Population ecology of subadult southern bald eagles in Florida post-fledging ecology, migration patterns, habitat use, a...

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Title:
Population ecology of subadult southern bald eagles in Florida post-fledging ecology, migration patterns, habitat use, and survival
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English
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Wood, Petra Bohall
Collopy, Michael W.
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University of West Virginia -- West Virginia Cooperative Fish and Wildlife Research Unit
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Nongame Wildlife Program, Florida Game and Freshwater Fish Commission
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Tallahassee, Fla.
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Table of Contents
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Full Text







MAHSIUN SUktNUL LIBRARY











Population Ecology of Subadult
Southern Bald Eagles in Florida:
Post-Fledging Ecology, Migration Patterns,
Habitat Use, and Survival






Petra Bohall Wood1
Michael W. Collopy2


Department of Wildlife and Range Sciences
University of Florida
118 Newins-Ziegler Hall
Gainesville, Florida 32611

ICurrent address:
West Virginia Cooperative Fish & Wildlife
Research Unit
West Virginia University
P.O. Box 6125
333 Percival Hall
Morgantown, West Virginia 26506-6125

2Current address:
Bureau of Land Management
Forest Sciences Laboratory
3200 SW Jefferson Way
Corvallis, Oregon 97331



Submitted as final report for
Nongame Wildlife Program project NG87-026


December 1995























This report is the result of a project supported by the Florida Game
and Fresh Water Fish Commission's Nongame Wildlife Program. It has
been reviewed for clarity, style, and typographical errors, but has not
received peer review. Any opinions or recommendations in this report
are those of the authors and do not represent policy of the Commission.

















Suggested citation:

Wood, P.B., and M.W. Collopy. 1994. Population Ecology of Subadult
Southern Bald Eagles in Florida: Post-Fledging Ecology, Migration
Patterns, Habitat Use, and Survival. Fla. Game and Fresh Water Fish
Comm. Nongame Wildl. Program Final Rep. 111 pp + vi. Tallahassee, Fla.













POPULATION ECOLOGY OF SUBADULT SOUTHERN BALD

EAGLES IN FLORIDA: POST-FLEDGING ECOLOGY,

MIGRATION PATTERNS, HABITAT USE, AND SURVIVAL


Petra Bohall Wood1
Michael W. Collopy2

Department of Wildlife and Range Sciences, University of Florida,
118 Newins-Ziegler Hall, Gainesville, Florida 32611


Abstract: 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. 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 Ifi-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. Minimum survival
rates through 4fi years of age was 50% and did not vary by sex.

Once subadults returned to the study area in the fall, individuals continued to use the same general areas
each year. Temporally and locally abundant food sources on the study area resulted in temporary small
concentrations of eagles. Certain portions of the study area were used consistently each year by large
numbers of eagles. Management for subadult populations must include these concentration areas as
survival of subadults may be affected if a highly used area becomes unsuitable.

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. They avoided main roads and developed areas. Immature eagles (I year olds)
tended to be located closer to eagle nests than 2-4 year olds.

On Lochloosa Lake, we determined that boating activity significantly reduced the numbers of all age classes
of eagles using the lake. Boating activity concentrated during midday, however, did not affect eagle use of Lake
Wauberg. High boat use of lakes resulted in eagles perching farther from the shoreline and flushing at a greater
distance. We suggest that a public education effort be developed to reduce boating disturbance on eagles.

West Virginia Cooperative Fish & Wildlife, Research Unit, West Virginia University
P.O. Box 6125, 333 Percival Hall, Morgantown, West Virginia 26506-6125
2 Bureau of Land Management, Forest Sciences Laboratory
3200 SW Jefferson Way, Corvallis, Oregon 97331






NONGAME WILDLIFE PROGRAM FINAL REPORT


ACKNOWLEDGEMENTS

Financial support for this study 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
Miller Brewing Company. 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 work. Carolyn Sekerak also spent many hours collecting
fledgling behavior data. We 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 for banding
and radio-tagging. We also thank the many landowners who graciously
granted us access to eagle nests on their property. Pilot Steve Schwikert
skillfully and safely flew aerial surveys. Many individuals reported sightings
of marked bald eagles from this study outside of Florida. We 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. We thank Steve Nesbitt of FGFWFC for offering advice on
various phases of this study. Jim Weimer, Florida DNR, provided water level
data for Alachua Lake. 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. John Richardson and Leonard
Pearlstine provided expertise for the analyses involving a geographic
information system.


______ ____






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy





TABLE OF CONTENTS


A BSTRA CT .................................................. xi

ACKNOWLEDGMENTS ......................................... ii

CHAPTER 1: OVERVIEW AND GENERAL BACKGROUND
Introduction ................................................ 1
Study Area ...................................................2
Study Organization ............................................. 4

CHAPTER 2: POST-FLEDGING MOVEMENTS AND HABITAT USE
Introduction ................................................. .6
O objectives ....................................... ........... 6
Methods
Banding and Radio-tagging Procedures ......................... .6
Intensive Nest Observations ................................. 7
Extensive Radio-tracking Surveys ............................ .10
Climate and Water Level Data ............................... .10
Results
Extensive Radio-tracking Surveys ........................... .11
Intensive Nest Observations ................................ 21
D discussion ................................................ 25

CHAPTER 3: MIGRATORY PATTERNS OF SUBADULT BALD EAGLES
Introduction ............................................... 29
Objectives ..................................... ........... .29
M ethods ...................................... .......... 30
Results
Timing of Migration ...................................... 30
Long-distance M ovements .................................. .35
D discussion ................................................ 39

CHAPTER 4: MOVEMENTS AND HABITAT USE
Introduction ................................................ 44
O objectives ...................................... .......... 45
M ethods ........................................ .......... .45
Results ................................................... 49
M ovem ents ............................................ 50
Interactions ............................................ 56
Perch Characteristics ...................................... .60
H habitat use .............................................. 61
Discussion
Movements ............................................. 68
H habitat U se ............................... ............ 69






NONGAME WILDLIFE PROGRAM FINAL REPORT





CHAPTER 5: EFFECTS OF BOATING ACTIVITY
Introduction .......................... .......... ............ 71
Objectives ..................................... ........... .71
M ethods ....................................... ........... .71
Results ...................................................73
Discussion ................... .............. .. ............78

CHAPTER 6: SURVIVAL OF SUBADULT BALD EAGLES
Introduction ................................................ 81
Objectives .............................. ...... .......... .... 81
Methods .................................. ............... 82
Results ................................................... 83
Discussion ................................................ 89

CHAPTER 7: SYNTHESIS AND CONCLUSIONS ................... .92

LITERATURE CITED .......................................... 95

APPENDICES ............................................... .102
































vi






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


CHAPTER 1: OVERVIEW AND GENERAL BACKGROUND

Introduction

To fully understand the population dynamics of a species, all aspects of
the life history must be examined. For many avian species, the period from
fledging or independence to breeding is the least well understood. Similarly,
very little is known about seasonal movements, habitat requirements, or
survivorship of the various age classes in subadult bald eagle (Haliaeetus
leucocephalus) populations. 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 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.

