Habitat use, movements, migration patterns, and survival rates of subadult bald eagles in North Florida

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Title:
Habitat use, movements, migration patterns, and survival rates of subadult bald eagles in North Florida
Physical Description:
xiii, 123 leaves : ill., maps ; 29 cm.
Language:
English
Creator:
Wood, Petra Bohall
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Subjects

Subjects / Keywords:
Bald eagle -- Habitat -- Florida   ( lcsh )
Bald eagle -- Migration -- Florida   ( lcsh )
Forest Resources and Conservation thesis Ph. D
Dissertations, Academic -- Forest Resources and Conservation -- UF
Genre:
bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1992.
Bibliography:
Includes bibliographical references (leaves 110-115)
Statement of Responsibility:
by Petra Bohall Wood.
General Note:
Typescript.
General Note:
Vita.

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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 001757975
oclc - 26699666
notis - AJH1022
System ID:
AA00003287:00001

Full Text









HABITAT USE, MOVEMENTS, MIGRATION PATTERNS,
AND SURVIVAL RATES OF
SUBADULT BALD EAGLES IN NORTH FLORIDA
















By

PETRA BOHALL WOOD


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

1992


UNIVERSITY OF FLORIDA LIBRARIES













ACKNOWLEDGMENTS

Many people contributed to the success of this research and to all of them I am

grateful. I especially thank my graduate advisor, Dr. Michael W. Collopy. Mike rescued me

from South Florida in 1985 and gave me an interesting and challenging opportunity to study

bald eagles. He persuaded me to go back to graduate school for a PhD, recommended me for

a CEA position, and has made many contributions throughout this study. I also thank my

committee members, Drs. Jack Kaufman, Franklin Percival, Joe Schaefer, and Mel Sunquist,

for their contributions to this study and for reviews of this dissertation.

Financial support was provided by the Nongame Wildlife Program of the Florida

Game and Fresh Water Fish Commission from fall 1987 to fall 1991. Radio-tracking

equipment and initial work in spring 1987 was funded by the International Association of Fish

and Wildlife Agencies and the Miller Brewing Company. The Cooperative Research Units

Center, USFWS, provided funds for my CEA position with the Florida Cooperative Fish and

Wildlife Research Unit.

Research assistants Myra Noss and Rick Sullivan spent numerous hours assisting with

boat surveys. Research assistants Carolyn Sekerak and Linda Waite were invaluable in radio-

tracking, banding, and data entry. Carolyn Sekerak also spent many hours collecting

fledgling behavior data. I also thank the many volunteers who assisted with banding eagles.

Dave Buehler shared his expertise in attaching radio-transmitters. Tree climbers Tony

Steffer, Wayne Norton, and Craig Koppie made this study possible by retrieving nestling

eagles from nests in very tall trees for banding and radio-tagging. Pilots Steve Schwikert and







Tom Campbell skillfully and safely flew aerial surveys and telemetry flights. Dr. John

Smallwood conducted aerial nest surveys and banded nestlings in Ocala National Forest in

1989. Laura Cochran, Ocala N.F., conducted aerial nest surveys in Ocala National Forest in

1990 and 1991. Thanks also go to the many landowners who graciously granted access to

eagle nests on their property. Many individuals reported sightings of marked bald eagles

from this study outside of Florida-to all of them I am grateful. I especially thank Dave

Buehler, Jim Fraser, Charlie Todd, Russ Treadwell, Peter Nye, and Mark King for their

efforts in locating eagles on the Chesapeake Bay, in Maine, and in New York.

Steve Nesbitt, FGFWFC, offered advice on various phases of this study. Jim

Weimer, Florida DNR, provided water level data for Alachua Lake and assistance at nests on

Paynes Prairie State Preserve. The Office of Environmental Services, FGFWFC, provided

the Landsat satellite imagery data and the Alachua County Department of Information

Services provided a digitized data file of roads in Alachua County. The Florida Cooperative

Fish and Wildlife Research Unit provided access to computer equipment and John Richardson

and Leonard Pearlstine provided expertise for the analyses involving a geographic information

system. Joann Boyko, Robert Coney, Monica Lindberg, Stan Ainsworth, Harry Buck, and

Woody Christie of the University of Florida kept the paperwork under control.

Thanks to Cyndi, Brian, Nolls, Peter, Ruthe, Jeff, Tim, Cathy, Kathleen, Steve, and

others for the welcome diversions of canoeing, the Hipp, and Risk. Finally, I especially

thank John Wood for his support during my graduate career and his help on all aspects of this

study.














TABLE OF CONTENTS


page


ACKNOWLEDGMENTS .....................


LIST OF TABLES ............................................. vi


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


ABSTRACT

CHAPTERS


..... ..... ........ .... ........ .. .... ....... .. xii


1 STUDY OVERVIEW .....................


Introduction ................
Study Area ................
Study Organization ...........


. . . 1

. . . 1
. . .. 3
... ... .... ..... .. .. 4
.~4


2 POST-FLEDGING MOVEMENTS AND HABITAT USE ..................


Introduction ..............................
Methods
Banding and radio-tagging procedures ........
Intensive nest observations ...............
Extensive radio-tracking surveys ............
Climate and water level data ..............
Results
Extensive radio-tracking surveys ............
Intensive nest observations ...............
Discussion ..............................


3 MIGRATORY PATTERNS OF SUBADULT BALD EAGLES .............. 35


Introduction ..............................
M ethods ................................
Results


Timing of migration ..


. . . . 38


Long-distance movements ..............................
D discussion ............................................


S8
S9
10
11








4 MOVEMENTS AND HABITAT USE ............................. 59

Introduction ............................................. 59
Methods ............................................... 61
Results
Movements ...................................... 65
Interactions ................. .................... 66
Perch characteristics ................................. 67
Habitat use ....................................... 67
Discussion
Movements ...................... ... ............ 70
Habitat use ........................................... 71

5 SURVIVAL OF SUBADULT BALD EAGLES ........................ 92

Introduction ............................................. 92
Methods ............................................... 93
Results ............................................... 95
Discussion ............................................. 96

6 SYNTHESIS AND CONCLUSIONS .............................. 105

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

APPENDIX: BANDING DATA ................................... 117

BIOGRAPHICAL SKETCH ...................................... 123












LIST OF TABLES


Table page

1-1 Habitat types identified from a Landsat image and amount of each available on
the study area in north-central Florida ............................ 5

2-1 Bill depth (mm) and length of foot pad (mm) for 8-week-old bald eagle nestlings
in north-central Florida from 1987 to 1991 ........................ 19

2-2 Hatching dates, migration dates, and perch distance for bald eagle nestlings fitted
with solar-powered radio transmitters, 1987 to 1990, in north-central Florida ... 20

2-3 Distance (km) fledgling bald eagles were located from their natal nest prior to
initiation of migration by sex, number of chicks fledged from nest, and timing
of fledging, 1987 to 1990 in north-central Florida .................... 22

2-4 Earliest and latest dates fledgling bald eagles last located on the study area in
north-central Florida, 1987 to 1990 ............................. 22

2-5 Age (number of days after hatch) at which fledgling bald eagles left the north-
central Florida study area in relation to sex, timing of fledging, number of
chicks fledged from a nest, and order of hatch, 1987 to 1990 ............. 23

2-6 Interactions observed for fledgling bald eagles at 2 nests in north-central Florida
after 11 weeks of age, 1991 .................................. 23

2-7 Prey deliveries post-fledging at 2 bald eagle nests in north-central Florida in 1991.
Nestlings were considered fledged at 11 weeks of age (AL32A = 4 April 1991,
MRl7D = 22 April 1991) .................................. 24

2-8 Distance (m) from the natal nest in north-central Florida that fledgling bald eagles
were located in relation to days post-fledging in 1991. Nestlings were considered
fledged at 11 weeks of age. For each nest, means with the same letter are not
significantly different (Waller-Duncan K-ratio t-test) .................. 25

3-1 Timing of arrival and departure from the north-central Florida study area for 1
to 4 year old subadult bald eagles, fall 1987 to spring 1991 .............. 45

3-2 Number of days gone from the north-central Florida study area for 1 to 4 year
old subadult bald eagles, spring 1987 to spring 1991 .................. 45







3-3 Tracking history and out-of-state locations and recoveries of subadult bald eagles
(March 1987 to June 1990). Locations are in Alachua and Marion counties study
area, unless indicated otherwise ............................... 46

3-4 Maximum distance (km) from the north-central Florida study area an individual
was located at each age, spring 1987 to fall 1990. Within a variable, means with
the same letter are not significantly different (Waller-Duncan K-ratio t-test) .... 52

3-5 Distance migrated (km) and speed of movements for radio-tagged bald eagles
from north-central Florida during their initial migration, spring 1987 to
fall 1990 ............................................... 52

4-1 Habitats used by perched, flying, and soaring eagles located on the study area
in north-central Florida from fall 1987 to spring 1991 ................. 73

4-2 Variables used in logistic regression analyses comparing used with random
locations, sex, and age for bald eagles in north-central Florida from fall
1987 to spring 1991 ....................................... 74

4-3 Mean percent of bald eagle locations on the study area in north-central Florida
as an index of philopatry weighted for repeated locations of individuals, fall
1987 to spring 1991 ........................................ 75

4-4 Mean distance (km) 1- to 4-year-old bald eagles were located from their natal
nest in north-central Florida from fall 1987 to spring 1991 ............... 76

4-5 Perch use (%) of radio-tagged eagles 1 to 4 years of age located in north-central
Florida from fall 1987 to spring 1991 ............................ 77

4-6 Comparision of use and availability of 8 habitats for subadult bald eagles in
north-central Florida from fall 1987 to spring 1991 ................... 77

4-7 Percent habitat availability in 1-ha and 10-ha plots centered on locations of radio-
tagged eagles (n=519) in north-central Florida, fall 1987 to spring 1991 ...... 78

4-8 Comparison of used bald eagle locations and random points in north-central
Florida for various features of the landscape using logistic regression, fall
1987 to spring 1991. Variables are defined in Table 4-3. All variables
included in univariate analyses; variables with P < 0.25 included in full
logistic regression model; variables with P < 0.05 included in reduced model 79

4-9 Comparison by sex of bald eagle locations in north-central Florida for various
features of the landscape using logistic regression, fall 1987 to spring 1991.
Variables are defined in Table 4-3. All variables included in univariate
analyses; variables with P < 0.25 included in full logistic regression model;
variables with P < 0.05 included in reduced model ................. 80







4-10 Comparison of bald eagle age classes 1 and 2 for various features of the
landscape using logistic regression, north-central Florida, fall 1987 to spring
1991. Variables are defined in Table 4-3. All variables included in
univariate analyses; variables with P < 0.25 included in full logistic
regression model; variables with P < 0.05 included in reduced model ....... 81

5-1 Survival rates through May 1991 for each cohort of radio-tagged bald eagles
from 1987 to 1990, north-central Florida ......................... 100

5-2 Survival rates of radio-tagged bald eagles by age class in north-central Florida,
1987 to 1991 ........................................... 100


5-3 Survival rates through 1 1/2 years of age in relation to sex, timing of hatch,
number of chicks in the nest, and hatch order, north-central Florida, 1987
to 1991. N is the number at risk during the sampling period ..........


... 101


5-4 Survival rates for 3 bald eagle populations in the eastern United States ....... 101













LIST OF FIGURES


Figure pg

1-1 Study area in north-central Florida for bald eagle radio-tracking study, 1987
to 1991 ................................................ 6

2-1 Patagial marker attached to nestling bald eagles in north-central Florida and
its location on the right wing of the bird .......................... 26

2-2 Measurements of bill depth (mm) and foot pad (mm) for 8-week old nestling
bald eagles in north-central Florida ............................. 27

2-3 Mean monthly water levels (in feet NGVD) from January 1987 to June 1991
for Alachua, Lochloosa, Newnans and Orange Lakes, Florida. Migration dates
are for radio-tagged fledgling bald eagles on their initial migration from the
study area .............................................. 28

2-4 Average monthly maximum and minimum temperatures (C) measured at
Gainesville Regional Airport from January 1987 to January 1991. Migration
dates are for radio-tagged fledgling bald eagles on their initial migration from
the study area ........................................... 29

2-5 Total monthly precipitation (mm) measured at Gainesville Regional Airport from
January 1987 to January 1991. Migration dates are for radio-tagged fledgling
bald eagles on their initial migration from the study area ................ 30

2-6 Mean number of fish electro-shocked on monthly transects on Newnans Lake
(data from Edwards 1987) in relation to mean departure (D) dates for radio-
tagged fledgling bald eagles on their initial migration away from the study area 31

2-7 Relationship between age radio-tagged fledgling bald eagles began their initial
migration from the study area and timing of hatching .................. 32

2-8 Perch use of fledgling bald eagles under intensive observation in 1991 at 2 nests
in north-central Florida ..................................... 33

2-9 Activity of fledgling bald eagles under intensive observation in 1991 at 2 nests
in north-central Florida ..................................... 34







3-1 Mean monthly water levels (in feet NGVD) from January 1987 to June 1991 for
Alachua, Lochloosa, Newnans and Orange lakes, Florida. Migration dates are
for radio-tagged bald eagles 1 to 4 years of age ..................... 53

3-2 Total monthly precipitation (mm) measured at Gainesville Regional Airport from
January 1987 to January 1991. Migration dates are for radio-tagged bald eagles
1 to 4 years of age ....................................... 54

3-3 Average monthly maximum and minimum temperatures (C) measured at
Gainesville Regional Airport from January 1987 to January 1991. Migration
dates are for radio-tagged bald eagles 1 to 4 years of age ............... 55

3-4 Mean number of fish electro-shocked on monthly transects on Newnans Lake
(data from Edwards 1987) in relation to mean departure (D) and arrival (A)
dates on the study area for radio-tagged 1 to 4 year old bald eagles ......... 56

3-5 Locations of radio-tagged or marked bald eagles 1 to 4 years of age outside of
Florida. Numbers indicate several locations; circled points indicate dead birds.
Star indicates banding location ................................ 57

3-6 Water temperatures averaged from 4 sampling stations on Lochloosa, Newnans
and Orange Lakes, 1988-1991 (unpubl. data, Florida Game and Fresh Water
Fish Commission) ........................................ 58

4-1 Bald eagle telemetry locations in Florida but outside of the study area in north-
central Florida, 1987 to 1991 ................................. 82

4-2 Telemetry locations of 1 to 4 year old bald eagles in north-central Florida from
September 1987 to July 1988 ................................. 83

4-3 Telemetry locations of 1 to 4 year old bald eagles in north-central Florida from
September 1988 to July 1989 ................................. 84

