscs, crustaceans, and insects (Barr 1997). Large alligators tend to eat fish, mammals, birds, reptiles, and amphibians (Barr 1997). However, alligators are opportunisti c feeders and readily consume any available prey (Delany and Abercrombie 1986, Barr 1997). Alligators have been reported as opportunistic scavengers below wading bird nesting colonies and as active predators of mammals that prey upon wading bird eggs and young. Dusi and Dusi (1968) noted that alligators inhabiting the marsh below a bird colony in Alabama ate nestlings that fell into the water in addition to restricting other predators from moving through the trees. Jenni (1969) also noted several instanc es of young birds from an island colony in Florida falling out of nests being preyed upon by alligators. Although they are top carnivores in the Everglades ecosystem, alligators in the region are known to be seasonally foodlimited (Jacobsen and Kushlan 1989, Mazzotti and Brandt 1994, Fujisaki et al. 2009). They grow more slowly, take longer to reach maturity, and are smaller at maturity than in other regions of their range (Jacobsen and Kushlan 1989, Mazzotti and Brandt 1994). Barr (1997) found that bir ds were the most important prey item by mass for adult and subadult alligators in the Everglades, although they were found in few samples. This indicates that capturing a bird could be nutritionally important to the foodlimited alligators of the Everglades. Barr also found evidence of alligator predation on raccoons ( Procyon lotor ) and several rat

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51 species (Florida water rat, Neofiber alleni ; rice rat, Oryzomys palustris ; cotton rat, Sigmodon hispidus ) all of which are potential wading bird nest predator s. In addition to consuming birds and their nest predators, all size classes of alligators also eat crayfish ( Procambrus sp.) and grass shrimp ( Palaemonetes intermedius Barr 1997), two prey groups that adult Egretta herons commonly regurgitate for young at the nest, and that may spill into the water below (B.Burtner, personal observation) Wading bird nesting aggregations are associated with alligator holes significantly m ore often than chance (Chapter 1), probably as a result of active choice by birds. Based on the reported eating habits of alligators, I hypothesized that alligators benefitted from associating with wading birds through direct food inputs (dropped eggs, chicks, grass shrimp, crayfish) from the colony. Because Everglades alligators may b e food limited, I hypothesized that the amount of food input from the colony could be nutritionally significant to the alligators. I examined this question by measuring potential food inputs from birds, and estimated the nutritional content that might be used by alligators. Alligators could also benefit from associating with wading birds through opportunistic predation of wading birds themselves as well as wading bird nest predators. However, quantifying raccoons being eaten by alligators was beyond the scope of this study. Methods I measured potential food for alligators from wading bird nests by measuring dropped regurgitant, nestlings, and other protein below nests. I placed throughfall traps below 57 Egretta heron nests in 4 colonies in 2010 and 2011 (Table 31) to quantify the amount of food an alligator could derive from a typical colony. Throughfall traps were constructed from heavy duty black plastic garbage bags modified into the shape of a

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52 funnel, mounted on a 75 cm diameter frame of chicken wi re. The traps had a window screen drain inserted into the bottom to allow for water drainage. Traps were centered directly below nests and secured with string affixed to surrounding branches (Figure 31). Since young herons and egrets spend at least hal f the time from hatching to independence (30 40 days, Werschkul 1979, Erwin et al. 1996 a,b, Frederick 1997) in the colony but away from the nest, I also measured throughfall at 20 additional traps placed at random locations about the colony. To minimi ze bird disturbance and risk of colony abandonment, I installed traps during a 1hour maximum visit to each colony when the majority of nests in the colony were in incubation phase with full clutches (34 eggs/ nest). In 2010, 40 traps were divided between 2 colony islands in WCA 3A. One island had 12 traps below nests and 12 traps in random locations within the nesting area. The second island had 8 traps below nests and 8 traps in random locations. In 2011, all 40 traps (20 under nests, 20 random locat ions) were initially placed on one of the two islands used in 2010. However, nestlings were repeatedly preyed upon by a great horned owl ( Bubo virginianus ) and the colony slowly abandoned during a two week period. I moved the traps to a second colony where unhatched nests were found and installed 24 traps (12 under nests, 12 random). I returned to the colony every 47 days to monitor the traps and nest contents. In 2011, 5 nests failed after the first week of study. I moved the traps below those nests to other nests with full clutches of unhatched eggs. During each visit I recorded number of eggs and chicks present and their species. I removed all debris from the trap and measured fresh weight to the nearest gram and counted all food items that could be