The state of Florida supports over half of the breeding population of bald
eagles (H. 1. leucocephalus) in the southeastern United States (Wood et al.
1990). 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 would yield
results generally more applicable to other eagle populations than do studies on
small, relict, or reestablished populations.

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 non-
breeding and subadult eagles overlap with breeding areas.

Currently, primary management emphasis and protection is 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 effective, 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, as
well as for breeding adults. Furthermore, habitats and habitat characteristics
important to the non-breeding eagle population have not been identified.






NONGAME WILDLIFE PROGRAM FINAL REPORT


In summary, very little is known about subadult eagle populations,
particularly in areas where large winter concentrations do not occur. In this
study, we 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, we characterized migration patterns, movements,
habitat use, and survival rates of subadult bald eagles in north-central Florida
to provide data for effective management of this segment of the eagle
population. We also evaluated how these eagles are affected by various types
of human disturbance such as development and boating activity. Objectives
for each segment of the study are enumerated in the pertinent chapter.

Study Area

Research focused on eagles found in the study area comprising southern
Alachua, northern Marion, western Putnam and eastern Levy counties (AMC),
south of Gainesville, Florida (Fig. 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
wet prairies. 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.

Human disturbance from boating activity was evaluated on 3 lakes located in
Alachua County: Newnans, Lochloosa, and Wauberg. These lakes were selected
because they support large numbers of eagles and they are readily accessible.
They also are representative of the larger lakes found on the study area.

Newnan's Lake, located on the eastern edge of Gainesville, is a hyper-
eutrophic lake of 2,433 ha (Shannon and Brezonik 1972). The mean depth is
1.5 m (maximum = 4.0 m). Lochloosa Lake is a 3,561 ha meso-eutrophic lake
about 20 km southeast of Gainesville. Its mean depth is 2.7 m. Both Newnans
and Lochloosa have control structures that regulate water depth. They are
rimmed primarily with bald cypress (Taxodium distichum) and hardwoods.
Lake Wauberg is a 101 ha eutrophic lake about 11 km south of Gainesville.
The mean depth is 3.8 m (maximum = 5.2 m). It is surrounded primarily with
hardwoods and sweet gum (Liquidambar styraciflua) swamp.

The secondary study area included Ocala National Forest and private
lands on the east side of Lake George (ONF). 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.














































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






NONGAME WILDLIFE PROGRAM FINAL REPORT


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

SAND 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


Study Organization

The results from this study were organized into 6 chapters. Chapter 2
examines post-fledgling movements, habitat use, and migration. Timing of
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. We also examine
speed and distance of migratory movements and discuss out-of-state locations
for 1- to 4-year-old eagles.

In Chapter 4, we include 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. We also address several human disturbance
factors that may relate to movements and habitat use.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy 5




Boating activity on 3 major lakes in the study area and the effects on all
age classes of bald eagles are addressed in Chapter 5. We compare numbers
of eagles, age class distribution, perch distance from shoreline, and flush
distance for high boat use days (weekends) and low boat use days (weekdays).

Chapter 6 examines survival rates of 1- to 4-year-old eagles and
contrasts survival in 3 eagle populations in relation to migratory tendencies.
We also discuss survival in relation to sex, timing of nesting, number of
chicks, hatch order, and dispersal age. In Chapter 7, we summarize findings
from the entire study.


rl






NONGAME WILDLIFE PROGRAM FINAL REPORT


CHAPTER 2: POST-FLEDGING MOVEMENTS AND HABITAT USE

Introduction

Little is known about the post-fledging movements and habitat use of bald
eagles prior to their first migration. Timing of initial migration and factors
responsible for 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) are adequate for the needs of fledglings. Because of
the difficulty in locating and observing the increasingly active fledglings, we
decided that radio-tracking was the most efficient method for obtaining these
data on a large number of fledgling eagles.

Radio-tracking studies of fledgling eagles have been 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.

We developed a study of fledgling eagles to address the following
objectives using a two-fold approach. We collected extensive data on radio-
tagged nestlings and supplemented these data with intensive observations of
nestlings at 2 nests.

Objectives

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. Examine the spatial use of the nest area by quantifying the distance
fledglings range from 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 water levels.

Methods

Banding and radio-tagging procedures.-All known nests 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.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


From 1987 to 1991 on the 2 study areas, 122 nestlings were banded 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, we
attached wrap-around patagial markers made of Herculite fabric 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 2-digit number
indicating the individual (Fig. 2-1). They are highly visible from an airplane
and allow much easier identification of young once they have fledged.

Additionally, 44 young on the AMC study area were fitted with radio
transmitters. Transmitters (manufactured by Telemetry Systems, Inc.,
Mequon, Wisconsin) were back-pack mounted with 1-cm wide tubular teflon
ribbon (Balley Ribbon Mills, Balley, 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 a 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. All nestling eagles were approximately 8-9 weeks old when
transmitters were attached.

While we had the birds in hand, we took a variety of physical
measurements, evaluated each bird's condition, and estimated the fullness of its
crop. Bortolotti (1984) determined that bill depth and length of foot pad were
the 2 best indicators of sex for bald eagle nestlings, and length of the eighth
primary was the best measure for estimating age. At nests with 2 chicks, we
estimated hatch order using length of the eighth primary. We initially judged the
sex of each nestling in the field using these measurements and the relative size
and appearance of each bird. We then used discriminant function analysis on
bill depth and length of the foot pad to distinguish between the sexes (Fig. 2-2).
We used these techniques on all nestlings handled.

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 large pasture with few
scattered pines. Nest MR17D was in a pine tree in a small patch of woods
surrounded by small patches of pasture, marsh, and woods. Observations
began when nestlings were approximately 9 weeks old and continued until the






NONGAME WILDLIFE PROGRAM FINAL REPORT


bright yellow tag

green shape

white number


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








Female


145



140



135


E
,t130
"0


0125
U-


120



115



110


*

*
**


*I'~ **
~4~*

i~*"K i


**c 5$

,$)f ~

~t
si~ ~~*

j it*
*c~i
* *
**c *~


**
*t


** *


29 30
Bill depth (mm)


I


31 32 33
31 32 33


0
>




r:1
0
a
>

o







0-

r


e
0-











0
3
a


o






PI
o






34 0
5"


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


Male


*


i I I
26 27 28


*V- -V-


-I -






NONGAME WILDLIFE PROGRAM FINAL REPORT


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 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.

We 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 of the 44
nestlings fitted with transmitters were monitored from 8 weeks of age until
they left the study area (3 died prior to fledging Chapter 6). Fledglings
were considered to have initiated migration when they could not be located
within the study area by aerial radio-tracking. Each of the 44 birds 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. We used analysis of variance to test for a year
effect on migration age. We 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. We 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.-We 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). We plotted mean monthly water levels for each of these lakes (Fig.2-3).






Sdrought

0,0 J p Ii =
065 -0-0 -0 .-0
z I I



-, '-BBs ,
60 -

55 -




I ., II I I '
,




4 I . . I I . . . . I . . .. .

1/87 7/87 188 7/88 1/89 7/89 1/90 7/90 1/91
40
1/87 7/87 1/88 7/88 1/89 7/89 1/90 7/90 1/91 0

Alachua Lake Lochloosa Lake Newnans Lake Orange Lake
-----------------------
-0--o



Fig. 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.