4-4 Telemetry locations of 1 to 4 year old bald eagles in north-central Florida from
September 1989 to July 1990 ................................. 85

4-5 Telemetry locations of 1 to 4 year old bald eagles in north-central Florida from
September 1990 to July 1991 .................................. 86

4-6 Telemetry locations in north-central Florida of subadult bald eagle 165.675 at 1
to 4 years of age, fall 1987 to spring 1991. Ages l=closed rectangle, 2=open
rectangle, 3= closed circle, 4= open circle ......................... 87

4-7 Telemetry locations in north-central Florida of subadult bald eagle 164.798 at
1 and 2 years of age, fall 1989 to spring 1991. Ages 1 =closed rectangle,
2= closed circle .................... ..................... 88







4-8 Consecutive radio locations of eagle 165.675 from 1 to 4 years of age on the
north-central Florida study area, 1987 to 1991 ...................... 89

4-9 Habitat use of radio-tagged bald eagles from 1 to 4 years of age in north-central
Florida, fall 1987 to spring 1991 ............................... 90

4-10 Habitat use by male and female radio-tagged bald eagles from 1 to 4 years of
age in north-central Florida, fall 1987 to spring 1991 .................. 91

5-1 Survival curves for male and female bald eagles through 4 1/2 years of age,
north-central Florida, 1987 to 1991 ............................. 102

5-2 Survival curves for bald eagles fledged from nests with 1 versus 2 chicks
through 1 1/2 years of age, north-central Florida, 1987 to 1991 ........... 103

5-3 Survival curves of first and second hatched bald eagles fledged from nests with
2 chicks through 1 1/2 years of age, north-central Florida, 1987 to 1991 ...... 104












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


HABITAT USE, MOVEMENTS, MIGRATION PATTERNS,
AND SURVIVAL RATES OF
SUBADULT BALD EAGLES IN NORTH FLORIDA


By

Petra Bohall Wood

May 1992

Chairman: Michael W. Collopy
Major Department: Forest Resources and Conservation (Wildlife and Range Sciences)

The state of Florida supports over half of the breeding population of bald eagles

(Haliaeetus leucocephalus leucocephalus) in the southeastern United States; this represents a

significant resource for the Southeast and for Florida. Currently, primary management

emphasis and protection is focused on bald eagle nest sites. No habitat protection or

management activities are aimed at foraging, roosting or loafing areas for subadult eagles. In

fact, habitats and habitat characteristics important to subadults have not been quantified. In

this study, we examined various aspects of eagle biology that might be pertinent to survival or

management of the Florida subadult eagle population. Specifically, using radio telemetry, we

examined post-fledging habitat needs, factors affecting timing of migration, local movements,

habitat use, and survival in north-central Florida from spring 1987 through spring 1991.

Fledgling eagles (birds prior to their initial migration) remained dependent on adults

and remained within 4 km of the natal nest until they initiated migration (approximately 7







weeks post-fledging). It was determined that habitat protection within the 229 m primary

protection zone used in Florida was not sufficient to meet the habitat needs of fledgling eagles

and that the protection period should extend until fledglings initiate migration in the summer.

Timing of migration for fledgling and 1- to 4-year-old eagles appeared to be correlated more

with prey availability than with temperature, although both factors appear to affect migration.

Locations of radio-tagged eagles outside of Florida ranged from South Carolina to

Prince Edward Island, Canada. Data suggest that eagles are philopatric to summering areas,

which emphasizes the need for protection of significant summering areas. Known and

assumed mortality occurred primarily during migration in northern states. The 1 1/2 year age

class had the lowest survival. Survival was significantly lower for eagles fledged from 1-

chick nests and for the younger chick in 2-chick nests. The minimum survival rate through 4

1/2 years of age was 50% and did not vary by sex.

After subadults returned to the north-central Florida study area in the fall, individuals

continued to use the same general areas each year. Temporally and locally abundant food

sources resulted in temporary small concentrations of eagles. Certain portions of the study

area were used consistently each year by large numbers of eagles. Subadult eagles were not

distributed randomly over the study area. Logistic regression analyses revealed that eagles

tended to be located close to large water bodies and eagle nests, were frequently in cypress

and marsh habitats, and avoided main roads and developed areas. Immature eagles (1-year

olds) tended to be located closer to eagle nests than 2- to 4-year-olds. Thus, management for

subadult populations must include these heavily used concentration areas that supply the

habitat features preferred by subadults. Survival of subadults may be affected if a highly used

area becomes unsuitable.













CHAPTER 1

STUDY OVERVIEW


Introduction

To fully understand the population dynamics of a species, all aspects of the life

history must be examined and incorporated into deterministic or stochastic population models

(Young 1968, Grier 1980). For many avian species, however, the period from fledging or

independence to breeding is the least well understood. Similarly, little is known about

seasonal movements, habitat requirements, or survivorship of the various age classes in

subadult bald eagle (Haliaeetus leucocephalus) populations. In a population modeling study,

Grier (1980) determined that survival rates were the most important factor affecting growth of

long-lived, slow-breeding bird populations such as the bald eagle. In fact, eagle populations

were twice as sensitive to changes in survival compared to fecundity (Grier 1980).

Consequently, it is important to document survival rates for all age classes and to examine

factors that might affect survival.

Previous studies in Florida addressed nesting habitat requirements (McEwan and Hirth

1979, Wood et al. 1989). Studies throughout the United States have examined eagle nesting

ecology, in particular documenting fecundity. In winter concentration areas, various aspects

of wintering habitat and ecology also have been addressed. Wintering habitats used by

subadults had not been studied in an area such as Florida where eagles do not form large

winter concentrations and where environmental factors, such as climate, are extremely

different from temperate regions.






2

Bald eagles in Florida are at the extreme southern end of the range for the species and

exhibit some life history characteristics that are distinct from more northern populations.

Most notably, they breed in the winter and migrate north in the summer. Consequently,

wintering areas for large numbers of nonbreeding and subadult eagles overlap with breeding

areas.

Florida supports over half of the breeding population of bald eagles (. 1.

leucocephalus) in the southeastern United States (Wood et al. 1990) with 601 known breeding

pairs in 1991 (S. Nesbitt, pers. commun.). Consequently, Florida eagles represent an

important part of the total southeastern population. Understanding the dynamics of Florida's

eagle population, therefore, is necessary in understanding that of the southeastern population.

In addition, studies on a large, stable population should yield results generally more applicable

to other eagle populations than do studies on small, relict, or reestablished populations.

Currently, primary management emphasis and protection are focused on active bald

eagle nest sites (U.S. Fish and Wildlife Service 1987), because it has been recognized that

disturbance at nest sites can decrease productivity. Increasing productivity is not an effective

management strategy, however, if immatures subsequently do not survive due to lack of

adequate foraging, roosting, and loafing areas. At this time in Florida, there are no habitat

protection measures aimed at foraging or roosting habitats that are important to nonbreeding

subadults, or to breeding adults. Furthermore, habitats and habitat characteristics important

to the non-breeding eagle population have not been identified.

In summary, very little is known about subadult eagle populations, particularly in an

area such as Florida where large winter concentrations do not occur. In this study, I

examined various aspects of eagle biology that might be pertinent to survival or management

for the non-breeding segment of Florida's eagle population. Specifically, I characterized

migration patterns, movements, habitat use, and survival rates of subadult bald eagles to 4 1/2






3

years of age in north-central Florida to provide data for effective management of this segment

of the eagle population. Objectives for each segment of the study are enumerated in the

pertinent chapter.



Study Area

Research focused on eagles found in the AMC study area comprising southern

Alachua, northern Marion, western Putnam and eastern Levy counties, south of Gainesville,

Florida (Figure 1-1). This area contains 340 bodies of open water ranging from 0.4 ha to

2,702 ha (mean = 12.6 ha) in size and numerous marshes and wet prairies (see Hartman

1978; also classified as palustrine persistent emergent wetland by Cowardin et al. 1979).

Habitat availability was identified on the study area from a Landsat satellite image (courtesy

of the Office of Environmental Services, Florida Game and Fresh Water Fish Commission).

The 14 habitat types on the image were condensed into the 8 habitat classes (Table 1-1)

considered in this study. Hartman (1978) described these Florida habitats.

Four large lakes occur on the study area that are used extensively by bald eagles.

Newnan's Lake, located on the eastern edge of Gainesville, is a hyper-eutrophic lake of 2,433

ha (Shannon and Brezonik 1972) with a mean depth of 1.5 m (maximum = 4.0 m). Lake

Lochloosa is a 2,235 ha meso-eutrophic lake about 20 km southeast of Gainesville with a

mean depth of 2.9 m. Orange Lake is a 3,324 ha meso-eutrophic lake with a mean water

depth of 1.8 m (maximum = 3.0 m) located about 20 km south of Gainesville. These 3 lakes

have control structures that regulate water depth. They are rimmed primarily with

baldcypress (Taxodium distichum) and hardwoods. Extensive areas of pine (Pinus s.)

forests surround Newnans and Lochloosa; Orange Lake is surrounded by pine forests and

improved pasture. Lake Wauberg is a 101 ha eutrophic lake 11 km south of Gainesville;








mean depth is 3.8 m (maximum = 5.2 m). It is surrounded primarily with hardwoods,

cypress, and sweet gum (Liquidambar styraciflua) swamp.

The secondary study area included Ocala National Forest and private lands on the east

side of Lake George. This area also contains numerous lakes and wet prairies. The major

water body, Lake George, is located on the St. Johns River system and is surrounded by bald

cypress, hardwoods, and pine forests. The lands surrounding Lake George are used primarily

for timber production.



Study Organization

The results and conclusions from this study are organized into 5 chapters. Chapter 2

examines post-fledging movements and habitat use. Timing of initial migration is related to

various climatic factors, sibling aggression, and food availability. The post-fledging period

begins when young fledge and ends when they leave the nest area to initiate migration.

Migration data are summarized in chapter 3. This chapter relates timing of migration

to various climatic factors and prey availability. I also examine speed and distance of

migratory movements and discuss out-of-state locations for 1- to 4-year-old eagles.

In chapter 4, I present movement and habitat use data for subadult eagles that have

returned to the Florida study area and examine these data with respect to sex and age

differences. I also address several human disturbance factors that may relate to movements

and habitat use.

Chapter 5 examines survival rates of 1 to 4 year old eagles and contrasts survival in 3

eagle populations in relation to migratory patterns. I also discuss survival in relation to sex,

timing of nesting, number of chicks, hatch order, and dispersal age. In chapter 6, I

synthesize and discuss findings from all aspects of the study.






5

Table 1-1. Habitat types identified from a Landsat image and amount of each available on the
study area in north-central Florida.



Percent of
Habitat Description Hectares study area


PINE Pinewoods and mixed pine/hardwoods 36,723 26.9

Sandhill 840 0.6

HDWD Hardwoods (hammocks, forests, swamps) 17,203 12.6

CYPR Cypress swamp 5,545 4.0

MARS Freshwater marsh/wet prairie/shrub swamp 16,945 12.5

WATR Open water 9,020 6.6

GRAS Grassland/improved pasture 24,455 17.9

CLCT Clearcuts, shrub and brushland 14,986 11.0

DEVL Developed (includes major roads) 10,934 8.0

Total 136,652 100.0







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CHAPTER 2

POST-FLEDGING MOVEMENTS AND HABITAT USE


Introduction

Management emphasis and protection for bald eagles are focused at nest sites (U.S.

Fish and Wildlife Service 1987) because it has been recognized that disturbance at nest sites

can decrease productivity. Restrictions specified near nests remain in effect until young

fledge. However, little is known about the post-fledging movements and habitat use of

fledgling bald eagles prior to their first migration. Timing of initial migration and factors that

influence it also are not well understood. These data are needed to determine if habitat

management guidelines for eagle nests (U.S. Fish and Wildlife Service 1987) adequately

address the habitat needs of fledglings. Because of the difficulty in locating and visually

observing the increasingly active fledglings, I used radio-tracking as the most efficient method

for obtaining movement and habitat use data for a large number of fledgling eagles.

Previous radio-tracking studies of fledgling eagles were conducted on the Chippewa

National Forest, Minnesota (Harper 1974, Kussman 1976) and in Maine (McCollough 1986).

A study using marked nestlings was conducted in Saskatchewan (Gerrard et al. 1974). All of

these studies, however, dealt with birds in extremely different environments than are found in

Florida.

I designed a study of fledgling eagles to address the following objectives using a

twofold approach. I collected extensive data on radio-tagged nestlings and supplemented these

data with intensive observations of nestlings at 2 nests. The objectives were:








1. Determine the length of time that fledgling eagles are dependent on their parents.

2. Characterize perch use, habitat use, and interactions with other eagles.

3. Determine the spatial needs of fledglings around the nest prior to migration.

4. Document the timing of initial migration by fledgling eagles and examine how this is

influenced by sibling aggression, prey deliveries, temperature, precipitation, and/or

water levels.



Methods

Banding and radio-tagging procedures

All known nests on the 2 study areas were surveyed once every 2 weeks between

November and May, 1987-1991, with a Cessna 172 or 152. Surveys were used to document

productivity of all breeding pairs and to determine dates for banding young and attaching

radio transmitters.

A total of 122 nestlings were banded at 7-9 weeks of age with USFWS aluminum

rivet bands (size 9) on the right leg. Eagles were removed from the nest by an experienced

tree climber and lowered to the ground in a duffel bag where all banding and measuring took

place. In 1987, a band tag with an alpha-numeric code was attached to each band. These

tags were very difficult to see on free-flying birds. Consequently from 1988 to 1991, I

attached wrap-around patagial markers made of Herculite fabric (Young and Kochert 1987) on

the right wing. Each tag is yellow with a green shape indicating the year (1988=circle,

1989=square, 1990=triangle, 1991 =circle) and a white two-digit number indicating the

individual (Figure 2-1). They are highly visible from an airplane and allow much easier

identification of young once they have fledged.

Additionally, 44 of the nestlings on the AMC study area were fitted with radio

transmitters at 8-9 weeks of age. Transmitters (manufactured by Telemetry Systems, Inc.,








Mequon, Wisconsin) were back-pack mounted with 1-cm wide tubular teflon ribbon (Balley

Ribbon Mills, Bailey, Pennsylvania). All 4 harness ends were sewn together at one point on

the breast with nylon thread to prevent loss of the package. Transmitters have a combination

of solar panels that theoretically should last indefinitely, and rechargeable nickel-cadmium

batteries with an estimated life of 4 years. Each transmitter package weighed approximately

55 g, about 1% of the weight of an average fledgling. They also were equipped with a

mercury activity switch that facilitated locating individuals. A change in the signal pulse

indicated when birds were moving rather than stationary.