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53 consumed by alligators. All nests were followed until failure or young had left the nest structure Results In 2010 I found grass shrimp, crayfish, whole Egretta eggs, and an Egretta chick in the throughfall traps. The number of collection days from a single nest ranged from 7 35 days. Of the 20 nests I followed in 2010, 10% (2 nests) failed before hatching (Table 3 2). Of the 18 nests that produced chicks, 72.2% (13 nests) fledged at least one chick. All of the food items were found in traps belo w nests and no items were found in the traps placed in random locations about the colony. The total wet mass of all food items collected from 20 nests over 440 collection days in 2010 was 299 grams (Table 33). Total shrimp input was 39 grams (39 shrimp) Total crayfish mass was 53 grams (31 crayfish). Total egg mass was 73 grams (3 eggs). Total chick mass was 134 grams (1 little blue heron chick). Average throughfall per nest was 14.95 grams (standard deviation: 31.23 grams) and per nest per day was 0.034 grams (standard deviation: 1.59 g/nest/day) When extrapolated to a typical sized colony of Egretta herons in the Everglades (50 nests) average input per nest, per day was 1.70 grams per day (standard deviation: 79.35 grams). After 20, 40, and 60 days, a 50nest colonys average input would be 33.98, 67.95, and 101.93 grams of food, respectively (Table 35 ). In 2011, I found far fewer food items in throughfall traps than in 2010. However, there were a large number of nests that had been preyed upon and abandoned, and a high proportion that failed before chicks hatched. Of the 37 nests that I followed, 67.6% (25 nests) failed prior to hatching. Ten (83.3%) of the 12 nests that produced chicks

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54 were preyed upon or failed less than a week after hatchi ng. The remaining two nests were preyed upon or failed less than two weeks after hatching. I found heron eggs, shrimp, and half of a fish in the throughfall traps in 2011. Total food mass collected was 144 grams over 695 collection days (Table 34). Eggs made up the vast majority of the food input (142 grams, 6 eggs). The two shrimp weighed 1 gram. The half of a fish also weighed 1 gram. The shrimp and fish were found in traps below nests with chicks. The eggs were all found in traps below predated or failed nests Average throughfall per nest was 3.89 grams (standard deviation: 10.48 grams) and per nest per day was 0.0056 grams (standard deviation: 1.34 g/nest/day). When extrapolated to a typical sized colony of Egretta herons in the Everglades (50 nests) average input per nest, per day was 0.28 grams per day (standard deviation: 66.78 grams). After 20, 40, and 60 days, a 50nest colonys average input would be 5.60, 11.20, and 16.80 grams of food, respectively (Table 35 ). Discussion The amount of food dropped from the nests we monitored in 2010 suggests that a typical Egretta heron colony of 50 nests has the potential to provide nontrivial food benefits to an alligator foraging below. The amount of heron eggs, shrimp, crayfish and occasional chic k falling from the colony could certainly contribute to the overall body condition of the alligator. Additionally, the colony is a reliable food source positioned directly above the alligator hole where the alligator resides. Rather than expending energy to forage, the alligator only has to sit and wait for food to fall from the trees. However, as evidenced by the mass nest failures and predation by raccoons in 2011, it is possible that an Egretta heron colony could provide very little in the way of food to the alligator.

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55 Although Egretta heron chicks leave the nest as early as two weeks after hatching, they remain at the colony and are fed by their parents until approximately 5058 days post hatching. In addition to regurgitated food being dropped when it is transferred from adult to chick, the young begin to learn to forage in and around the alligator hole (Werschkul 1979, Rodgers and Nesbitt 1979). It seems likely that alligators would also be able to prey upon these nave, poorly flighted birds. Adult Egretta herons do not usually forage in alligator holes. The hydrological conditions that I noted during my study represent two extremes of normal Everglades weather and hydrology. Water levels in the Everglades were higher than usual in 2010 and the typical dry season decline in marsh water depth never occurred. Alligators living in the Everglades rely on the seasonal marsh dry down to concentrate aquatic prey (Rehage & Trexler 2006) High water levels in the marsh during 2010 could have made the f ood input from bird colonies a more important food source for the alligators as their aquatic prey were probably more dispersed than a typical year. In contrast in 2011, the marsh was so dry by May that it limited the alligators movements, allowing raccoons to infiltrate the colony we were monitoring. Raccoons decimated the colony, and may have deprived alligators of the additional food benefits nesting birds provide. However, alligators could have benefitted from raccoon presence in the marsh by opport unistically preyed upon them. Wading birds are apparently attracted to alligators (Chapter 1) and probably reap benefits from the association through protection from raccoons (Chapter 2). In this chapter, I have shown that alligators appear to receive food benefits from the association as well. Together, these lines of evidence suggest that the association

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56 between the two taxa may be driven by benefits to both. I would therefore term the relationship between nesting Egretta herons and alligators in the Everglades a mutualism.