NONGAME WILDLIFE PROGRAM FINAL REPORT


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

Results

Over the 5 years that we banded eagles, 58 of the banded nestlings were
male and 54 were female (Fig. 2-2). Discriminant function analysis of bill
depth and foot pad measurements found that only 1 individual was originally
misclassified. Consequently, this individual classified 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 of 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%).

We observed 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, we 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








Maximum Minimum
A---- <--




ui


A

'5;


35



030
4-


CL
S25 -

E





0
> 20 -


5 -




0


AA
4' '


Ir




f
/ A
A





I I




I I
U



U


A 'I
A











I

'U
UO
U,


J A J 0 J A J 0 J A J 0 J A J 0 J A


1987


1988


1989


1990


1991


Fig. 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.


A

:ri


A ,



I :




1'U
CO


1

pli*
t


t









1


a
A





#2


AA
A' '


4


A
I'
A


C
0


jr;t


SI


/u

r










EE E

E 2.5

0-0
f i r' lI 1 *\






1987 1988 1989 1990 1991
Fig. 2-5. Total monthly precipitation (mm) measured at Gainesville Regional Airport from January 1987 to January 199 Migration dates are for radio-tagged
l l l l o l .

!iii
0.5 ;^ I


0 o
J A J 0 J A J 0 J A J 0 J A J 0 J A
1987 1988 1989 1990 1991

Fig. 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.







BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy 15






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 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. Estimated 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 165.100 M 3/9/87 7/30/87 145 524
LV-25 165.520 M 3/7/87 7/10/87 129 243
AL-7A 165.616 M 3/9/87 7/30/87 145 423
AL-27 165.675 F 2/5/87 7/2/87 151 802
AL-35 165.718 F 2/5/87 6/3/87 122 0
MR-108 165.861 F 2/5/87 6/10/87 126 639
AL-17C 165.881 F 2/9/87 6/16/87 132 368
AL-35 165.942 F 2/5/87 5/27/87 115 0
AL-33 165.958 F 3/14/87 7/2/87 113 0
AL-19 165.999 F 2/5/87 6/3/87 122 0

1988
AL-15A 165.081 M 1/28/88 5/11/88 108 0
AL-10 165.145 F 2/22/88 6/23/88 124 0
MR-107 165.155 F 2/12/88 6/17/88 127 412
AL-17B 165.180b F 2/22/88
AL-14 165.212b F 2/22/88
AL-40 165.241 M 2/22/88 7/7/88 138 0
AL-40 165.262 M 2/22/88 7/7/88 138 0
AL-24A 165.418 M 3/8/88 6/28/88 112 0
AL-24A 165.561 F 3/8/88 7/13/88 129 294
AL-33 165.593 M 3/11/88 7/25/88 137 505
AL-32A 165.957 M 1/11/88 5/6/88 119 0
AL-32A 165.998 F 1/11/88 5/6/88 119 0

1989
AL-33 164.197 M 2/1/89 6/23/89 143 582
AL-28C 164.399 F 1/14/89 5/11/89 121 3968
AL-10 164.496 M 2/1/89 5/26/89 117 0






NONGAME WILDLIFE PROGRAM FINAL REPORT


Table 2-2. Continued.

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

AL-40 164.738 M 2/8/89 5/19/89 104 0
AL-26A 164.756 M 2/5/89 5/3/89 ? (radio out?) 0
AL-28C 164.798 M 1/14/89 5/18/89 124 681
AL-40 164.814 M 2/8/89 6/21/89 137 2996
AL-I A 164.895 M 2/28/89 6/28/89 123 0
AL-24A 164.897b M ???
AL-3B 165.210 M 1/19/89 5/26/89 130 566
AL-17A 165.698 M 1/2/89 4/23/89 117 1076
AL-17A 165.755 M 1/2/89 6/1/89 153 1137
AL-3B 165.933 M 1/19/89 5/26/89 130 566

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-IA 165.992 F 2/8/90 6/21/90 137 450


a Hatch date estimated from nesting chronology and age of nestlings when banded. Siblings likely
hatched 1-3 days apart; error in estimated hatch date could be 3 days.
b Recovered dead prior to fledging.



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). Fledglings tended to migrate when
water levels were dropping (Fig. 2-3), when temperatures were approaching or
at their annual high (Fig. 2-4), and when fish abundance was at an annual high
and beginning to decline (Fig. 2-6). Precipitation (Fig. 2-5) did not appear to
be correlated with migration. The average date immatures left the study area
was 8 June. 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 the major lakes and
resulted in many wetlands becoming completely dry. The estimated age
fledglings migrated from the study area varied from 104 to 153 days of age
(Table 2-2), with an average of 127 days (approximately 7 weeks after







BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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 wna 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


a wn = sample size when weighted for repeated observations on individuals
b Late = last 25%; 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






















S -
-t-


- D ---


- D ----


I I I I


I I 1 1 1 1 I | 1 I i I I 1
M A M J J A S O N D J F M A M J J A


1985


1986


Fig. 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.


30 F


25
.V
tl

E 20
z



15


10 F


\

I


II | I I I


I I






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


fledging) (Table 2-5). There was no year effect on migration age (F = 0.64, P
= 0.60), so data for the 4 years were combined. 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). The largest difference in migration age occurred for
order of hatch. First hatched birds tended to migrate earlier at a younger age.

We expected late-fledging young to migrate from the study area at an
earlier age; however, a linear regression analysis showed no such relationship
(R2 = 0.009) (Fig. 2-7). This suggests that the timing of migration is not tied
to seasonal changes in temperature but more to prey abundance. If migration
were tied to temperature one would expect late-fledging young to leave more
quickly (at a younger age) before temperatures became high. We suggest that
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.






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

a Late = last 25%; Peak = first 75% of all clutches laid within a breeding season.













160




150




a 140





C
0


0 120




110


R = 0.009


*
*

*
*
* *


* *


*+ *
*


( *


1UU II I

1 Jan 21 Jan 10 Feb 2 Mar

Hatch Date




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


'











40 .--- .-----------------.--------..................... ...- ... .................. ... ... .......... ................ ...........


O 30 -
-
0o

S20 -
0.


-. --- ---....-174...................... ... ......................... ....
174


.............................108..................................... ........
108


10 -4---------------------------------------


1 6


Ground


Hardwood


Snag


140








. . . .


Perch Type


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


279




I


Nest limb


I


I


F


I


.L


OW








626

4(


Perch


Exercise


...-.-.-.....- ............... ..... f-------4-- ---- -


9 7 11

-, ,4N


Feed


Soar


Nest limb


Activity


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


80 -4----


60 4 ----


40 -4---


20 ----


( I I I I I I






NONGAME WILDLIFE PROGRAM FINAL REPORT


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


a C = chick, A = adult, O = 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 Assumeda

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


a Assumed prey delivery because young observed feeding on fresh prey when nest observations began.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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). 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 1 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. Movement to the distant perch site
appeared to be a precursor to migration.