I recorded a variety of physical measurements and estimated the fullness of the crop

for each bird handled. Bortolotti (1984) determined that bill depth and length of foot pad

were the two best indicators of sex for bald eagle nestlings, and length of the eighth primary

was the best measure for estimating age. I initially judged the sex of each nestling in the field

using these measurements and the relative size and appearance of each bird. I then used

discriminant function analysis of bill depth and length of the foot pad to confirm the sex of

each bird (Figure 2-2). At nests with 2 chicks, I estimated hatch order using length of the

eighth primary.

Intensive nest observations

Post-fledging behavior and habitat use were recorded for fledglings at 2 nests (AL32A

and MR17D) on the study area in 1991. Nest AL32A was in a pine tree located in a 57 ha

pasture with a few scattered pines. Nest MR17D was in a pine tree in a small patch of woods

(0.7 ha) surrounded by small patches of pasture, marsh, and woods. Observations began

when nestlings were approximately 9 weeks old and continued until the young left the nest

area. At nest AL32A, young appeared to have initiated migration when they left the nest

area. At nest MR17D, the patchy landscape made it difficult to follow the movements of

fledglings; observations were discontinued prior to migration.








Two young at each nest site were observed 2 consecutive half days each week to

document spatial use of the nest area and behavioral interactions between parents and young

prior to dispersal. This resulted in 1 full day of observations from just before dawn to just

after dusk per week. Data recorded every 15 minutes included location (measured from

topographic maps with a 45.7 m grid) and activity of each fledgling and number of adults

present. In addition, all interactions (between fledglings, between fledglings and adults, or

between fledglings and other species) and all prey items delivered were recorded.

I calculated the distance each fledgling was located from the nest during each 15

minute observation. Analysis of variance and Waller-Duncan K-ratio t-tests were used to

examine distance from the nest in relation to age for fledglings at each nest. Intensive data

collected from birds at these 2 nests were used to supplement the extensive data collected

from radio-tagged nestlings.

Extensive radio-tracking surveys

From 1987 to 1990, 41 nestlings fitted with transmitters were monitored from 8-

weeks-of-age until they left the study area (3 died prior to fledging-Chapter 5). Fledglings

were considered to have initiated migration when they could not be located within the study

area by aerial radio-tracking. Each bird was located from 1 to 22 (x = 8) times using aerial

and ground surveys for a total of 372 locations. For each location, data were recorded on

date, time, location, activity, habitat, and association with other eagles.

Age at migration was calculated as the mid-point between the last date an individual

was located on the study area and the date of the next survey when the individual was not

located. I used analysis of variance to test for a year effect on migration age. I used t-tests

to examine mean distance to the natal nest and age of migration in relation to sex, number of

chicks in the nest, timing of fledging and hatch order. I used linear regression to examine the








relationship between age at migration and hatching date. Data were weighted to account for

repeated observations on individuals where appropriate.

Climate and water level data

I obtained water level data for Alachua Lake from Paynes Prairie State Preserve,

Florida Department of Natural Resources. Water level data for Lochloosa, Newnans and

Orange lakes were obtained from the U.S. Geological Survey (U.S. Geological Survey 1987-

1991). I plotted mean monthly water levels in feet NGVD (National Geodetic Vertical Datum

of 1929) for each of these lakes (Figure 2-3). Maximum and minimum daily temperatures

and daily precipitation recorded at Gainesville Regional Airport (National Oceanic and

Atmospheric Administration 1987-1991) were summarized into monthly mean temperatures

(Figure 2-4) and total monthly precipitation (Figure 2-5).



Results

Over the 5 years that I banded eagles, 58 of the banded nestlings were male and 54

were female (Figure 2-2). Discriminant function analysis of bill depth and foot pad

measurements found that only one individual was originally misclassified. Consequently, this

bird identified as a male in the field was reclassified as a female because its measurements

(bill depth = 30.4 mm, foot pad = 129 mm) were more within the range found for all

females (Table 2-1). For radio-tagged individuals, 26 were classed as male and 18 were

female (Table 2-2).

Extensive radio-tracking surveys

Prior to fledging, all observations of radio-tagged young during aerial surveys were

birds perched in the nest or in the nest tree. Nestlings were considered to have fledged at 11

weeks of age. After fledging, 91% of all sightings (n=289) were of perched birds, 8% were

flying, and 1% were soaring. Of the 202 observations of perched fledglings, 45% were in








the nest. Fledglings perched in pine trees accounted for 39% of all observations. This

reflects the preponderance of eagle nests in pine trees. All other observations of perched

birds occurred in snags (6%), cypress (1%), hardwoods (1%), other trees (1%), and on the

ground (1%).

I recorded 60 instances during aerial surveys where radio-tagged fledglings were

perched with other eagles. Eagles were perched with siblings on 75% of the observations,

22% with adults, and 3% with both adults and siblings. The adults present during these

observations most likely were the parents as they were perched at or very near the natal nest

site.

Two birds were located > 10 km (11.5 and 32.2 km) from the nest on the last date

each was located on the study area. Since neither was located on the next aerial survey, I

assumed the previous location represented the initiation of migration. These 2 locations and

locations of birds prior to fledging were excluded from calculations of the mean distance

fledglings ranged from the nest.

Radio-tagged fledglings generally were located close to the nest. The mean distance

for 40 individuals was 0.2 km (Table 2-3). The maximum distance a fledgling was located

from its nest was 4 km. There was no difference in relation to sex or timing of fledging;

however, fledglings with a sibling tended to range significantly farther from the nest than

those from nests with only 1 young (Table 2-3).

Over the 4 years, the earliest date a fledgling was last seen on the study area was 23

April 1989; the latest date was 30 July 1987. The earliest and latest dates varied each year

(Table 2-4). The average date immatures left the study area was 8 June. Fledglings tended

to migrate when water levels were dropping (Figure 2-3), when temperatures were

approaching or at their annual high (Figure 2-4), and when fish abundance was at an annual

high and beginning to decline (Figure 2-6). Precipitation (Figure 2-5) did not appear to be








correlated with migration. The mean and maximum dates for migration in 1989 and 1990

were 1 month earlier than in 1987 and 1988, possibly related to a prolonged drought. The

drought caused water levels to drop on all lakes and wetlands in 1989; in 1990, many

wetlands became completely dry.

The estimated age fledglings migrated from the study area ranged from 104 to 153

days of age (Table 2-2), with an average of 127 days (7 weeks post-fledging) (Table 2-5).

Data for the 4 years were combined because there was no year effect on migration age

(F = 0.64, = 0.60). There also was no difference in the age at which young migrated

from the study area in relation to sex, timing of fledging, number of chicks that fledged from

a nest, or order of hatching (Table 2-5). Similarly, linear regression analysis of migration

age and hatch date showed no relationship (R2 = 0.009) (Figure 2-7). The largest difference

in migration age occurred for order of hatch. First-hatched birds tended to migrate at a

younger age than second-hatched birds.

Intensive nest observations

As I observed with radio-tagged young, nestlings under intensive observation were

perched during the majority of the 727 observations (97.4%). Fledglings were most often

perched in the nest tree, either in limbs adjacent to the nest (39.4%) or in the nest itself

(19.8%) (Figure 2-8). I recorded 5 activities for bald eagles during the nest observations:

exercise, fly, feed, perch, and soar (Figure 2-9). Fledglings spent most of their time perched

(85.1%). They spent very little time flying or soaring. The second most common activity

was feeding, accounting for 10.2% of the observations.

I recorded 50 associations/interactions between fledglings, fledglings and adults, and

fledglings and other birds (Table 2-6). A fledgling perched with its sibling accounted for

40% of the interactions. It was uncommon for fledglings to perch with their parents (12% of

observations), as adults rarely were present at or near the nest. Twelve aggressive






14

interactions (aggressive posturing and biting) between siblings were observed. All 8 cases of

aggressive posturing occurred at nest AL32A; 5 of these were associated with the possession

of prey. The older and larger female in all 5 food-related cases dominated the younger male

sibling. The 3 instances of aggressive posturing of the younger male towards the female

occurred when the female returned to the nest from a bout of flying. The 2 biting interactions

at this nest also were initiated by the female during food conflicts. All aggressive interactions

occurred on or before 30 April, at least 1 week before the older sibling migrated. At nest

MR17D, 2 biting interactions were observed, one initiated by each sibling. No aggressive

interactions between chicks and adults were observed at either nest.

A fledgling eagle was observed interacting with other bird species only 3 times (Table

2-6). A red-tailed hawk (Buteo iamaicensis) and 2 loggerhead shrikes (Lanius ludovicianus)

stooped on this fledgling. Turkey vultures (Cathartes ar were chased by this same

fledgling.

I observed 15 prey deliveries at the 2 nests under observation (Table 2-7). In

addition, at least 9 other prey items had already been delivered to the nest when observation

sessions began. When young were observed feeding on fresh prey before any deliveries were

recorded for the day, the prey item being consumed was considered a delivery for the day,

although it might have occurred late the day before.

At nest AL32A, 0 to 3 prey items were delivered during each half-day observation

(Table 2-7). The older sibling from this nest left the nest area between 7 and 13 May when

prey still was being delivered by the adults. The younger sibling left the area on 13 or 14

May. On its last day of observation, no prey deliveries were observed and it frequently

vocalized and followed the adult male. Prey deliveries were difficult to observe at nest

MR17D due to thick vegetation and became impossible once fledglings began spending

considerable time away from the nest tree.








Young at both nests were first observed away from the nest tree at approximately 13

weeks of age, and continued using the nest tree up to 16 (MR17D) and 17 (AL32A) weeks of

age. The distance that fledglings were observed from the nest varied significantly with age

for both nests (AL32A: F = 38.26, P = 0.0001; MR17D: F = 40.32, P = 0.0001; Table

2-8). As young became older and more skillful at flying, they moved farther from the nest

(Table 2-8). At nest AL32A, fledglings remained significantly closer to the nest prior to 18

days post-fledging than they did at older ages.

The maximum distance any fledgling was observed perched from either of the 2 nests

was 0.9 km (with 1 exception, see below). They undoubtedly ranged farther than this,

however, because those at nest AL32A were lost from view on a few occasions while soaring

and those at nest MR17D were lost from view in the dense trees. An immature eagle

believed to be one of the fledglings from nest AL32A was seen perched approximately 8.3 km

from the nest, near the area its parents used for foraging. At this time (5/14/91), the

fledgling would have been approximately 17 weeks of age. This was the last time this eagle

was observed. Attempts to locate it in the vicinity of the nest on the following day were

unsuccessful. The distant perch site appeared to be a precursor movement to migration.



Discussion

Nestlings at 8 weeks of age in this study had slightly smaller bill depth and foot pad

measurements (Table 2-1) than eagles of a similar age hatched from eggs collected in Florida

and raised in captivity. The mean bill depth for captive-raised birds was 31.3 mm for females

(n=20) and 29.0 mm for males (n=28) (A. Jenkins, pers. commun.). Likewise, mean foot

pad length was 134.6 mm for females and 123.2 mm for males. Fledglings raised in captivity

were supplied with unlimited amounts of food and probably attained maximum size for

southern bald eagles, unlike wild nestlings that might have experienced periods of food








shortage. In contrast, the northern subspecies (i. 1. alascanus) is larger than either wild or

captive-raised H. 1. leucocephalus nestlings. Bill depth in H. 1. alascanus is approximately

33.3 mm for females and 30.1 mm for males, while foot pad length is 147.1 mm for females

and 131.8 mm for males (Bortolotti 1984).

In Maine, young fledged over a 34 day span (11 July to 14 August; McCollough

1986); in Florida, fledging occurs over a longer time span. Radio-tagged birds in this study

fledged from about 25 March to 27 May, a 63 day span. Because I did not make daily

observations, these fledging dates are not precise.

The distance fledglings perched from the nest tree in Maine ranged from 0.4 to 2.7

km (McCollough 1986), while the maximum distance young were observed to perch from the

nest in Minnesota was 6.9 km (Kussman 1976). Gerrard et al. (1974) reported that most

fledglings stayed within 1.6 km of the nest for approximately 7 weeks after fledging. In this

study, the maximum distance a radio-tagged fledgling was located from its nest was 4 km.

Intensive observations at 2 nests showed that fledglings remained within 400 m of the nest for

approximately 5 weeks post-fledging. Generally, it appears that fledglings remain in close

proximity to the nest site until they initiate migration.

McCollough (1986) observed a decrease in food deliveries 6 to 8 weeks after young

fledged. He concluded that reduced feeding likely initiated migration. Herrick (1934) also

observed a reduction in food deliveries at one nest. However, at the same nest the following

year and at a second nest, he saw no decrease in food deliveries. Kussman (1976) observed

no decrease in the number of prey items brought to a nest in Minnesota. It is unknown if the

biomass of food delivered in these studies changed over time during post-fledging. At one

nest, I observed no decrease in the number of food deliveries before the older sibling

migrated; however, food deliveries decreased before the younger sibling migrated. Perhaps if

young do not leave on their own, food deliveries are decreased by adults, possibly in response






17

to declining prey availability. Fish availability declined on at least one lake on the study area

during the later months fledglings leave on migration (Figure 2-6).

As I observed in this study, Harper (1974), Kussman (1976), and McCollough (1986)

all indicated that fledgling eagles relied almost exclusively on their parents for food and

observed no active hunting before initiation of migration. Only Kussman (1976) observed 3

instances of scavenging of fish on sand beaches of nearby lakes. The earliest age at which

scavenging was observed was 46 and 60 days post-fledging.

The age at which fledglings left their natal sites varied in other studies as well,

ranging from 112 to 147 days of age (Harper 1974, Kussman 1976, McCollough 1986) with

only 1 individual leaving after 196 days of age (Harper 1974). The range of ages young left

the Florida study area was similar (104-153). Apparently, a certain amount of time is

required for fledglings to initiate migration regardless of their geographic origin, perhaps to

reach a physical condition capable of sustaining them during long-distance migration. The

older chick generally migrated first. It frequently dominated prey delivered to the nest and

likely builds up adequate reserves for migration more quickly.

I expected late-fledging young to migrate from the study area at a younger age;

however, a linear regression analysis showed no such relationship. This suggests that the

timing of migration is tied more to prey abundance than to seasonal changes in temperature.

If migration was tied to temperature one would expect late-fledging young to leave more

quickly (at a younger age) before temperatures became high. Perhaps at nests with greater

food availability, fledglings might migrate at a younger age because they can reach peak

condition more quickly. However, additional prey availability data are needed to test this

hypothesis.