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57 Table 31 Locations of colonies in WCA 3A where throughfall traps were placed in 2010 and 2011. Colony location Number of traps Year N25 52.105 W80 48.398 8 2010 N26 7.62 4 W80 44.715 12 2010 N25 52.105 W80 48.398 20 2011 N2 5 52.614 W80 45.318 12 2011

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58 A B Figure 31. Throughfall trap set up below an Egretta heron nest with three eggs. A) view from the side, B) view from above. Photo courtesy of author.

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59 Table 3 2. The fate of nests with throughfall traps in 2010 and 2011. Percent values are the percent of total number of nests at that stage. Numbers of nests at each stage are in parentheses. 2010 2011 Nests failed pre hatching 10.0% (2) 67.6% (25) Nests failed pre fledging 27.8% (5) 100% (12) Nests fledged at l east 1 chick 72.2% (13) 0% (0) Table 33. The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall traps beneath 20 Egretta heron nests for 440 collection days in 2010. Food item Number of items Tota l mass Grams per nest Percent of total mass Shrimp 39 39 1.95 13.04 Crayfish 31 53 2.65 17.73 Egg 3 73 2.65 24.41 Chick 1 134 6.7 44.82 Total 74 299 14.95 100 Table 3 4 The number, total mass, grams per nest, and percent of total mass of all food items recovered from throughfall traps beneath 37 Egretta heron nests for 695 collection days in 2011. Food item Number of items Total mass Grams per nest Percent of total mass Shrimp 2 1 0.03 0.69 Fish 1 1 0.03 0.69 Egg 6 142 3.84 98.61 Total 9 144 3. 89 100

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60 Table 35. The potential fresh food mass input from a 50nest Egretta heron colony in 2010 and 2011 after 20, 40, and 60 days. Number of Days 2010 (grams) 2011 (grams) 20 33.98 5.60 40 67.95 11.20 60 101.93 16.80

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66 Rice, A. N. 2004. Diet and condition of American alligators ( Alligator mississippiensis ) in three central Florida lakes. M.S. thesis, University of Florida. Richardson, D. S. & Bolen, G. M. 1999. A nesting association between semi colonial Bullock's orioles and yellow billed magpies: evidence for the predator protection hypothesis. Behavioral Ecology and Sociobiology 46, 373 380. Robinson, S. K. 1985. Coloniality in the yellow rumped cacique as a defense agains t nest predators. Auk 102 506519. Rodgers, J. A. 1987. On the antipredator advantages of coloniality a word of caution. Wilson Bulletin 99 269 271. Rodgers, J. A. & Nesbitt, S. A. 1979. Feeding energetics of herons and ibises at breeding colonies. Proceedings of the Conference of the Colonial Waterbird Group, 3 128132. Ruckdeschel, C. & Shoop, C. R. 1987. Aspects of wood stork nesting ecology on Cumberland Island Georgia USA. Oriole 52, 21 27. Seppanen, J., Forsman, J., Monkkonen, M. & Thomson, R. 2007. Social information use is a process across time, space, and ecology, reaching heterospecifics. Ecology 88 16221633. Shoop, C. R. & Ruckdeschel, C. A. 1990. Alligators as predators on terrestrial mammals. American Midland Naturalist 124 407412. Strong, A. M., Sawicki, R. J. & Bancroft, G. T. 1991. Effects of predator presence on the nesting distribution of whitecrowned pigeons in Florida Bay. Wilson Bulletin 103, 415 425. Uchid a, H. 1986. Passerine birds nesting close to the nests of birds of prey. Japanese Journal of Ornithology 35, 25 32. Urban, D. 1970. Raccoon populations, movement patterns, and predation on a managed waterfowl marsh. Journal of Wildlife Management 34, 372 382. Werschkul, D. F. 1979. Nestling mortality and the adaptive significance of early locomotion in the little blue heron. Auk 96 116 130. Willey, J. S., Biknevicius, A. R., Reilly, S. M. & Earls, K. D. 2004. The tale of the tail: limb function and loco motor mechanics in Alligator mississippiensis Journal of Experimental Biology 207, 553 563. Wilson, K. A. 1953. Raccoon predation on muskrats near Currituck, North Carolina. Journal of Wildlife Management 17, 113 119.

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67 BIOGRAPHICAL SKETCH Brittany Burt ner was born and raised in Keedysville, a rural community in western Maryland. She graduated with high honors from the University of Maryland College Park in 2006 with a B.S. degree in biology and a concentration in behavior, ecology, and evolution. Af ter a threeyear interlude studying oyster restoration in the Chesapeake Bay at the University of Marylands Horn Point Lab, she returned to her behavioral ecology roots for her M.S. thesis at the University of Florida. She completed her M.S. degree in De cember 2011.