Discussion

Nestlings at 8 weeks of age in this study had slightly smaller bill depth and
foot pad measurements than eagles of a similar age hatched from eggs
collected in Florida and raised in captivity. The mean bill depth for these 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. In contrast, bill depth in the northern subspecies (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), unlike in Florida where 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 we did not make daily observations, our
fledging dates are not precise.






NONGAME WILDLIFE PROGRAM FINAL REPORT


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 na 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

a n is the number of 15-minute locations.


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 of 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 1 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






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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 1 nest, we 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 to declining prey
availability. Fish availability declined on at least 1 lake on the study area during
the later months fledglings leave on migration (Fig. 2-6).

As we observed in this study of radio-tagged nestlings and intensively
monitored nestlings, 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. It appears that the older chick often migrates first. The older bird
frequently dominates prey delivered to the nest and likely builds up adequate
reserves for migration more quickly.

The bald eagle habitat management guidelines (U.S. Fish and Wildlife
Service 1987) specifies a primary protection zone with a boundary of 229-457
m (750-1500 feet) from any eagle nest used for breeding in Florida. 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, development
occasionally is permitted within the 229 m primary zone.

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). 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, we
believe the boundary of the primary zone should be at least 457 m from the nest.





NONGAME WILDLIFE PROGRAM FINAL REPORT


Restrictions in human activity around nest sites also must account for the
time fledglings remain dependent on the adults and the nest site, on average 7
weeks post-fledging. Currently, protection from disturbance as specified in
the bald eagle habitat management guidelines (U.S. Fish and Wildlife Service
1987) only extends 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 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 northward, in concert with little
foraging, emphasizes the need for fledglings to be in optimum physical
condition to withstand the rigors of migration.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


CHAPTER 3: MIGRATORY PATTERNS OF SUBADULT BALD EAGLES

Introduction

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 suggest 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, this study), through the Chesapeake Bay (Buehler et al. 1991b) to Canada's
Maritime Provinces and the Great Lakes (Broley 1947, Stocek 1985, this study).
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 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 I-year olds. In areas where
breeding densities are high (e.g., 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.).

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, we used radio-telemetry to address the following objectives.

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.






NONGAME WILDLIFE PROGRAM FINAL REPORT


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. We tracked approximately once per week from a Cessna
172, while any radio-tagged eagle was on the study area. We 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, we 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.

We 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. We 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. We 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 1 was located on the study area was 7 July. The
earliest that 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,
P = 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






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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. Gerrard et al. (1978) similarly
found that first-year birds lagged behind older birds on their return
(northward) migration to Saskatchewan. 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 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 (Fig. 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 (Fig. 3-1) each year of the study except 1991. In
January and February 1991, unusually high rainfall increased water levels in
area lakes. Precipitation (Fig. 3-2) did not appear to be correlated with
migration dates.







NONGAME WILDLIFE PROGRAM FINAL REPORT


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 I 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 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









/U
z- I drought




/,., >. oI. I '0-O J G

S- --- ,

.. 0 .. .. 0-
> r








50 S 8 I

'I I







1/87 7/87 1/88 7/88 1/89 7/89 1/90 7/90 1/91
0o
--- e-------- I I
















Alachua Lake Lochloosa Lake Newnans Lake Orange Lake


Fig. 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.
a3 a- a.





0 i t


W/8 1o (dry) a,




AlachuaLake Lochloosa Lake Newnans Lake Orange Lake


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













3


E
E 2.5
c











0.5
0







t-


0.5



0


J A J O J A J 0 J A J 0 J A J 0 J A
1987 1988 1989 1990 1991


Fig. 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.





BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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 (Fig. 3-3). Earliest
arrival dates on the study area occurred when maximum temperatures dropped
below 30 C.

We 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 our
mean arrival and departure dates for subadult eagles. Eagles tended to arrive
on the study area in the fall when fish abundance was low (Fig. 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 (Fig. 3-4).

Long-distance movements.-We obtained locations outside of Florida for
29 subadult eagles; 23 of these had radio-transmitters (Appendix A, Fig. 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 (Appendix A). 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 3-3). For these analyses, we 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 1 bird (165.081) resided more than a
month in 1988.











Maximum Minimum
-A-- --i-


40







6 30

c5
4)-


E
I-25 -


_ 20

o
0
2 15











0
5 -


br


. I r r l _ r r _ _


I .' . .I' I I' I I
J 0 J A J 0 J A
1988 1989


^A
A
a
'A
i
^ '



/ a







i / ,
A


/ /


/ :*

I: :1





*V a
/I'

U

U0
U0
rs


AA



A
A




a/a
I






r | ,
f





/ :


* -.
0,
0
aoa


J O J A J O J A
1990 1991


Fig. 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.


'A

'A

A



, /

S



a
U
a)






I
0>
CO


a


a


A
^-*


A


A



I


I I
J A
1987


i


:4
i i
a
'4
S At
a r,
*A



a i
I/







C
!
a

00


B
I
I



e





t




II















*
, *


30 -


- S^


b
' .


---D --


I-- D -
-- A


I I I I


I I I I I


I I I I I I I


MA M J J A S N D J M J
M A M J J A S 0 N D J F M A M J J A


1985


1986


Fig. 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.


S25


U'

20
z

z
5 15






NONGAME WILDLIFE PROGRAM FINAL REPORT


Fig. 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.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


Table 3-3. Maximum distance (km) from the north-central Florida study area an individual was located at
each age, spring 1987 to fall 1990.

Variable n x a 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

a Means with the same letter are not significantly different (Waller-Duncan K-ratio t-test).


Radio-tagged eagles appeared to move north in the spring fairly rapidly.
Data for 1-year olds indicated rapid movement northward (Table 3-4). 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. On
northward migration, 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 (Appendix A; D.
Buehler, pers. commun.), indicating more leisurely movements south.

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 we radio-tracked known-age eagles, we were able to
determine at what age adult plumage characteristics are expressed. Birds 32
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.






NONGAME WILDLIFE PROGRAM FINAL REPORT


Table 3-4. 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


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. We 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 34 C (Fig. 3- 3).
Hayes and Gessaman (1980) documented thermal stress in raptors at 34 C.
Adults frequently were observed during the summer months. We observed
several on summer boat surveys and have received reports from reliable
observers of adults on the study area during summer (see Chapter 5 for
methodology of boat surveys). One unmarked subadult was observed in
August during a boat survey, suggesting that subadult eagles can withstand
summer temperatures in Florida as well as adults. However, heat stress occurs
at lower ambient temperatures in birds when humidity is high (King and
Farner 1961), suggesting that high summer humidity may be more stressful.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


It is doubtful that eagles migrate entirely in response to declining food
availability. Edwards (1987) showed that fish abundance on 1 of the major
lakes in the study area reached its peak during the time most subadults left the
study area (Fig. 3-4). Water levels are dropping at this time (Fig. 3-1) which
should make fish prey more available for a short time by concentrating the
prey in a smaller area, although eventually, prey availability will decrease.
Increasing water temperatures (Fig. 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. Earliest migration occurred when waterfowl abundance had
decreased and few remained on area lakes.