The bald eagle habitat management guidelines (U.S. Fish and Wildlife Service 1987)

specify a primary protection zone with a boundary range of 229-457 m (750-1500 feet) from








any bald eagle nest used for breeding in the southeastern United States. Residential,

commercial, or industrial development, tree cutting, logging, and use of chemicals toxic to

wildlife are prohibited in this zone. Unauthorized human entry is restricted during the

breeding season. In Florida, the primary zone boundary is set at 229 m; development

occasionally is permitted within this zone. Restrictions are enforced until nestlings fledge.

The protection zones specified around nests in the habitat management guidelines are

not as large as the maximum distance of 4 km that radio-tagged fledglings were seen from the

nest. The mean distance of 0.16 km was well within the boundary of the smaller primary

zone (229 m), reflecting use of the nest by fledglings as the hub of their activities. Fledglings

observed intensively at 2 nests stayed within the 229 m boundary only until approximately 3

weeks post-fledging (Table 2-8). By approximately 6 weeks post-fledging these eagles had

ranged outside of the boundary of the larger (457 m) primary zone. To protect the amount of

habitat that is sufficient for the needs of fledglings and to prevent disturbance that may cause

premature migration, the boundary of the primary zone should be at least 457 m from the

nest.

Restrictions in human activity around nest sites also must allow for the time fledglings

remain dependent on the adults and the nest site, approximately 7 (range 4-11) weeks post-

fledging. Currently, protection from disturbance as inferred from the bald eagle habitat

management guidelines (U.S. Fish and Wildlife Service 1987) extends only until the time

young fledge. Disturbance near a nest while fledglings still are dependent on adults may

prevent adults from supplying adequate amounts of food to dependent young or cause

premature dispersal of young before they can build up adequate food reserves.

Migration is energetically demanding, particularly to young and inexperienced birds.

Fledglings in less than optimum physical condition when initiating migration may be less








likely to survive the energetic demands of migration. It appears that fledglings may not stop

to forage during their initial stages of migration (A. Jenkins, pers. commun.). At least 5

birds from this study traveled more than 100 km per day indicating rapid movement north

(see Chapter 3). This rapid movement north, in concert with little foraging, emphasizes the

need for fledglings to be in optimum physical condition to withstand the rigors of migration.
























Table 2-1. Bill depth (mm) and length of foot pad (mm) for 8-week-old
bald eagle nestlings in north-central Florida from 1987 to 1991.


Variable Sex n x SE Range


Bill depth F 54 31.1 0.11 29.4-33.3
M 58 28.3 0.12 26.4-30.6

Foot pad F 46 133.3 0.71 122-141
M 58 121.1 0.63 104-129








Table 2-2. Hatching dates, migration dates, and perch distance for bald eagle nestlings fitted
with solar-powered radio transmitters, 1987 to 1990, in north-central Florida.

Maximum
Nest Radio Sex Estimated Last date Estimated age at distance
number frequency hatch date in study area migration (days) from nest (m)


1987
AL-7A
LV-25
AL-7A
AL-27
AL-35
MR-108
AL-17C
AL-35
AL-33
AL-19

1988
AL-15A
AL-10
MR-107
AL-17B
AL-14
AL-40
AL-40
AL-24A
AL-24A
AL-33
AL-32A
AL-32A

1989
AL-33
AL-28C
AL-10
AL-40
AL-26A
AL-28C
AL-40
AL-1A
AL-24A
AL-3B
AL-17A
AL-17A
AL-3B


165.100
165.520
165.616
165.675
165.718
165.861
165.881
165.942
165.958
165.999


165.081
165.145
165.155
165.180"
165.212a
165.241
165.262
165.418
165.561
165.593
165.957
165.998


164.197
164.399
164.496
164.738
164.756
164.798
164.814
164.895
164.897"
165.210
165.698
165.755
165.933


3/9/87
3/7/87
3/9/87
2/5/87
2/5/87
2/5/87
2/9/87
2/5/87
3/14/87
2/5/87


1/28/88
2/22/88
2/12/88
2/22/88
2/22/88
2/22/88
2/22/88
3/8/88
3/8/88
3/11/88
1/11/88
1/11/88


2/1/89
1/14/89
2/1/89
2/8/89
2/5/89
1/14/89
2/8/89
2/28/89
?1/
1/19/89
1/2/89
1/2/89
1/19/89


7/30/87
7/10/87
7/30/87
7/2/87
6/3/87
6/10/87
6/16/87
5/27/87
7/2/87
6/3/87


5/11/88
6/23/88
6/17/88


7/7/88
7/7/88
6/28/88
7/13/88
7/25/88
5/6/88
5/6/88


6/23/89
5/11/89
5/26/89
5/19/89
5/3/89 ?
5/18/89
6/21/89
6/28/89

5/26/89
4/23/89
6/1/89
5/26/89


145
129
145
151
122
126
132
115
113
122


108
124
127


138
138
112
129
137
119
119


143
121
117
104
(radio out?)
124
137
123

130
117
153
130


524
243
423
802
0
639
368
0
0
0


0
0
412


0
0
0
294
505
0
0


582
3968
0
0
0
681
2996
0

566
1076
1137
566








Table 2-2 cont.


Maximum
Nest Radio Sex Estimated Last date Estimated age at distance
number frequency hatch date in study area migration (days) from nest (m)


1990
AL-50 164.011 M 1/16/90 5/20/90 126 0
AL-32A 164.033 M 1/16/90 5/15/90 121 0
AL-26A 164.666 F 2/8/90 6/10/90 124 349
AL-32A 164.902 M 1/16/90 5/15/90 121 0
AL-43 164.963 F 1/22/90 5/26/90 126 0
AL-29A 164.969 F 3/5/90 6/21/90 112 240
AL-3B 165.570 M 1/16/90 5/26/90 132 137
AL-3B 165.580 M 1/16/90 5/26/90 132 137
AL-1A 165.992 F 2/8/90 6/21/90 137 450

aRecovered dead prior to fledging.








Table 2-3. Distance (km) fledgling bald eagles were located from their natal nest
prior to initiation of migration by sex, number of chicks fledged from nest, and timing
of fledging, 1987 to 1990 in north-central Florida.


Variable n wn" x SE t P


Sex
F 110 15 0.12 0.06
M 182 25 0.18 0.06 -0.66 0.52

Number of chicks
1 104 14 0.05 0.02
2 186 26 0.22 0.07 -2.35 0.03

Timing of fledgingb
Late 169 18 0.12 0.05
Peak 121 22 0.19 0.08 -0.74 0.46

Total 292 40 0.16 0.05

"wn = sample size when weighted for repeated observations on individuals
bLate = last 25% and peak = first 75% of all clutches laid within a breeding season.


Table 2-4. Earliest and latest dates fledgling bald eagles were last
located on the study area in north-central Florida, 1987 to 1990.


Year n x Minimum Maximum


1987 10 24 June 27 May 30 July

1988 10 18 June 6 May 25 July

1989 12 26 May 23 April 28 June

1990 9 31 May 15 May 21 June

Total 41 8 June 23 April 30 July








Table 2-5. Age (number of days after hatch) at which fledgling bald eagles left the
north-central Florida study area in relation to sex, timing of fledging, number of
chicks fledged from a nest, and order of hatch, 1987 to 1990.


n x SE Minimum Maximum t P


Sex
Female 17 125 2.3 112 151 -1.09 0.28
Male 23 129 2.6 104 153

Timing of fledging"
Late 18 128 2.8 104 145 0.54 0.59
Peak 22 126 2.4 108 153

Number of chicks
1 13 127 3.6 108 151 -0.10 0.92
2 27 127 2.1 104 153

Order of hatch
1 15 124 2.6 104 145 -1.56 0.13
2 13 131 2.9 115 153

Total 40 127 1.8 104 153

'Late = last 25% and peak = first 75% of all clutches laid within a breeding season.


Table 2-6. Interactions observed for fledgling bald eagles at 2 nests
in north-central Florida after 11 weeks of age, 1991.


Interaction C C C-A O-C


Stooped on 2
Chased by 1
Perch together 20 6
Flying together 4 2
Fed by 3
Aggressive posturing 8
Bites at 4

"C = chick, A = adult, 0 = other (red-tailed hawk, loggerhead
shrike, turkey vulture). First letter is individual that initiated
interaction; second letter is individual that received it.








Table 2-7. Prey deliveries post-fledging at 2 bald eagle nests in north-
central Florida in 1991. Nestlings were considered fledged at 11 weeks
of age (AL32A = 4 April 1991, MR17D = 22 April 1991).



Nest Days Number prey deliveries
number Date post-fledging Observed Assumed"


AL32A 4/11/91 7 0 0
4/12/91 8 0 1
4/16/91 12 0 1
4/17/91 13 2 0
4/22/91 18 1 1
4/23/91 19 2 0
4/29/91 25 1 0
4/30/91 26 2 0
5/6/91 32 1 1
5/7/91 33 2 1
5/13/91 39 0 1
5/14/91 40 0 0

MR17D 4/27/91 5 0 0
5/1/91 9 1 1
5/8/91 16 1 1
5/9/91 17 1 1
5/15/91 23 1 0


feeding on fresh prey


"Assumed prey delivery because young observed
when nest observations began.








Table 2-8. Distance (m) from the natal nest in north-central Florida that fledgling
bald eagles were located in relation to days post-fledging in 1991. Nestlings were
considered fledged at 11 weeks of age. For each nest, means with the same letter
are not significantly different (Waller-Duncan K-ratio t-test).



Nest Days
number post-fledging n x SE Range


AL32A 7 50 0 D 0.0 0-0
8 50 0 D 0.0 0-0
12 55 0 D 0.0 0-0
13 38 10 D 6.7 0-183
18 53 17 D 4.0 0-102
19 52 88 C 9.4 0-188
25 58 154 B 12.4 0-233
26 52 93 C 11.7 0-233
32 42 150 B 14.2 0-233
33 45 220 A 24.6 0-421
39 28 67 C 7.2 0-91
40 21 149 B 45.3 0-867

MR17D 5 28 0 D 0.0 0-0
9 58 0 D 0.0 0-0
16 29 0 D 0.0 0-0
17 42 16 C 4.3 0-102
23 45 52 B 7.1 0-129
30 10 0 D 0.0 0-0
36 11 101 A 3.3 91 -129
37 6 57 B 16.0 0-102








































bright yellow tag

green shape

white number










Figure 2-1. Patagial marker attached to nestling bald eagles in north-central Florida and its
location on the right wing of the bird.

















Female


5. 5
.5


145



140



135



130



125



120



115



110


27
27


S 2
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Figure 2-2. Measurements of bill depth (mm) and foot pad (mm) for 8-week old nestling bald
eagles in north-central Florida.


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Hatch Date

























Figure 2-7. Relationship between age radio-tagged fledgling bald eagles began their initial
migration from the study area and timing of hatching.


160




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Figure 2-8. Perch use of fledgling bald eagles under intensive observation in 1991 at 2 nests
in north-central Florida.















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Figure 2-9. Activity of fledgling bald eagles under intensive observation in 1991 at 2 nests in
north-central Florida.


Perch Exercise Feed Fly Soar
Activity













CHAPTER 3

MIGRATORY PATTERNS OF SUBADULT BALD EAGLES


Introduction

Avian migration patterns have been studied extensively (Gauthreaux 1982). Partial

migration, characterized by some individuals of a population remaining at or near the

breeding grounds, occurs in a large number of species (Lack 1954). In populations with this

migration strategy, predominantly young birds migrate and females migrate more than males

(Gauthreaux 1982). Bald eagles in different portions of their range exhibit various migration

patterns. Maine populations are partial migrants (McCollough 1986), Chesapeake Bay

populations do not migrate (Buehler et al. 1991b), while in some Canadian populations all

individuals appear to migrate (Gerrard et al. 1978). The migration strategy of the Florida

population is not well understood.

Immature eagles from Florida migrate northward, primarily along the coast, in late

spring and early summer (Broley 1947). Visual sightings of color-marked birds and band

returns from eagles hatched in South Carolina suggested a coastal route to the Chesapeake

Bay and then movement to Maine or the Great Lakes (Murphy et al. 1986). A secondary

route may follow the Appalachian Mountains. Summering areas for southern eagles range

from the Carolinas (Chester et al. 1990), through the Chesapeake Bay (Buehler et al. 1991b)

to Canada's Maritime Provinces and the Great Lakes (Broley 1947, Stocek 1985). Specific

areas along the migration corridor serve as stopover points or endpoints for migratory eagles.

These areas may play a significant role in the life history of Southern bald eagles. Although






36

northward migration pathways and some destinations are known, the timing of migration and

age-specific differences remain largely unknown. Few data were available for return

(southward) migration to Florida.

Gerrard et al. (1978) found that some subadult Saskatchewan eagles marked as

nestlings returned to the lake where they fledged. Observations of marked eagles establishing

territories near their natal area in Saskatchewan suggests a return of breeding age birds to

natal areas (Gerrard et al. 1980). Immature bald eagles tended to move farther from the

nesting region than subadult and adult bald eagles (Hodges et al. 1987, Stalmaster 1987). In

contrast, Gerrard et al. (1978) found that in Saskatchewan older immatures may migrate away

(south) from an area first, or migrate faster and farther than 1 year olds. In areas where

breeding densities are high (eg. Alaska and Florida), adults may remain near nesting areas

through the non-breeding season to retain possession of their nesting territories or to be in

position to move into a vacant territory (Hodges et al. 1987, W. Robertson, pers. commun.),

as is common for many avian species that exhibit partial migration (Gauthreaux 1982).

Eagles tagged with radio-transmitters permit repeated locations of a bird over several

years, rather than a single location when a bird dies and is recovered. Consequently, I used

radio-telemetry to address the following objectives:

1) Determine the timing of migration (arrival and departure dates) and examine

relationships with temperature, precipitation, water levels, and fish abundance.

2) Examine migration dates for age-specific and sex-specific differences.

3) Identify summering areas for Florida eagles.

4) Quantify the age-specific and sex-specific differences in migration distance.








Methods

Arrival and departure dates from fall 1987 to spring 1991 on the study area were

determined for 18 eagles radio-tagged as nestlings in north-central Florida. Radio-tagged

eagles rarely located on the study area were excluded from these analyses. Banding and

radio-tagging procedures are discussed in detail in Chapter 2. While any radio-tagged eagle

was on the study area, I tracked approximately once per week from a Cessna 172. I

continued to radio-track approximately once every 3 weeks when no radio-tagged birds were

on the study area to document their absence during the summer months. Beginning in early

September, I again radio-tracked approximately once per week to determine arrival dates on

the study area.

Locations from outside of Florida were compiled from recoveries of dead banded

birds, sightings of marked birds, and radio locations of eagles carrying radio transmitters.

Requests for information on marked eagles were mailed to the Hawk Migration Association of

North America, USFWS Cooperative Research Units, Audubon Society Chapters, and many

individuals and researchers in the eastern United States. Researchers in South Carolina,

Chesapeake Bay, New York, Maine, and New Brunswick monitored the radio frequencies

used for eagles marked in this study.

I used analysis of variance to test for differences in arrival dates, departure dates and

number of days gone from the study area for sex, age, and year. I also used analysis of

variance to examine migration distance in relation to sex and age. The Waller-Duncan K-

ratio t-test was used in multiple comparisons to determine where differences occurred. I

examined timing of migration in relation to water temperatures, air temperatures,

precipitation, water levels, and fish abundance. See Chapter 2 for sources of data.







Results

All radio-tagged eagles migrated away from the study area during the summer. The

latest date one was located on the study area was 7 July. The earliest that a radio-tagged

young returned was 14 September. However, an unmarked subadult was observed in August

during a boat survey of the shoreline on Newnans Lake.

Timing of migration

There was no difference between years (F = 1.37, E = 0.27) or sexes (F = 2.02,

P = 0.16) in the dates radio-tagged young arrived on the study area in the fall (Table 3-1).

There was, however, a significant difference in arrival dates by age (F = 7.94, P = 0.0003)

(Table 3-1). The Waller-Duncan multiple comparison test showed that 1-year old eagles

(i.e. young fledged the previous spring) returned to the study area significantly later than

individuals from older age classes. A wide range in arrival dates occurred for the 1 year age

class. The earliest arrival date was similar for all 4 age classes of eagles (mid-September).

The latest arrival dates varied considerably and were much later for 1-year old eagles.

Departure dates for the 3 year age class were slightly earlier than for the other age

classes, but the difference was not significant (F = 2.23, P = 0.10) (Table 3-1). There was

no significant difference in departure dates by sex (F = 2.62, P = 0.11), although females

left on average 18 days later than males. There also was no significant difference by year (F

= 1.32, P = 0.28), although eagles tended to leave somewhat earlier in each successive year

of the study. The latest departure dates in 1990 and 1991 were approximately 1 month earlier

than in 1988 and 1989, and the earliest departure dates also were earlier in 1989, 1990 and

1991, probably related to the drought. During the course of the study, water levels dropped

on all of the major lakes on the study area (Figure 3-1); many wetlands including Alachua

Lake became completely dry. In 1989, declining water levels probably concentrated fish in








smaller pools; by 1990, water levels were so low that fish availability undoubtedly was

reduced.

Since individuals in the 1 year age class tended to arrive on the study area somewhat

later in the fall, they were gone from the study area for a somewhat longer time period during

the summer (Table 3-2). There was, however, no significant difference by age (F = 0.80,

P = 0.50) or sex (F = 0.45, P = 0.50) in the number of days individuals were gone from

the study area during the summers of 1987 through 1990.

Only 3 individuals did not return to the study area until their second (n = 2) or third

(n = 1) year. The maximum amount of time away for these 3 birds was 843 days; the

minimum was 590 days. All other eagles returned to the study area each year.

Subadult eagles tended to migrate away from the study area when water levels were

declining (Figure 3-1) each year of the study except 1991. In January and February 1991,

unusually high rainfall increased water levels in area lakes. Precipitation (Figure 3-2) did not

appear to be correlated with migration dates.

Departure dates from the study area seemed to correlate with maximum weekly

temperature. The majority of radio-tagged birds left the study area before temperatures

exceeded and remained above 30"C (Figure 3-3). Earliest arrival dates on the study area

occurred after maximum temperatures dropped below 30C.

I assumed that arrival and departure dates during the 4 years of this study were

representative of other years to allow comparison of fish abundance for Newnans Lake in

1985 and 1986 (Edwards 1987) with mean arrival and departure dates for subadult eagles

from this study. Eagles tended to arrive on the study area in the fall when fish abundance

was low (Figure 3-4). Waterfowl are available from November through March with peak

abundance occurring in January. This prey resource undoubtedly compensates for the low







fish availability at this time. Several subadult eagles initiated migration in the spring when

fish abundance was at its annual peak (Figure 3-4).

Long-distance movements

I obtained locations outside of Florida for 29 subadult eagles; 23 of these had radio-

transmitters (Table 3-3, Figure 3-5). All but 1 location south of the Chesapeake Bay and

most of the locations north of the Bay were coastal. Birds located in Pennsylvania, New

York, and just west of the Bay appeared to have followed major river systems or a chain of

the Appalachian Mountains. It is unknown what routes were taken by the 4 birds located in

the Midwest. Three of these locations occurred in 1990.

One radio-tagged eagle (165.942) returned to the same general area each summer

from 1987 to 1990. The area was not checked in 1991. It was located near the coasts of

northern Maine and southern New Brunswick between 28 June and 23 September each year

(Table 3-3). Three of the locations were within 60 km of each other; 2 were within 15 km,

but all 3 occurred in different years.

Subadult eagles in this study migrated up to 2,403 km north of their natal area (Table

3-4). For these analyses, I used the farthest location for each individual at each age. The

mean maximum distance north that birds were located did not significantly vary by sex

(F = 0.32, P = 0.58) or age (F = 2.47, P = 0.09). There was no interaction between sex

and age (F = 0.50, P = 0.69). The farthest known summering areas for Florida eagles

were Prince Edward Island and Quebec, Canada. The closest known summering area was on

the South Carolina coast, where one bird (165.081) resided more than a month in 1988.

Radio-tagged eagles appeared to move north in the spring fairly rapidly. Data for

1-year-old eagles indicated rapid movement northward (Table 3-5). The most rapid

movement for an individual was 1,081 km in a maximum of 5 days (216 km/day). At least 5

individuals moved more than 100 km per day.








Discussion

Florida eagle populations are partial migrants with non-breeding subadults migrating

to northern latitudes during summer. Breeding adults probably do not migrate; many are

observed in Florida during summer months. Observations of bald eagles in adult plumage at

hawk migration counts probably are subadults that have attained adult plumage but are not yet

breeding. Because I radio-tracked known-age eagles, I was able to determine at what age

adult plumage characteristics are expressed. Birds 3 1/2 years of age showed considerable

white coloration in the head and tail. The dark osprey-like stripe through the eye and some

dark coloration in the tail was visible when these birds were in close proximity. They

reached adult plumage at about 4 years of age. One 6-year-old eagle that migrated had not

begun breeding.

Most subadults returned to Florida each fall, although 3 individuals remained away

from Florida for 2-3 years, similar to other species of partial migrants (Gauthreaux 1982).

One-year-old eagles returned to the study area significantly later than individuals from older

age classes. Gerrard et al. (1978) similarly found that first-year birds lagged behind older

birds on their return (northward) migration to Saskatchewan. I did not find any differences in

incidence or distance of migration for the sexes.

Several explanations for summer migration of subadult Florida eagles have been

proposed. One explanation is that northward migration in late spring occurs in response to

extreme summer heat and humidity. Most subadults left the study area before summer

temperatures peaked. Summer temperatures in Florida often exceed 30'C; they rarely reach

34C (Figure 3-3). Hayes and Gessaman (1980) documented thermal stress in raptors at

34"C. Adults frequently were observed during the summer months. I observed several on

summer boat surveys and have received reports from reliable observers of adults on the study

area during summer. One unmarked subadult was observed in August during a boat survey,








suggesting that eagles can withstand summer temperatures in Florida. However, heat stress

occurs at lower ambient temperatures in birds when humidity is high (King and Farner 1961),

suggesting that high summer humidity in Florida may be more stressful.

It is doubtful that eagles migrate entirely in response to declining food availability.

Earliest migration occurred when waterfowl abundance had decreased and few remained on

area lakes. The extent to which subadults used this prey base is unknown. Edwards (1987)

showed that fish abundance on one of the major lakes in the study area reached its peak

during the time most subadults left the study area (Figure 3-4). Water levels are dropping at

this time (Figure 3-1), which should make fish prey more available for a short time by

concentrating the prey in a smaller area, although prey availability will decrease eventually.

Increasing water temperatures (Figure 3-6) also may cause fish to move into deeper water,

again decreasing prey availability. Lowest water levels, and potentially lowest prey

availability occurred when the latest subadults migrated. Although fish kills generally occur

on Florida lakes in August and September, they usually occur infrequently and involve low

numbers of fish (J. Estes, pers. commun.). Thus, these kills are an unpredictable prey

resource and occur too late in the summer to prevent subadults from migrating.

The migration patterns of subadult Florida eagles are the same as conventional

migrants; they travel north in spring and south in fall. Unlike other migrants, these eagles

were raised in the south immediately prior to northward migration. Apparently bald eagles in

Florida retain the typical migratory instinct of most migrants, but timing of breeding has been

adjusted to take advantage of the more favorable weather and prey abundance conditions that

occur in winter.

Frequent observations of adult bald eagles in summer indicate that they remain on or

near breeding areas. Newton (1979) generalizes that adult raptors may spend a longer time

on the breeding range to maximize their time for breeding. In addition, he noted that adults






43

have a competitive advantage when prey becomes scarce. Thus immatures are more likely to

leave the area, as I observed in Florida subadult eagles.

Eagles radio-tagged as nestlings in Florida from this study arrived on the Chesapeake

Bay from 19 April to 22 July (mean=6 June) and departed from 19 June to 17 October

(mean=3 September; Buehler 1990). On their northward migration, Florida eagles often

were located on the Chesapeake Bay only 1 time, indicating that they did not remain there

long. On southward migration, the same individuals often were located several times (Table

3-3; D. Buehler, pers. commun.) indicating more leisurely movements south. Generally,

these birds passed through the Chesapeake region quickly with the mean stay on the Bay

being only 4.5 days (Buehler 1990).

Data suggest that subadult eagles are philopatric to summering areas. One individual

in this study (165.942; Table 3-3) was located in the same general area of Maine during 4

consecutive summers. Eagles radio-tagged in this study consistently frequented 2 areas of the

northern Chesapeake Bay (Buehler et al. 1991b). Gerrard et al. (1978) reported that a

marked Saskatchewan eagle migrated to the same wintering area in 4 successive years.

Thus, migration data allow identification of important summering areas for Florida

subadult eagles. Management plans can then incorporate provisions to ensure the protection

of these areas from habitat alteration and human disturbance, or public acquisition. For

example, the most significant summer concentration area in Virginia recently was acquired by

the Nature Conservancy for transfer and later sale to the U.S. Fish and Wildlife Service (M.

Byrd, pers. commun.). Several eagles marked in this study were observed using this area

(M. Byrd, pers. commun.).

The U.S. Fish and Wildlife Service currently is reviewing the status of bald eagles in

the lower 48 states to determine whether populations in any of the 5 recovery regions warrant

downlisting from endangered to threatened status. Three recovery regions occur in the East








(northern, Chesapeake, and southeastern) and are used at various times of the year by

southeastern eagles. Consequently, downlisting of eagle populations in the northern or

Chesapeake regions could negatively impact southeastern eagles. Downlisting of northern

eagle populations and the possible accompanying relaxation of habitat protection in northern

states could reduce survivorship of young from Florida and other southeastern populations.

Development is compromising habitat suitability for eagles on the Chesapeake (Buehler et al.

1991c) and in coastal Maine (Todd 1979), 2 areas eagles from this study are known to

frequent.

Four of 6 known dead subadult eagles from this study were recovered during or after

migration north of the study area (Figure 3-5). Three died within 4 months after banding.

One flew into a powerline and was electrocuted; the cause of death for the other 3 was not

determined. Given the fact that migration is physically demanding and hazardous, especially

to young and inexperienced birds, it is particularly important that high quality habitat be

protected along the migration route. Consequently, there is concern about the negative effects

that the proposed downlisting may have on the survival of southern bald eagles.








Table 3-1. Timing of arrival and departure from the north-central Florida study
area for 1 to 4 year old subadult bald eagles, fall 1987 to spring 1991.


Arrival dates Departure dates
n x Range n x Range


Age (years)
1 18 12/16 9/17-3/11 19 5/5 2/1-7/7
2 14 11/3 9/20-1/15 13 5/3 4/10-7/5
3 8 10/12 9/14 11/8 8 4/3 3/1 5/24
4 4 10/16 9/24 11/12 4 4/26 4/2 6/10

Sex
F 15 11/18 9/14-3/11 16 5/9 4/2-7/7
M 27 11/11 9/17-2/25 28 4/21 2/1-6/28

Year
1987-88 6 12/9 9/17- 3/1 6 5/25 4/15- 7/7
1988-89 7 11/15 9/25-1/5 7 5/2 2/1-7/5
1989-90 14 11/9 9/14- 1/24 13 4/28 3/7 5/24
1990-91 17 11/12 9/24-3/11 18 4/18 3/1-6/10

Total 44 11/15 9/14-3/11 44 4/28 2/1-7/7


Table 3-2. Number of days gone from the north-central Florida
study area for 1 to 4 year old subadult bald eagles, spring 1987
to spring 1991.


Days gone
n x SE Range


Age (years)
1 18 192 14.1 69-300
2 13 173 11.6 84-245
3 7 153 17.1 71-201
4 4 179 20.5 145-238

Sex
F 15 167 15.1 71-300
M 27 184 9.0 69-275

Total 42 178 7.9 69-300








Table 3-3. Tracking history and out-of-state locations and recoveries of subadult bald eagles
(March 1987 to June 1990). Locations are in Alachua and Marion counties study area, unless
indicated otherwise.


Nest Radio Probable Tracking history
number frequency sex Dates Location


1987 Fledglings


AL 7A 165.100 M


11/11/87
2/8/88 5/18/88
10/11/88 4/14/89
10/16/89 10/20/89
2/6/90 3/7/90
6/28/90
10/25/90 11/24/90
12/5/90 2/12/91
2/19/91 4/10/91


SE of study area




S of Oromocto on St. Johns River, NB

S of study area


LV 25 165.520 M 9/17/87 5/24/88

AL 7A 165.616 M none since 7/30/87

AL 27 165.675 F 11/12/87- 6/10/88
3/17/88
3/20/88
10/6/88
11/29/88 4/14/89
10/4/89 5/2/90
8/90
9/24/90 10/25/90
10/31/90 4/22/91

AL 35 165.718 F 12/1/87, 12/2/87

4/20/88


MR 108 165.861


F 10/15/87


11/12/87 7/7/88
7/25/88
10/20/88 4/23/89
5/3/89 7/5/89
9/14/89
9/20/89


in SE Georgia
near Gainesville, FL
N Florida/S Georgia


Lake Erie, OH (unconfirmed)
N of study area


SW of study area past Otter Creek
toward Gulf Hammock
upper Chesapeake Bay

upper Rappahannock River, 20 miles
upstream from Chesapeake Bay

upper Rappahannock River, Virginia

SE of study area


S on Lake Harris








Table 3-3 (cont.)