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). These birds
generally 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 somewhat philopatric to summering
areas. One individual in this study 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 either to ensure the protection of these areas from habitat alteration
and human disturbance or to implement 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 that 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








40




35



30
30


2
(D

0)
F- 25




20




15




10


NJ MM J S N J MM J SN J MM J S N
1988 1989 1990


J MM J S
1991


Fig. 3-6. Water temperatures averaged from 4 sampling stations on Lochloosa, Newnans and Orange lakes, 1988-1991 (Fla. Game and Fresh Water Fish Comm..
unpubl. data).






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy 43




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 (Fig. 3-5). 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, inexperienced birds, it is particularly
important that high quality habitat be protected along the migration route.
Consequently, we are concerned about the negative effects that the proposed
downlisting may have on the survival of southern bald eagles.





NONGAME WILDLIFE PROGRAM FINAL REPORT


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, certain areas are experiencing declining
populations and productivity. The southwest coast of Florida 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 eagle use to areas with protected status, such as state
and federal lands. Only 28% of 116 nests examined in Florida were on public
lands (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 (600 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






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


productivity is not effective, 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, as well as
breeding adults. Furthermore, habitats important to the non-breeding eagle
population have not been identified.

In this portion of the study, we identified age-related movement patterns,
habitat use, and features of the environment that influence use of the study
area by subadults. We addressed the following objectives:

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 habit 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 (e.g., 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).






NONGAME WILDLIFE PROGRAM FINAL REPORT


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. We 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). For most analyses, woods was combined with pinewoods,
and palm hammocks was included with cypress because very few
observations occurred in woods or palm hammocks. Activity was recorded
as perched, flying, or soaring. Flying birds were differentiated as such to
indicate a direct association with the habitat in which they were located.
Soaring birds were not considered to be associated with a particular habitat
for data analyses. When possible, we also determined percent canopy cover
class (0-25%, 26-50%, 51-75%, 76-100%) and perch location (canopy, super
canopy, edge) 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.

We 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. We 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. 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. We did not calculate home ranges because eagles wander
widely and because White and Garrott (1990) suggest that tests on the actual
data are preferable. We 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. We used arcsine transformation to assure 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






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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


assure that no more than 5% of the habitat types had bird counts of 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.
Overall habitat use was analyzed with a Chi-square goodness-of-fit test
(Siegel 1956:42-47) comparing use and availability. The 11 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 a 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).

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. Nine habitat types were identified on the
study area (Table 1-1). Of these, the sandhill habitat was very rare and was
never used by eagles. It was excluded from analyses.

We also assembled a GIS data file containing all major and minor roads
on the study area. Data from Alachua County were 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





NONGAME WILDLIFE PROGRAM FINAL REPORT


topographic maps. The categories of roads we defined included city streets,
county roads (which includes dirt roads and low-use paved roads), main
roads (heavily used, paved, 2-lane roads), and 4-lane roads (4-lane and
interstate highways).

All GIS data files were overlaid and analyzed using ERDAS (Earth
Resources Data Analysis System) software. We measured the percent of each
habitat type within 1 ha and 10 ha sized plots centered on each eagle location
and on 300 random points. We also measured distances to 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; 4-lane road; and developed habitat.
Distances to roads and development were used as indicators of tolerance to
human disturbance. In addition, we 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, we 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, we 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.

We 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. We examined all
variables initially, even those correlated with each other, so as not to
prematurely eliminate a variable that might be important. We 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).






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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.


Variable Description

Distance (m) to nearest:
FORLANE four-lane roads and interstate highways
MAIN main roads (heavily used, paved, two-lane)
CORD county roads (dirt and low-use paved roads)
CITY city streets
ALLRDS road of any type
DEVEL developed habitat
NEST bald eagle nest occupied by a breeding pair
WATER open water body greater than 0.4 ha in size

WATHA 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:
SAND1 Sandhill and xeric oak scrub
PINE Pinewoods and mixed pine/hardwoods
HDWD1 Hardwoods (hammock, forest, and swamp)
CYPR I Cypress
MARS1 Freshwater marsh and wet prairie
WATR1 Open water
GRAS1 Grassland with and without scattered trees
CLCT I Clearcut with some shrub and brushland
DEVL1 Developed lands (includes roads)






The AIC is calculated as:

AIC = -2 log likelihood + 2(number estimated parameters)

We used the same analyses to examine these variables in relation to sex and
age. We restricted age related analyses to age classes 1 and 2 because we had
the most individuals in these 2 classes.

Results

From March 1987 to June 1991, we conducted 326 aerial surveys; we
located 25 of the radio-tagged eagles on 306 of these flights. Excluding
locations of nestlings and fledglings, we obtained 615 specific locations of
radio-tagged subadult eagles and identified 118 general locations. We also
received 250 signals that emanated some distance off of the study area but
were in Florida.






NONGAME WILDLIFE PROGRAM FINAL REPORT


Movements.-Individuals tended to wander on and off of the study area.
As a crude index of philopatry to the study area, we 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 in
Florida off the study area were north of Osceola and Polk counties (Fig. 4-1).

Over the 4 years of the study, eagles tended to use portions of the study
area more frequently than others (Figs. 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.





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" (%) 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

a Sample sizes weighted by individual.






















* Bald eagle
telemetry locations
outside of
study area
0m
\a












\ t-
o







a


\ '-o


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
















































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













































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
















































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















































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





NONGAME WILDLIFE PROGRAM FINAL REPORT


Of the 7 individuals with large sample sizes, 2 significantly altered their
use of the study area in successive years (P < 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 (P < 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 (Fig. 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 (Fig. 4-8). 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. Dates of return to the
study area after migration (Chapter 3) also indicated that the 1-year age class
tended to wander more than older age classes.

Interactions.-We 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%). We 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
radio-tagged subadults were located in association with adult eagles. Griffin
and Baskett (1985) found that immature and adult eagles used overlapping
foraging areas.
















































Fig. 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.
















































Fig. 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.









Radio Frequency 165.675


Age 3


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






NONGAME WILDLIFE PROGRAM FINAL REPORT


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 wna 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

a Sample size weighted by number of individuals.


Perch characteristics.-Perches used most often by subadult eagles were
pine and cypress trees (Table 4-5). 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 51-75%, and 41% at
76-100% canopy cover.

We 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 (z2= 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%.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


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


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) (Fig. 4-9) or age
(F = 0.70, P = 0.79) (Fig. 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, 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 (P < 0.05). The cypress habitat undoubtedly
was used so frequently because of its location along most lake edges.

We 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, we 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 (CYPR1), hardwoods (HDWD1), and marsh


































Pasture Pasture Clearcut Pine Hardwood Cypress
w/ trees
Habitat Type


Water Marsh Developed


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













25 -


-O rri
20 .------------------------- ..- ---- -.. .. .. .. ...


5 ------------------------------ ---------- ------------------------------------.....................................................
'-3


20 -.---------------------------------------------------------------------. .......................
o M












Pasture Pasture Clearcut Pine Hardwood Cypress Water Marsh Developed
w/ trees Habitat Type


Fig. 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.
2-






Fig. 4-10. Habitat use by male and female radio-tagged bald eagles from I to 4 years of age in north-central Florida, fall 1987 to spring 199 1.