Nest Radio Probable Tracking history
number frequency sex Dates Location


165.861
(cont.)


10/16/89 1/15/90
1/17/90 5/24/90
10/31/90 6/10/91


AL 17C 165.881 F 9/17/87
11/12/87- 5/18/88
10/11/88 3/13/89
5/25/89
9/20/89 4/17/90
9/24/90 4/2/91


mostly SW of study area
about half SW of study area

N of Jasper, Georgia


N Florida/S Georgia


F 5/27/87
6/7/87


9/23/87
10/15/87

11/20/87
3/1/88, 3/2/88, 4/15/88
8/18/88
11/22/88
12/7/88
8/17/89
10/28/89
11/8/89
2/21/90
4/9/90
6/28/90


AL 33 165.958 F 7/7/87


AL 19 165.999 F 5/27/87
6/25/87
8/30/87

AL 39 M 2/4/91


upper Chesapeake Bay, Aberdeen
Proving Ground, MD
northern ME, near New Brunswick
lower Chesapeake Bay S of Potomoc
River mouth
lower Chesapeake Bay, James River

Hog Island, Machias Bay, ME
Potomac River
James River, Virginia
Machias River, town of Machias, ME
Chesapeake Bay
N of study area
SE of Lake Monroe, FL
N of study area
Ross Isl., E of Grand Manon, NB

died and recovered in Surf City, NC
(flew into powerline)


Potomac River, Westmoreland S.P., VA
recovered dead, Cape Jouriman, NB

recovered dead near Flemington, FL
(hit by car)


AL 35 165.942








Table 3-3 (cont.)


Nest Radio Probable Tracking history
number frequency sex Dates Location


- M 9/4/89


- M 4/16/89


Unidentified 1987 bird


6/23/88


recovered dead near Rhinelander,
Oneida Co., WI

remains found near Rochelle, FL

Mongaup River, Forestburgh,
Sullivan Co, NY


1988 Fledglings

AL 15A 165.081


M 6/23 7/29/88
9/29/88
11/8/88 1/24/89
3/16/89 6/28/89
9/20/89 5/8/90
10/2/90 4/12/91


SC coast, Santee River
N Florida/S Georgia
S of study area

mostly S and W of study area
mostly S and W of study area


AL 10 165.145 F 1/5/89
1/15/90


near Crystal River


MR 107 165.155


AL 17B 165.180

AL 14 165.212


F 6/17/88
6/22/88


mouth of Potomac River


F died prior to fledging

F died prior to fledging


AL 40 165.241 M 7/4/88
10/25/90 12/5/90
2/9/91- 3/11/91


AL 40 165.262

AL24A 165.418


M 7/17/88

M 8/24/88

9/5/88
9/21/88
11/29/88
8/8/89 9/4/89
2/11/90
2/24/90, 5/30/90
9/5/90


W side Chesapeake Bay below Baltimore

Naudua Creek, VA, N of Bay
Bridge/Tunnel
near Easton, MD
near Fredricksburg, VA
N Florida?
Chesapeake Bay
Horseshoe Beach, FL


S James River, VA


PU 18


VO 34








Table 3-3 (cont.)


Nest Radio Probable Tracking history
number frequency sex Dates Location


AL24A 165.561 F 10/31/88
1/17/89
3/20/89

AL 33 165.593 M 10/20/88
1/5/89 4/14/89
11/17/89 5/8/90
10/31/90, 2/3/91 3/11/

AL 32A 165.957 M 8/24/88
9/25/88 2/1/89
10/4/89 4/13/90
4/23/90 5/13/90

10/2/90, 10/9/90
10/31/90 3/1/91

AL 32A 165.998 F 5/22/88

Unidentified 1988 or 7/12/89
1989 bird
(with patagial and transmitter)

1989 Fledglings

AL 33 164.197 M none since 6/23/89

AL28C 164.399 F 5/11/89
2/12/91

AL 10 164.496 M none since 5/26/89

AL 40 164.738 M 5/19/89
12/30/90
2/5/91 3/1/91

AL 26A 164.756 M none since 5/3/89

AL 28C 164.798 M 10/16/89 5/4/90
10/31/90 4/10/91

AL 40 164.814 M 12/1/89 4/29/90
10/9/90 5/15/91


W of Aberdeen Proving Ground, MD
Prospect Bay, MD
Chesapeake Bay

N Florida/S Georgia


91

mouth of Patuxent River, Chesapeake Bay


Rappahannock River, VA/MD
(3 locations)
N of study area
mostly S of study area

Susquehana River, Bradford Co., PA

Catfish Lake, Croatan N.F.,
Craven Co., NC







SW L. Wauberg, far




S of study area
S of study area






half of locations S of study area
half of locations S of study area








Table 3-3 (cont.)


Nest Radio Probable Tracking history
number frequency sex Dates Location


AL 1A 164.895 M none since 6/28/89

AL 24A 164.897 M died prior to fledging

AL 3B 165.210 M 6/11/89
6/12/89
10/4/89 10/16/89
11/17/89 5/4/90
6/26/90 9/21/90

10/9/90 5/15/91

AL 17A 165.698 M 12/14/89
1/15/90 4/23/90
10/9/90 4/22/91

AL 17A 165.755 M 12/1/89 5/2/90
5/4/90
7/26/90 9/5/90
10/9/90
10/25/90 4/22/91


AL 3B 165.933 1






Unidentified 1989 bird


1990 Fledglings

AL 7B 164.011


M 8/17/89
12/1/89, 12/14/89
1/15/90
1/24/90 5/8/90
10/31/90
11/24/90 4/22/91

6/6/90 6/25/90


M 6/7/90 6/17/90
10/14/90
10/24/90


Susquehanna River
Chesapeake Bay
S of study area

Potomac River, Chesapeake Bay
(6 locations)


SE toward Rodman Reservoir

half of locations S and W of study area


far N of Newnans Lake
Potomac River, MD/VA (4 locations)
NW of Newnans Lake


Machias River, ME
S of study area
W of study area

N of study area


near Hillsborough River, Prince
Edward Island (3 locations)


Smutty Nose Island, Isles of Shoals, NH
Back Creek Neck, MD
S James River, VA


AL32A 164.033 M


10/3/90


York River, VA


Fletcher, Miami Co, OH


AL 26A 164.666


F 9/9/90 9/10/90








Table 3-3 (cont.)


Nest Radio Probable Tracking history
number frequency sex Dates Location


AL 32A 164.902 M 3/11/91 4/10/91

AL 43 164.963 F 10/3/90

AL 29A 164.969 F 7/4/90
11/19/90 4/12/91

AL 3B 165.570 M 10/31/90
2/25/91 6/10/91

AL 3B 165.580 M 5/26/90
5/30/90

AL 1A 165.992 F 4/10/91

MR 57 F 7/19/90

Unidentified 1990 bird 1/3/91
(with patagial and transmitter)


N James River, VA

Onslow Co., NC, on New River


NW of study area



N of Waldo, FL



found dead in Sutton, Ontario

Mattamuskeet NWR, NC








Table 3-4. Maximum distance (km) from the north-central Florida study
area an individual was located at each age, spring 1987 to fall 1990.
Within a variable, means with the same letter are not significantly
different (Waller-Duncan K-ratio t-test).


Variable n x SE Range


Sex
F 18 1296 A 158.4 155-2355
M 15 1313 A 124.8 483-2261

Age
1 23 1276 B 121.0 155-2403
2 7 1131 B 215.5 160-2125
3 4 1526 AB 285.1 976-2129
4 2 2207 A 54.7 2152-2261

Total 37 1313 97.5 155-2403


Table 3-5. Distance migrated (km) and speed of movements for radio-tagged
bald eagles from north-central Florida during their initial migration, spring
1987 to fall 1990.


Last date Location Speed
Frequency on study area date Kilometers (km/day)


164.011 5/20/90 6/7/90 1796 100
164.666 6/10/90 9/9/90 1174 13
164.969 6/21/90 7/4/90 727 56
165.081 5/11/88 6/23/88 483 11
165.155 6/17/88 6/22/88 1081 216
165.210 5/26/89 6/11/89 1240 77
165.262 7/7/88 7/17/88 1190 119
165.418 6/28/88 8/24/88 1040 18
165.933 5/26/89 8/17/89 2125 26
165.942 5/27/87 6/7/87 1217 111
165.957 5/6/88 8/24/88 1115 10
165.958 7/2/87 7/7/87 684 137
165.998 5/6/88 5/22/88 1254 78














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CHAPTER 4

MOVEMENTS AND HABITAT USE


Introduction

Identification of habitats important to Florida's eagle population is necessary to

develop management guidelines, particularly since much of Florida is undergoing rapid

development and recreational use of major lakes is increasing. The human population of

Florida grew 27% between 1980 and 1988, and is projected to increase by an additional 20%

by the year 2000 (U.S. Bureau of the Census 1990). Detrimental impacts on bald eagle

populations have been documented. For example, although the overall number of breeding

pairs has increased in Florida, eagle populations on the southwest coast have declined in

numbers and productivity (S. Nesbitt, pers. commun). This area has undergone particularly

rapid development; 62% of the 45 active nests in the area were on lands owned by

development corporations, and 32% of 28 nests examined had urban development as the

major land use in the secondary protection zone (Wood et al. 1989).

Habitat alteration, including disturbance at nest sites, is the most significant factor

limiting recovery of eagle populations (U.S. Fish and Wildlife Service 1989). This includes

cutting nest trees (Weekes 1974), logging and other human disturbance near nest sites leading

to nest abandonment (Juenemann 1973), destruction of important perches and winter roosts

(Stalmaster 1976, Stalmaster and Newman 1979, Hansen et al. 1981), and habitat losses to

real estate development (Wood et al. 1989) or flood-control projects (Shapiro et al. 1982).

Shoreline development poses a particularly significant threat to bald eagle habitat.

Eagles on the northern Chesapeake Bay tended to avoid developed shoreline areas (Buehler et








al. 1991c). Continued development of otherwise suitable habitat may limit use by eagles to

areas with protected status, such as state and federal lands. Only 28% of 116 nests examined

in Florida were on public lands; the majority (17%) were located on a single national forest

(Wood et al. 1989). Concentration of eagles in these habitat islands would increase

competition for limited food resources, nest sites, and roost sites. Public lands could not

support the number of breeding pairs currently inhabiting Florida (601 in 1991; S. Nesbitt,

pers. commun.). Although eagles may habituate to human activity and development, as have

several nesting pairs in Florida, it is unknown how this affects their future survival and

productivity, or if a nest will continue to be used once the habituated pair dies.

Currently, habitat protection zones are enforced around active bald eagle nests (U.S.

Fish and Wildlife Service 1987) because it has been recognized that disturbance at nest sites

can decrease productivity. Increasing productivity is not an effective management strategy,

however, if immatures subsequently do not survive due to lack of adequate foraging, roosting,

and loafing areas. At this time in Florida, there are no habitat protection measures aimed at

foraging or roosting habitats that are important to non-breeding subadults, or to breeding

adults. Furthermore, habitats important to the non-breeding eagle population have not been

identified.

In this portion of the study, I identified age-related movement patterns, habitat use,

and features of the environment that influence use of habitats by subadults. I addressed the

following objectives:

1. Identify concentration areas for subadult bald eagles within the study area.

2. Examine philopatry to the study area and natal nest in relation to sex and age.

3. Examine frequency and occurrence of interactions between various age classes of

eagles.

4. Determine habitat preference of subadult eagles in relation to sex and age.








5. Determine if locations of radio-tagged eagles differ from random locations on the

study area, with respect to distance from various types of human disturbance (eg.

developments and roads), occupied eagle nests, and various features of the

environment.

6. Evaluate use of a GIS in the study of bald eagle habitat use.



Methods

Data on movements and habitat use of subadult eagles were collected primarily from

eagles radio-tagged as nestlings in Florida (see Chapter 2). Some additional data were

collected from southern eagles trapped and marked on the Chesapeake Bay. Radio telemetry

is particularly useful in a study of movements and habitat use because it is possible to

eliminate biases caused by differences in detectability of an animal in various habitats. It also

is useful in relocating individuals that have moved long distances as eagles frequently do.

From September 1987 to June 1988, radio-equipped young were tracked twice weekly

from the air using a single-engine Cessna 172 or 152, and periodically from a boat or truck.

Beginning in fall 1988 and continuing throughout the remainder of the study, radio-equipped

subadult eagles were tracked approximately once a week from the air. The majority of the

locations were from aerial tracking. Aerial tracking minimizes the bias associated with

frequent locations obtained in accessible areas when ground tracking (Samuel et al. 1985).

The pilot and an observer homed in on radio signals with a scanning receiver and H-

type antennas (Telonics, Inc., Mesa, AZ) mounted on each wing strut. I attempted to

pinpoint each signal on the study area to a specific location and obtained a visual location

whenever possible. When it was not possible to obtain a specific location, a general location

on the study area was recorded. Signals emanating from radio-tagged eagles off of the study








area generally were not followed due to time constraints, but a general direction was

recorded.

Data recorded included date, time, location, activity, habitat, and association with

other eagles. Eleven habitat types were defined on the study area (Table 4-1). Activity was

recorded as perched, flying, or soaring. Flying birds were differentiated as such to indicate

an association with the habitat in which they were located. Soaring birds were not associated

with a particular habitat type. When possible, I also determined percent canopy cover class

(0-25%, 26-50%, 51-75%, 76-100%) and perch location (canopy, supercanopy, edge of

canopy, edge of supercanopy) for perched birds. Locations were plotted in the field on

1:2,000 aerial photographs or topographic maps. Location data were converted to Universal

Transverse Mercator grid coordinates (Grubb and Eakle 1988) using 1:24,000 topographic

maps.

I used the multiple response permutation procedure (MRPP) test (White and Garrott

1990) to determine if individuals changed their use of the study area in successive years. This

non-parametric test makes no assumptions about the underlying distribution of the data, does

not require equal sample sizes, and detects subtle changes in the distribution of an animal's

locations. I had more than 1 year of data and large sample sizes (n = 10-53) on the study

area for 7 individuals and small sample sizes (n = 5-10) for 4 additional individuals. I did

not calculate home ranges because eagles wander widely and because White and Garrott

(1990) suggest that tests on the actual data are preferable. I used analysis of variance

weighted by individuals to examine sex and age differences in the mean distance subadults

were located from their natal nest.

Habitat use as determined from telemetry locations was first examined for differences

in sex or age by using multivariate analysis of variance (MANOVA) to test if the proportion

of observations in the habitats differed for sex or age. I used arcsine transformation to assure







that the distribution of the data approximated a normal distribution (Sokal and Rohlf

1969:386-387). Habitats used infrequently were pooled with structurally similar habitats to

assure that no more than 5% of the counts in each habitat were less than 5; this resulted in 6

categories. Developed habitat was available on the study area but never used by eagles and

was excluded from sex and age analyses.

Habitat availability was measured from a geographic information system (GIS) data

file (courtesy of the Office of Environmental Services, Florida Game and Fresh Water Fish

Commission) that identified habitats on the study area from a Landsat satellite image with a

resolution of 30 m2. Nine habitat types were identified on the study area (Table 1-1). Of

these, the sandhill habitat was rare and was never used by eagles. It was excluded from

analyses.

Habitat preference was analyzed with a Chi-square goodness-of-fit test (Siegel

1956:42-47) comparing use and availability. The eleven habitats used on the study area (Table

4-1) were combined structurally into 8 categories and compared to the available 8 habitat

categories measured from the Landsat image (Table 1-1). The Bonferroni z statistic was used

to examine each habitat to determine which contributed significantly to the overall Chi-square

statistic (Neu et al. 1974).

I also assembled a GIS data file containing all major and minor roads on the study

area. Data from Alachua County was obtained courtesy of the Alachua County Department of

Information Services. Additional roads on the study area outside of Alachua County were

digitized from 1:24,000 topographic maps. The categories of roads I defined included city

streets, county roads (which includes dirt roads and low-use paved roads), main roads (heavily

used, paved, two-lane roads), and four-lane roads (four-lane and interstate highways).

All GIS data files were overlaid and analyzed using ERDAS (Earth Resources Data

Analysis System) software. I measured the percent of each habitat type within 1-hectare and








10-hectare sized plots centered on each eagle location and on 300 random points. I also

measured distances from eagles and random points to the nearest bald eagle nest occupied by

a breeding pair, open water body greater than 0.4 ha in size, city street, county road, main

road, four-lane road, and developed habitat. Distances to roads and development were used

as indicators of tolerance to human disturbance. In addition, I determined the size of the

nearest open water body greater than 0.4 ha in size.

To examine the effect breeding pairs had on the distribution of subadult eagles, I used

analysis of variance on the distance individuals were located from nests occupied by a

breeding pair in relation to age, sex, and month. Multiple comparisons were made with the

Waller-Duncan K-ratio t-test.

To examine landscape level habitat preference, I used logistic regression on 19

variables (Table 4-2) to determine which characteristics differentiated between used and

random locations following the model selection strategy suggested by Hosmer and Lemeshow

(1989). Logistic regression is particularly useful when modeling a binary response variable

(Hosmer and Lemeshow 1989) such as presence/absence. Locations (n=519) of perched and

flying radio-tagged birds identified used areas. Locations of random points (n=300) in the

study area were generated using a random number generator in SAS.

I initially included each variable in a univariate logistic regression model. Those

variables with P < 0.25 were included in a multivariate logistic regression model as potential

variables for the final model. I examined all variables initially, even those correlated with

each other, so as not to prematurely eliminate a variable that might be important. I then

included the variables with P < 0.05 in a reduced multivariate logistic regression model and

compared the Akaike Information Criterion (AIC) values of the original and reduced models

(Lebreton et al. 1992, Burnham and Anderson in press). Variables were added or deleted

until the model with the lowest AIC was found. The lowest AIC results in the best








compromise between goodness-of-fit of the model and the cost of including an excessive

number of variables in the model (SAS Institute Inc. 1990). The AIC is calculated as

AIC = -2 log likelihood + 2(number estimated parameters)

I used the same analyses to examine these variables in relation to sex and age. I restricted

age-related analyses to age classes 1 and 2 because I had the most individuals in these 2

classes.



Results

From March 1987 to June 1991, I conducted 326 aerial surveys; I located 25 of the

radio-tagged eagles on 306 of these flights. Excluding locations of nestlings and fledglings, I

obtained 615 specific locations of radio-tagged subadult eagles and identified 118 general

locations. I also received 250 signals that emanated from a location off of the study area but

in Florida.

Movements

Individuals tended to wander on and off of the study area. As a crude index of

philopatry to the study area, I compared the number of locations on and off of the study area

for sex and age (Table 4-3). There was no interaction of sex and age (F = 0.70, P = 0.56)

so they were examined separately. No differences were found by sex (F = 1.90, P = 0.16)

or age (F = 0.11, P = 0.96). All eagles for which a specific location was obtained off of the

study area, but in Florida, were north of Osceola and Polk counties (Figure 4-1).

Over the 4 years of the study, eagles tended to use portions of the study area more

frequently than others (Figures 4-2 to 4-5). On Lake Lochloosa, 2 areas (Burnt Island and

Allen's Point) were frequented by large numbers of eagles, both marked and unmarked. On

Newnans Lake, radio-tagged eagles frequented Palm Point and the eastern shore of the lake.

Several small lakes and wetlands on the study area were heavily used for short periods each








year. One wetland east of Lake Lochloosa was used by local commercial fishermen and

hunters for disposal of waste fish parts; eagles frequented the area.

Of the 7 individuals with large sample sizes, 2 significantly altered their use of the

study area in successive years (E < 0.05). One individual (165.675) was located on the

study area for 4 years. Its use at age 1 was significantly different than at ages 2, 3, or 4 (

< 0.01). This resulted primarily from its frequent use of Lake Wauberg during 1988 (at age

1) that did not occur in subsequent years (Figure 4-6). The second individual (164.798) that

changed its use of the study area used the southeastern portion of the study area during its

second year (Figure 4-7). One of the 4 individuals with small sample sizes changed its use of

the study area. At age 1, eagles tended to wander throughout the study area (Figure 4-7). At

older ages, they restricted their movements to a smaller portion of the study area. Eagle

165.675, for example, restricted her movements primarily between Newnans and Lochloosa

lakes.

Although 1- to 4-year-old females tended to be located farther from their natal nests

than males, the difference was not significant (F = 2.06, P = 0.16) because of the high

variation found among females (Table 4-4). This pattern is consistent with the general trend

among birds for males to remain closer to the natal site. There was no difference between the

age classes (F = 1.06, P = 0.38) and no sex/age interaction (F = 0.82, P = 0.21). The

range for 1-year-old eagles, however, was larger than for the other age classes, indicating that

this age class tended to wander throughout the study area.

Interactions

I observed 242 interactions of radio-tagged subadult eagles with all age classes of

eagles, ospreys, and vultures. The interactions recorded included: chasing (5.8%), chased by

(3.3%), perching together (35.5%), flying together (35.1%), located in the vicinity of

(19.8%), and feeding with (0.4%). I observed a 2-year-old bird feeding with an adult. The








majority of the observations involved eagles perching or flying together and located in the

vicinity of another eagle.

Of the 14 interactions with vultures, 10 observations were of eagles soaring with

them, and 4 were eagles perched near vultures. Only 8 interactions with ospreys were

observed; 2 of these involved an osprey chasing an eagle and 2 were an eagle chasing an

osprey. Of the 120 interactions involving eagles where age class was identified, 46.7% of the

locations of radio-tagged subadults were in association with adult eagles. Griffin and Baskett

(1985) found that immature and adult eagles used overlapping foraging areas.

Perch characteristics

Perches used most often by subadult eagles were pine and cypress trees (Table 4-6).

Four locations were of eagles perched in unused eagle nests; 3 sightings were 1- and 2-year-

old birds. A 1-year-old eagle was located perched in an unused osprey nest.

Percent canopy cover was not different at perch sites used by males as compared to

females (X2 = 2.97, P = 0.40). Overall, 48% were located at perches with 0-25% canopy

cover, 2% at 26-50%, 9% at 56-75%, and 41% at 76-100% canopy cover.

I determined perch location (i.e. canopy, supercanopy, edge of canopy, or edge of

supercanopy) for 224 locations of subadult bald eagles. The distribution of eagles perched in

the 4 categories was not different for males and females (x2 = 4.66, P = 0.20). Overall,

eagles were perched in the canopy on 16% of the observations, on the edge of the canopy

(43%), in a supercanopy tree (30%), and on the edge of the supercanopy (10%).

Habitat use

There was no interaction between sex and age in habitat use (F = 1.05, P = 0.44),

therefore sex and age were examined separately. Habitat use on the study area did not differ

by sex (F = 0.49, P = 0.81) (Figure 4-9) or age (F = 0.70, 1P = 0.79) (Figure 4-10).

Consequently, data were pooled in analyses of habitat preference. There was a significant








difference between habitat use and availability (x2 = 603.5, P < 0.0001). Marsh, open

water, and hardwoods habitats were used in proportion to availability (Table 4-6). Cypress

was used significantly more than expected, while all other habitats were used significantly less

than expected (E < 0.05). The cypress habitat undoubtedly was used so frequently because

of its location along most lake edges.

I compared the habitat composition of a 1 ha and 10 ha square plot around each

location of a perched or flying eagle and found no difference (Table 4-7). The mean percent

of each habitat was similar for both sized plots. Consequently I used only data for 1-ha sized

plots in subsequent analyses. All habitats except developed comprised from 0% to 92% of

the 1-ha plot. The low percentage of developed habitat in the 1-ha plot (0% to 40%)

indicated avoidance of this habitat type.

The features of the landscape that discriminated between used and random points were

distance to main roads (MAIN), water (WATER), nest (NEST) and developments (DEVEL),

size of water (WATHA), and amount of developed (DEVL1), cypress (CYPRI), hardwoods

(HDWD1), and marsh (MARS1) habitats in a 1 ha plot (Table 4-8). Eagle locations tended to

be closer to water and nests, were nearer larger bodies of water, and had more cypress and

marsh habitat within the 1 ha plot than random locations. Eagle locations tended to be farther

from main roads and developed areas, and had less developed and hardwood habitat within

the 1 ha plot than random locations. The addition of other variables to the reduced model

resulted in an increase in the AIC, indicating that the reduced model was the most

parsimonious for the data.

The relationship between time of year (month) and size of the closest body of open

water was significant (F = 4.35, P = 0.001). All age classes of subadult bald eagles tended

to be located closest to smaller water bodies during October, November, January, February,

and March and nearer larger bodies of water in December, April, and May during the 4








years. Distance to the 3 largest lakes on the study area (Lochloosa, Newnans, and Orange)

varied significantly by month (F = 3.41, P = 0.0004) and was greatest in October, January,

February, and March.

The features of the landscape identified by logistic regression that differentiated

between locations of males and females were distance to main roads (MAIN), county roads

(CORD), all roads (ALLRDS), water (WATER) and eagle nests (NEST), and amount of

developed (DEVL1) and marsh (MARS1) habitats (Table 4-9). However, examination of the

data revealed that although these variables differed the most between the sexes, the actual

differences were not large.

Comparisions of age classes 1 and 2 showed that distance to eagle nests (NEST) and

development (DEVEL), size of water (WATHA), and amount of cypress (CYPR1) habitat

differentiated between locations of individuals in these 2 age classes (Table 4-10). Birds in

age class 1 tended to be closer to nests and developed areas, near smaller lakes, and had more

cypress habitat in the 1 ha plot than those in age class 2. The largest difference occurred in

distance to the nearest nest. Consequently, I examined distance to the nearest nest for all 4

age classes with analysis of variance.

Distance to the nearest nest occupied by a breeding pair of eagles varied significantly

between age classes 1 to 4 (F = 7.73, P = 0.0001), but not by sex (F = 1.19, P = 0.31).

There also was no interaction between age and sex (F = 0.53, P = 0.71). The Waller-

Duncan K-ratio t-test revealed that birds in age class 1 (x = 1,711 m) were located

significantly closer to nests than older age classes (2: x = 2,357 m; 3: x = 2,644 m;

4: x = 2,774 m). Distance to nests did not vary by month (November to June) (F = 1.63,

P = 0.12).








Discussion

Movements

A concentration of locations on Lake Wauberg (Figure 4-1) occurred from 23 January

to 22 February 1988, although eagles were located there in other months and years. Not only

were all radio-tagged eagles located there, but many unmarked eagles as well. Up to 80

individuals used a roost near the lake during this time. This is a fairly small lake, 101 ha in

size; however, a major gizzard shad (Dorosoma cepedianum) die-off occurred there during

this time, resulting in an abundant but temporary food source. Following the fish-kill, eagles

dispersed throughout the study area. Other temporally and locally abundant food sources on

the study area have resulted in smaller concentrations of eagles. Additionally, subadult eagles

were located closer to small lakes during most of the breeding season. Consequently, these

smaller water bodies are also important to subadult eagles. In Minnesota, Fraser et al. (1985)

also detected shifts of subadult eagles over a large area in response to temporally abundant

food sources, and subadult eagles in Saskatchewan increased their use of small water bodies

when spawning fish were available (Gerrard et al. 1990).

During 1988-89 few radio-tagged eagles returned to the study area. I believe that

several subadult eagles may have wintered farther north because of the warm winter weather.

The winter of 1988-89 was relatively mild on the Chesapeake Bay (D. Buehler, pers.

commun.). One radio-transmittered bird (165.561; Table 3-3) was located on the Chesapeake

Bay twice that winter, once in October and once in January, a time of year when southern

eagles normally do not occur there (Buehler et al. 1991b).

After subadults returned to the study area, they continued to use the same general

areas each year. The multiple response permutation procedure indicated that few birds

changed their spatial use of the study area in subsequent years. They generally used areas

near their natal nests (x = 19 km). Certain portions of the study area were used consistently








each year by large numbers of eagles. Thus, management for subadult populations must

include protection and management measures aimed at these important concentration areas.

Survival of subadults may be affected if a part of the area an individual uses becomes

unsuitable. The bald eagle habitat management guidelines (U.S. Fish and Wildlife Service

1987) suggest habitat protection measures for important roosting, loafing, and foraging areas.