64 NONGAME WILDLIFE PROGRAM FINAL REPORT






Table 4-6. Comparison of use and availability of 8 habitats for subadult bald eagles in north-central Florida
from fall 1987 to spring 1991. Habitat variables are defined in Table 4-2.


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


a Expected proportions are based on habitat availability.
b < 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. Habitat variables are defined in Table 4-2.


1-ha plot 10-ha plot

Habitat R SE Minimum Maximum 3 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






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


(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. 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 and most
nests contain young chicks January through 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.

Comparisons 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, we 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 ( = 1,711 m) were located significantly closer to nests than older age







NONGAME WILDLIFE PROGRAM FINAL REPORT


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
4-2. 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 R SE R SE Pa pa

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
PINE 20.0 1.1 25.9 1.9 0.51
HDWDI 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
WATR 1 16.2 1.3 6.5 1.3 0.52
GRAS1 9.5 1.0 18.9 1.8 0.52
CLCTI 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


a Multivariate 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 4-2. 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 K SE SE Pa pa

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







BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


Table 4-9. Continued.


Female Male Full model Reduced model

Variable 2 SE 3 SE Pa pa

HDWDI 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
WATRI 15.9 2.1 17.1 1.8 0.24
GRAS1 8.9 1.5 10.0 1.5 0.25
CLCTI 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


a 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 4-2. 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 i SE SE Pa pa

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
PINEI 19.0 1.4 17.7 2.0 0.01
HDWDI 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


a Multivariate analyses






NONGAME WILDLIFE PROGRAM FINAL REPORT


classes (2: K = 2,357 m; 3: j= 2,644 m; 4: R= 2,774 m). 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%-year-old individuals in this study had significant amounts of adult
plumage. Distance to nests did not vary by month (November to June) (F =
1.63, P = 0.12). Gerrard et al. (1990) found that immatures tended to be
farther from nests with eggs or young present.

Because we were radio-tracking known-age eagles, we were able to
determine at what age adult plumage characteristics are expressed. Birds 3V2
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.

Discussion

Movements.-The concentration of locations on Lake Wauberg (Fig. 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. 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. We
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; Appendix A) was located on the Chesapeake Bay twice that winter,
once in October and once in January, a time of year that southern eagles
normally do not occur on the Chesapeake (Buehler et al. 1991b).

Once 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 ( = 19 km).
Certain portions of the study area were used consistently each year by large






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


numbers of eagles. Thus, management for subadult populations must include
these important 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 this 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.

Although subadults frequently were located near lakes and often near
eagle nests, habitat protection around nests occupied by a breeding pair
probably is not sufficient to also serve the needs of subadults. The average
distance subadults were located from nests was 2,055 m. As we discussed
in detail in Chapter 2, the boundary of the primary protection zone 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 to increase.





70 NONGAME WILDLIFE PROGRAM FINAL REPORT




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.





BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


CHAPTER 5: EFFECTS OF BOATING ACTIVITY

Introduction

Increasing recreational use of lakes in Florida may pose problems for
eagles. The number of boats registered in Florida increased from 436,348 in
fiscal year 1975-1976 to 718,054 in 1989-1990 (data courtesy of Florida
Marine Patrol, Department of Natural Resources). In Alachua and Marion
counties, the number of registered boats increased from 13,704 to 23,179
during the same period. In addition, it is impossible to estimate how many
unregistered boats use Florida waters each year.

In North Carolina, there was evidence that human use precluded use of
sections of Jordan Lake by wintering eagles (Chester et al. 1990). McGarigal
et al. (1991) reported a reduction in the use of highly used foraging areas on
the Columbia River estuary in response to a single stationary boat. In South
Dakota, bald eagles avoided traditional wintering sites when fishing boats
were in the area (Steenhof 1976). Knight and Knight (1984) found no age-
specific differences in flushing response to boats in Washington. In contrast,
Stalmaster and Newman (1978) reported that adults were more sensitive to
disturbance than younger eagles and preferred areas with lower human
activity. Disturbed eagles did not return to the same feeding area until several
hours after the disturbance ceased (Stalmaster and Newman 1978). Both of
these studies of wintering bald eagles reported habituation by birds to
disturbance, although Knight and Knight (1984) were unable to exclude
decreased food abundance as the reason for a decrease in flushing response.

All of these studies, except McGarigal et al. (1991), involved wintering
concentrations of eagles. In Florida, winter populations include non-breeding
subadults and adults, as well as breeding adults. Disturbance at feeding sites
may pose different problems for breeding adults than for wintering eagles.
McGarigal et al. (1991) concluded that boating activities restrict use of certain
foraging areas by breeding eagles and ultimately may affect productivity.

We conducted a study to examine the effects of boating activity on the use
of shorelines by bald eagles and addressed the following objectives.

Objectives

1. Determine if the number of bald eagles using a segment of shoreline
differs between high and low boat use days.





NONGAME WILDLIFE PROGRAM FINAL REPORT


2. Determine if differences exist in response to boat disturbance by different
age classes by counting number of eagles in each age class.

3. Determine if distance perched from the shoreline or flush distance
changes in response to boat disturbance.

4. Determine if differences in activity, habitat use, perch use, or interactions
occur in response to boating activities.

Methods

The effects of boating activity on eagles were evaluated on 3 lakes in
Alachua County: Lochloosa, Newnans, and Wauberg. Lochloosa and
Newnans are large major fishing lakes in the region, while Wauberg is a small
lake with restricted access for gasoline-powered. motorboats but heavily used
for recreational activities. These lakes were surveyed every 2 weeks on
Sunday and Mondays, from 28 February to 9 May 1988, and on Sundays and
Tuesday from 11 December 1988 to 4 April 1989, to compare days with high
human use (Sundays) to low-use days (weekdays).

On Newnans and Lochloosa, a route of approximately 7 km was surveyed
from ajohnboat by driving slowly about 100 m from the shoreline. We began
the first survey shortly after dawn, and finished the last survey near dusk. We
surveyed each lake twice each sampling day and began each survey at the
same time on the 2 paired sample days. Because Wauberg is a smaller lake,
we surveyed the entire shoreline from a stationary boat at the center of the lake.

Data recorded for each eagle observed included location, age-class,
activity, habitat, perch type, distance perched from edge, interactions, and with
whom the interaction occurred. The age classes we identified based on
plumage characteristics included adults (all white head and tail), late subadults
(some brown in head and tail), early subadults, immatures (first-year eagles),
subadults (birds that could not be classed as early or late), and unknowns. Age
classes are after McCollough (1989). Locations of eagles and boats were
plotted on topographic maps. When birds were flushed by a boat, the flush
distance was estimated.

We first determined the sample size needed to test the hypothesis that
boating activity is reducing use of lakeshores using the pre-specified variance
method (Gilbert 1987:51-52). Data were analyzed with a paired difference t-
test to avoid problems with temporal changes in eagle and boat abundance.
Data were analyzed separately for 1988 and 1989 for each lake.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


We used a t-test to examine the effect of boating activity on the distance
eagles perched from the edge of the shoreline and on the estimated flush
distance. Analysis of variance was used to examine month and age variations
in flush distance. We used X2 contingency tests (Winkler and Hays 1975: 825-
829) to examine the distributions for age of eagles observed, habitat use, perch
types, activity, and interactions on weekends versus weekdays.