Habitat Use

Subadult eagles were not distributed randomly over the study area. Logistic

regression analyses revealed that eagles tended to be located close to large water bodies and

eagle nests, and were frequently in cypress and marsh habitats. Because most lakes on my

study area are fringed with cypress, subadults likely preferred cypress because of its

proximity to water. In other parts of Florida or the Southeast, other tree-dominated habitats

near water might be preferred. Structural height diversity appeared important to subadults;

73% of perch locations occurred on the edge of the canopy or in the supercanopy. They

avoided main roads and developed areas. Habitat management and protection measures must

be aimed at areas that exhibit the habitat characteristics preferred by subadult eagles and that

act as important loafing and foraging sites for subadult eagles. As human populations and

lakeshore developments continue to increase, protected areas will become increasingly

important to the subadult population.

A high proportion of dead eagles recovered in Florida were hit by cars (Wood et al.

1990). One of 2 known causes of mortality for birds in this study was collision with a car.

The numerous roads and the increasing volume of traffic in Florida make this a significant

problem that appears to be increasing. The close proximity of radio-tagged eagles to the 4

categories of roads on the study area further highlights the problem.

Subadults frequently were located near large lakes; however, distance to the 3 largest

lakes was greatest when most eagle nests contain young chicks (January through March).








This may relate to temporal changes in prey availability or to greater aggression of nesting

adults to subadult eagles near nests. Most eagle nests are located close to large bodies of

water. Subadult eagles in age class 1 were located much closer to nests occupied by a

breeding pair than were older subadults. One-year olds may be trying to maintain an

association with adult bald eagles. Alternatively, adults may direct more nest defense

behavior towards older subadults since 3 1/2-year-old individuals in this study had significant

amounts of adult plumage. Gerrard et al. (1990) found that immatures tended to be farther

from nests with eggs or young present.

Although subadults often frequented areas near eagle nests, they wandered over large

areas. Thus, habitat protection around nests occupied by a breeding pair probably is not

sufficient to also serve the habitat needs of subadults. The average distance subadults were

located from nests was 2,055 m. As discussed in detail in Chapter 2, the boundary of the

primary protection zone generally used in Florida is 229 m from a nest. Outside of the

primary zone, there are few restrictions to habitat development.

Identifying the habitat requirements of nonbreeding eagles will provide the potential

for improving remaining habitat or identifying potential future habitat. Both of these

management practices will become increasingly important for mitigating loss of eagle habitat

as eagle populations continue their post-DDT recovery (Grier 1982) and as human demands

on the landscape continue their increase.

Use of a satellite image in a Geographic Information System (GIS) in this study of

bald eagle habitat use allowed examination of various features of the landscape. Proximity

analyses and habitat composition, for example, are measures that are extremely difficult and

time consuming by hand, particularly when dealing with a large number of locations on a

large study area. Thus, the GIS allowed more in-depth analyses of landscape habitat

characteristics.







Table 4-1. Habitats used by perched, flying, and soaring eagles located on the study area in
north-central Florida from fall 1987 to spring 1991.


Total Perched Flying Soaring
Habitat n % n % n % n %


Clearcuts 22 3.6 10 2.7 5 3.4 7 7.9
Cypress 120 19.8 101 27.4 16 10.8 3 3.4
Developed 0 0.0 0 0.0 0 0.0 0 0.0
Hardwoods 95 15.7 76 20.7 12 8.1 7 7.9
Lake 72 11.9 3 0.8 47 31.8 22 24.7
Marsh 85 14.0 35 9.5 37 25.0 13 14.6
Palm Hammock 6 1.0 5 1.4 1 0.7 0 0.0
Pasture 24 4.0 7 1.9 6 4.1 11 12.4
Pasture with trees 59 9.8 41 11.1 12 8.1 6 6.7
Pinewoods 116 19.2 85 23.1 11 7.4 20 22.5
Woods 6 1.0 5 1.4 1 0.7 0 0.0

Total 605 368 148 89








Table 4-2. Variables used in logistic regression analyses comparing used with random
locations, sex, and age for bald eagles in north-central Florida from fall 1987 to spring
1991.


Description


Distance (m) to nearest:
four-lane roads and interstate highways
main roads (heavily used, paved, two-lane)
county roads (dirt and low-use paved roads)
city streets
road of any type
developed habitat
bald eagle nest occupied by a breeding pair
open water body greater than 0.4 ha in size

Size (ha) of nearest open water body greater than 0.4 ha in size

Amount of each habitat found in a 1 ha square centered on each location:
Sandhill and xeric oak scrub
Pinewoods and mixed pine/hardwoods
Hardwoods (hammock, forest, and swamp)
Cypress
Freshwater marsh and wet prairie
Open water
Grassland with and without scattered trees
Clearcut with some shrub and brushland
Developed lands (includes roads)


Variable


FORLANE
MAIN
CORD
CITY
ALLRDS
DEVEL
NEST
WATER

WATHA


SAND1
PINE1
HDWD1
CYPR1
MARS1
WATR1
GRAS1
CLCT1
DEVL1







Table 4-3. Mean percent of bald eagle locations on the study area in
north-central Florida as an index of philopatry weighted for repeated
locations of individuals, fall 1987 to spring 1991.


Sex Age n wn" x(%) SE Range


F 356 21 68.3 8.1 0.0- 100.0
M 513 32 69.8 5.4 0.0- 100.0

1 427 24 67.3 7.8 0.0- 100.0
2 259 17 69.6 7.8 0.0- 100.0
3 119 9 65.3 12.0 0.0- 100.0
4 69 4 70.6 4.3 61.9-81.8

F 1 180 9 60.7 15.7 0.0- 100.0
2 51 5 82.3 8.7 55.6- 100.0
3 68 4 65.5 23.2 0.0- 100.0
4 57 3 71.9 5.7 61.9-81.8

M 1 242 14 76.4 7.3 0.0- 100.0
2 208 12 64.3 10.3 0.0- 100.0
3 51 5 65.2 14.1 25.0- 100.0
4 12 1 66.7 66.7-66.7

Total 874 54 68.0 4.6 0.0- 100.0

"Sample sizes weighted by individual.








Table 4-4. Mean distance (km) 1- to 4-year-old bald eagles were located from their
natal nest in north-central Florida from fall 1987 to spring 1991.


Sex Age n wn" x SE Range


F 269 17 27.5 9.2 0.7-571.4
M 332 29 13.9 1.3 0.3 155.8

1 345 19 19.3 4.8 0.3 571.4
2 158 14 12.8 1.9 0.5- 171.7
3 63 9 29.8 15.0 1.6- 149.3
4 35 4 14.1 0.5 2.5-40.4

F 1 160 6 27.0 14.4 1.4-571.4
2 31 4 16.1 5.7 0.7- 171.7
3 48 4 49.3 33.4 1.6- 149.3
4 30 3 14.4 0.4 3.1-40.4

M 1 185 13 15.7 2.7 0.3- 155.8
2 127 10 11.5 1.5 0.5- 40.8
3 15 5 14.2 1.8 1.8 30.4
4 5 1 12.9 2.5-23.4

Total 601 46 18.9 3.6 0.3 571.4

aSample size weighted by number of individuals.








Table 4-5. Perch use (%) of radio-tagged eagles 1 to 4 years of age located in north-central
Florida from fall 1987 to spring 1991.


Sex Age
Perch F M 1 2 3 4 Total


Snag 7.0 3.9 5.3 2.3 11.1 8.3 5.3
Pine 32.5 30.6 33.9 29.6 22.2 33.3 31.4
Cypress 25.5 24.4 24.3 21.6 27.8 37.5 24.9
Hardwood 15.9 16.7 13.2 20.4 27.8 8.3 16.3
Palm 1.9 2.8 2.1 4.6 0.0 0.0 2.4
Eagle nest 2.6 0.0 1.1 1.1 0.0 4.2 1.2
Osprey nest 0.6 0.0 0.5 0.0 0.0 0.0 0.3
Ground 7.6 7.8 7.4 10.2 5.6 4.2 7.7
Tree 3.8 12.2 9.0 10.2 2.8 4.2 8.3
Shrub 0.0 0.6 0.5 0.0 0.0 0.0 0.3
Fencepost 2.6 1.1 2.6 0.0 2.8 0.0 1.8

n 157 180 189 88 36 24 337


Table 4-6. Comparision of use and availability of 8 habitats for subadult
bald eagles in north-central Florida from fall 1987 to spring 1991.


Habitat Observed Expected" 95% CI


CLCT 0.029 0.110 0.009 0.049 < *b
CYPR 0.241 0.041 0.189 0.292 > *
DEVL 0.0 0.081 0.000 0.000 < *
GRAS 0.127 0.180 0.087- 0.167 < *
HDWD 0.170 0.127 0.124 0.215
MARS 0.139 0.125 0.097 0.180
PINE 0.199 0.270 0.151 -0.246 < *
WATR 0.096 0.066 0.061 0.132

n 519 135,811

"Expected proportions are based on habitat availability.
'< use less than availability; > use greater than availability; = use
equal to availability. Asterisks indicate observed significantly different
than expected, Bonferroni z statistic (P < 0.05).








Table 4-7. Percent habitat availability in 1-ha and 10-ha plots centered on locations of
radio-tagged eagles (n=519) in north-central Florida, fall 1987 to spring 1991.


1-ha plot 10-ha plot
Habitat x SE Minimum Maximum x SE Minimum Maximum


PINE 20.0 1.10 0 100 19.8 0.88 0 97
SAND 0.0 0.00 0 0 0.0 0.00 0 1
HDWD 9.6 0.83 0 92 9.6 0.62 0 71
MARS 14.7 1.02 0 100 15.0 0.86 0 100
CYPR 16.1 1.12 0 100 12.1 0.70 0 100
WATR 16.2 1.35 0 100 19.3 1.15 0 100
GRAS 9.5 1.04 0 100 10.0 0.91 0 100
CLCT 5.4 0.62 0 100 5.3 0.50 0 86
DEVL 1.3 0.22 0 40 1.7 0.20 0 30








Table 4-8. Comparison of used bald eagle locations and random points in north-central
Florida for various features of the landscape using logistic regression, fall 1987 to spring
1991. Variables are defined in Table 4-3. All variables included in univariate analyses;
variables with P < 0.25 included in full logistic regression model; variables with P < 0.05
included in reduced model.


Used Random Full model Reduced model
Variable x SE x SE P P


FORLANE 3374.0 97.1 3890.0 186.7 0.16
MAIN 1829.0 63.4 1457.0 77.1 0.0009 0.00001
CORD 488.0 16.3 476.0 26.0
CITY 5309.0 110.5 5378.0 201.9 0.31
ALLRDS 445.0 16.2 348.0 19.5 0.40
DEVEL 456.0 17.4 323.0 21.5 0.21 0.01
NEST 2055.0 87.6 4306.0 179.3 0.00001 0.00001
WATER 475.0 28.9 1396.0 72.7 0.00001 0.00001
WATHA 1308.0 51.3 476.3 54.6 0.001 0.0005
PINE1 20.0 1.1 25.9 1.9 0.51
HDWD1 9.6 0.8 14.0 1.5 0.58 0.05
CYPR1 16.1 1.1 3.0 0.6 0.31 0.00001
MARS1 14.7 1.0 9.3 1.3 0.47 0.06
WATR1 16.2 1.3 6.5 1.3 0.52
GRAS1 9.5 1.0 18.9 1.8 0.52
CLCT1 5.4 0.6 10.8 1.1 0.53
DEVL1 1.3 0.2 6.2 1.0 0.80 0.0053

AIC 753.76 744.28

aMultivariate analyses








Table 4-9. Comparison by sex of bald eagle locations in north-central Florida for various
features of the landscape using logistic regression, fall 1987 to spring 1991. Variables are
defined in Table 4-3. All variables included in univariate analyses; variables with P < 0.25
included in full logistic regression model; variables with P < 0.05 included in reduced
model.


Female Male Full model Reduced model
Variable x SE x SE P P


FORLANE 3255.0 136.8 3443.0 137.6 0.75
MAIN 2049.0 104.0 1629.0 79.4 0.005 0.0008
CORD 453.0 22.8 523.0 24.0 0.008 0.01
CITY 5463.0 162.4 5100.0 152.1 0.76
ALLRDS 430.0 22.4 462.0 24.2 0.02 0.03
DEVEL 431.0 22.7 485.0 26.8 0.30
NEST 2104.0 127.8 2014.0 121.1 0.10 0.07
WATER 517.0 44.9 431.0 38.9 0.04 0.09
WATHA 1308.0 76.6 1317.0 72.2 0.89
PINE1 21.3 1.7 18.1 1.4 0.24
HDWD1 9.7 1.3 9.8 1.1 0.23
CYPR1 16.6 1.7 15.3 1.6 0.24
MARS1 13.3 1.5 16.5 1.5 0.25 0.09
WATR1 15.9 2.1 17.1 1.8 0.24
GRAS1 8.9 1.5 10.0 1.5 0.25
CLCT1 5.7 1.0 5.1 0.8 0.25
DEVL1 1.9 0.4 0.8 0.2 0.16 0.007

AIC 621.06 603.11

"Multivariate analyses







Table 4-10. Comparison of bald eagle age classes 1 and 2 for various features of the
landscape using logistic regression, north-central Florida, fall 1987 to spring 1991. Variables
are defined in Table 4-3. All variables included in univariate analyses; variables with P <
0.25 included in full logistic regression model; variables with P < 0.05 included in reduced
model.


Age 1 Age 2 Full model Reduced model
Variable x SE x SE P P


FORLANE 3249.5 133.3 3520.6 179.1 0.30
MAIN 1671.0 67.8 1949.4 149.8 0.34
CORD 462.2 20.1 547.9 37.2 0.07
CITY 5191.7 147.9 5223.4 218.2 0.13
ALLRDS 426.9 20.1 492.2 37.7 0.09
DEVEL 433.3 20.2 535.3 44.7 0.10
NEST 1723.3 98.4 2356.7 189.3 0.0009 0.00001
WATER 430.0 33.0 466.5 66.0 0.29
WATHA 1228.0 67.2 1422.9 108.6 0.001 0.002
PINE1 19.0 1.4 17.7 2.0 0.01
HDWD1 10.1 1.1 9.0 1.8 0.01
CYPR1 17.3 1.5 13.6 2.1 0.01 0.13
MARS1 13.2 1.3 18.1 2.3 0.01
WATR1 17.0 1.9 17.8 2.8 0.01
GRAS1 9.9 1.4 9.3 2.1 0.01
CLCT1 5.4 0.8 5.5 1.3 0.01
DEVL1 1.0 0.2 1.3 0.5 0.01 0.01

AIC 490.16 479.01

aMultivariate analyses

















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