Results

On Lochloosa Lake, boats were more abundant on weekends than on
weekdays in both years (1988: t = 4.06, P < 0.005; 1989: t = 5.37, P < 0.005), while
more eagles were observed on weekdays than on weekends (1988: t = -3.36, P <
0.005; 1989: t = -2.38, P < 0.025) (Table 5-1). The total number of boats on any 1
survey ranged 4-50 in 1988 and 3-24 in 1989 (Fig. 5-1). The total number of
eagles counted on any 1 shoreline survey ranged 0-15 in 1988, and 0-20 in 1989.

Likewise on Newnans Lake, boats were more abundant on weekends than on
weekdays in both years (1988: t = 5.02, P < 0.005; 1989: t = 2.94, P < 0.01),
although the mean difference was not as large in 1989 (Table 5-1). The number
of eagles observed was not significantly different on weekdays than on weekends
(1988: t = -0.95, P > 0.10; 1989: t = -0.19, P > 0.10), although slightly more were
counted on weekdays. In 1989, the maximum number of boats observed on any
survey was 16 as compared to 36 in 1988. This small number of boats apparently
affects a very small portion of the shoreline available to eagles. In general, very
few boats were observed on Newnans Lake in 1989 (0-16) as compared to 1988
(2-36), but more eagles were observed (Fig. 5-2). In 1988, 0-11 eagles were
sighted on any 1 shoreline survey compared to 2-20 in 1989.




Table 5-1. Mean difference between weekend and weekday counts of bald eagles and boats on shoreline
surveys of 3 lakes in north-central Florida, 1988 and 1989. Asterisk indicates significant difference between
weekend and weekday counts.

Number of eagles Number of boats
Year Lake n difference SE n difference SE
1988 Lochloosa 10 -2.9 0.79 12 12.9 3.18
Newnans 12 -1.1 ns 1.14 12 7.8 1.56
Wauberg 10 -1.7 ns 1.02 10 3.2 ns 1.98
1989 Lochloosa 10 -3.7 1.56 10 10.1 1.88
Newnans 12 -0.2 ns 0.87 12 3.1 1.05
Wauberg 14 -0.4 ns 0.99 14 8.1 2.62













50
>,
0)
L

40

'a
Co
> 30
3)
U)
.0
0


E
:3
Z

10




0


Weekday


Fig. 5-1. Weekend and weekday boat survey counts of boats and bald eagles on Lochloosa Lake, Alachua County, Florida, 28 February to 9 May 1988 and 29
January to 8 March 1989.












SBoats 0 Eagles

I E Newnans Lake


2/28/88 5/8/88


I 1 12/11/88-4/2/89

Weekend


2/29/88 5/9/88


SI. UI l InI Ij ,lII L IJ yL : ,T 'l ,lit :l . -I.I P I A, l l


12/13/884/4/8
12/13/88 4/4/89


Weekday


Fig. 5-2. Weekend and weekday boat survey counts of boats and bald eagles on Newnans Lake, Alachua County, Florida, 28 February to 9 May 1988 and 11
December 1988 to 4 April 1989.


a 30





o
-





a,
I 20-
a)
.0
O
0
a)

E
S10
z


'lo


lo 7


- l -
' : '.< '





NONGAME WILDLIFE PROGRAM FINAL REPORT


On Lake Wauberg, there was no difference in the number of boats or
eagles observed on weekends versus weekdays in 1988 (t = 1.61 and t = -1.66
P > 0.05) (Fig. 5-3). In 1989 on Lake Wauberg, there were more boats
observed on weekends (t = 3.08, P < 0.01) because the warm winter increased
recreational use of the lake during the study. The number of eagles observed
was not different (t= -0.43, P > 0.10), probably because most recreational use
of Lake Wauberg occurred during early afternoon when eagles normally spend
less time fishing (Mersmann 1989).

To determine if eagles might perch far enough from the shoreline to be
out-of-sight on high boat use days (weekends) but still fly to the lake to forage,
we compared the number of eagles first observed flying on weekends versus
weekdays. Of the 816 eagles observed on all shoreline surveys, 44% were
first observed flying on high boat use days and 50% on low-use days, a non-
significant difference (t = -1.22, P = 0.24). This suggests that the lower
number of eagles seen on high boat use days was not a result of birds perching
farther from the shoreline and missed on the surveys but a real decrease in the
number of eagles using the lake.

During the shoreline surveys, we identified eagles using 5 types of
perches: snags, pines, cypress, hardwoods, and palms. There was no
difference in the distribution of eagles using these perch types on weekends
(high boat use days) compared to weekdays (X2 = 5.74, P = 0.33). The
majority of the 489 sightings of perched birds occurred in cypress (51.9%) or
hardwood (21.1%) trees.

We also distinguished 6 habitat types used by eagles: cypress, hardwood,
pinewoods, marsh, lake, and developed. Eagles used these habitats in the
same proportion on weekends as on weekdays X2 = 3.85, P = 0.57). The most
frequently used habitat was cypress (46% of 817 observations). Pinewoods
(22%) and lake (21%) also were commonly used habitats. We observed only
21 (3%) eagles in habitats we considered developed.

The 6 interactions we observed that involved 332 sightings of eagles
included: chasing or chased by (31%), perch together (44%), fly together
(20%), and stoop on, hit, and talon lock (5%). Because of small sample sizes,
stoop on, hit, and talon lock were combined into 1 category for analysis
purposes. Boating activity did not change the distribution of eagles engaged
in the various interactions (X2 = 3.56, P = 0.31). Most interactions occurred
between eagles of various age classes. Of eagles perched with another eagle,
48% of 27 immatures were perched with adults, 19% of subadults perched
with adults, and 77% of adult eagles perched with another eagle were perched
with adults. Only 33 interactions involved ospreys; 19 ospreys were chased
by eagles and 7 eagles were chased by ospreys.



































S 2128/88- 5/8/88 1/8/89- 4/2/89 2/29/88- 5/9/88 1/10/89 -4/4/89
I Weekend I I Weekday


Fig. 5-3. Weekend and weekday boat survey counts of boats and bald eagles on Lake Wauberg, Alachua County, Florida, 28 February to to 9 May 1988 and 8
January to 4 April 1989.






NONGAME WILDLIFE PROGRAM FINAL REPORT


The age distribution of eagles sighted on weekends was not significantly
different from that on weekdays (X2 = 4.01, P = 0.55). This indicates that
weekend boating activity reduced the numbers of all age classes and did not
differentially affect any particular age class.

The distance that eagles were perched from the shoreline edge was
significantly greater on weekends (t = -2.52, P = 0.01) when boating activity
was high (Table 5-2), although the difference was not large. Of 517 eagle
sightings in 1989, we observed only 59 instances of eagles flushing in
response to boats in 1989, the year we recorded flush distance. There was no
significant difference between weekends and weekdays (t = 0.05, P = 0.96);
however, there was a significant difference by month (F = 10.46, P = 0.0001).
The Waller-Duncan multiple comparison test indicated that eagles were
flushed by boats at a greater distance in January and February (Table 5-3).
Boating activity typically increased in January and February when winter
tourists from northern states visit Florida. There was no significant difference
in flush distance by age class (F = 1.23, P = 0.32).


Table 5-2. Distance (m) eagles were perched from the edge of the shoreline and flush distance on weekdays
(WD) and weekends (WE) during shoreline surveys of Lochloosa, Newnans, and Wauberg lakes in north-
central Florida, 1988 and 1989.

Variable na R SE Range t P
Distance to edge
WD 256 5.6 0.56 0-50 -2.52 0.01
WE 193 8.5 0.99 0 -75
Flush distance
WD 32 53.9 12.32 5 200 0.05 0.96
WE 27 52.9 12.73 5 200

a Number of eagle observations


Discussion

We observed a significant negative relationship between boat numbers and
eagle numbers on Lochloosa Lake. Boat use was highest on weekends and
eagle use was highest on weekdays. Buehler (1990) observed significantly
decreased use of shorelines by eagles on weekends when boat activity was
high on the Chesapeake Bay.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


Table 5-3. Estimated flush distance (m) by month for bald eagles sighted (n) on shoreline surveys of
Lochloosa, Newnans and Wauberg lakes in north-central Florida, in 1989.

Month n a SE Range
December 5 5 B 0.0 5- 5
January 13 80 A 21.9 5 200
February 20 99 A 14.2 5 200
March 5 5 B 0.0 5- 5
April 13 5 B 0.0 5- 5
May 3 5 B 0.0 5-5

a Means with the same letter are not significantly different (Waller-Duncan K-ratio t-test).


On Newnans Lake, the mean difference in the number of boats was not as
large as that observed for Lochloosa, particularly in 1989. The small
difference in boating activity between weekends and weekdays, although
significantly different, may not have been a true measure of boating effects on
eagles because of the overall low number of boats present. On Newnans Lake,
the maximum number of boats counted (1988 = 36, 1989 = 16) was much less
than on Lochloosa (1988 = 50, 1989 = 24). It is possible that there is a
threshold number of boats required present on a lake before eagles avoid an area.

A boat/eagle relationship also did not occur on Lake Wauberg. Because
this lake is used primarily for recreational activities other than fishing, boat
disturbance is concentrated during early afternoon with little disturbance in the
early morning and late evening. This allows eagles to forage undisturbed on
the lake for several hours.

During 1 of our boat surveys, we observed deliberate harassment (chasing)
of a bald eagle by a motor boat operator. In most situations, however, eagles were
excluded from an area by the presence of boats moving or stationary near the
shoreline. Moving boats seemed to have a greater effect than stationary boats.

Boating activity on 1 lake reduced the number of eagles using the
shoreline, and on all lakes increased the distance perched from the shoreline
and the flush distance. All of these factors may result in greater energy
expenditure by eagles when foraging and might negatively affect breeding
adults. Boating activity does not affect activity, perch use, habitat use,
interactions, or the age distribution of eagles.





NONGAME WILDLIFE PROGRAM FINAL REPORT


In our study, we observed only 59 instances of eagles flushing in response
to boats out of 517 eagle sightings on the 1989 shoreline surveys. McGarigal
et al. (1991) also observed few instances of eagles flushing in response to
boats. They found that it was more common for eagles to entirely avoid areas
where boats were present.

Our mean flush distance of 53 m was less than that reported in other
studies (McGarigal et al. 1991: 197 m; Buehler 1990: 175 m in summer and
265 m in winter). Buehler (1990) suggested that the difference in winter and
summer flush distances that he observed on the Chesapeake Bay might be a
difference in response by the northern migrant eagles inhabiting the
Chesapeake in the winter, compared to the southern migrants and Chesapeake
eagles present in summer. Because our flush distance was very low, possibly
from habituation to the high boat disturbance in January and February on our
study area, it is possible that eagles remain habituated to boat disturbance after
migration and respond less to boat disturbance on the Chesapeake Bay.

Because it is not possible to limit boats on lakes or to exclude them from
areas favored by eagles, public education may be the best means of reducing
boating disturbance on eagles. Most people are very interested in eagles and
will follow measures to reduce disturbance to foraging eagles if they know
their disturbance may have a negative effect.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


CHAPTER 6: SURVIVAL OF SUBADULT BALD EAGLES

Introduction

Survival rates for bald eagles are poorly documented (Grier et al. 1983), yet
may be the most important factor affecting growth of eagle populations (Grier
1980). Status reviews of bald eagle populations and population recovery goals
generally are based on trends in reproduction and on winter counts (Sprunt et al.
1973, U.S. Fish and Wildlife Service 1989). However, population inferences
based solely on productivity or age ratio data can be misleading (Grier 1979).
Survival estimates for all age classes are necessary to truly assess and
understand the dynamics of a population, particularly for a long-lived species
that requires several years to reach breeding age. Consequently, management
for bald eagles also must be aimed at all factors that influence population size
including those that affect survival.

Hodges et al. (1987) reported that 8 first-year bald eagles trapped and radio-
tagged on the Chilkat River, Alaska, had a known minimum 50% mortality rate.
Minimum survival rates reported for marked first-year eagles include 37%
(Gerrard et al. 1978) and 21% (Brown and Amadon 1968). These studies relied
on reobservation of marked birds and band returns; neither method produces
reliable survival estimates. Buehler et al. (1991a) reported 100% survival
through the first year of life for radio-tagged eagles (n=39) on the Chesapeake
Bay. Minimum survival to 3 years of age was 19% for wing-marked
Saskatchewan eagles (Gerrard et al. 1978) compared to 56% for color-marked
Maine eagles resighted at feeding stations (McCollough 1986). Eagles hacked
in New York had a minimum survival rate of 22% to adulthood (Nye 1988).
Only 2 of these studies (Hodges et al. 1987, Buehler et al. 1991a) calculated
survival rates for individuals from a large, stable to increasing population and
used radio telemetry. Both large population size and ease of relocating birds
fitted with radio transmitters likely contribute to higher survival rates in these 2
studies. No gender specific survival rates have been reported for bald eagles, nor
have the effects of hatch order, number of siblings, and timing of nesting been
examined. No survival rates for the pre-fledging period have been reported.

In this study, we examined survival rates for radio-tagged bald eagles from
8 weeks to 4Y years of age to address the following objectives:

Objectives

1. Determine annual and cumulative survival rates of subadult bald eagles
through 4A years of age.

2. Examine survival rates through 1% years of age in relation to sex, timing of
nesting, number of siblings, hatch order, and age at dispersal.






NONGAME WILDLIFE PROGRAM FINAL REPORT


Methods

Between 1987 and 1990, 44 8-week-old nestling eagles were fitted with
radio transmitters (Chapter 2). Three did not survive to fledging age and were
excluded from analyses of post-fledging survival. Of the 41 remaining, 25
were male and 16 were female (1987: 3 male, 7 female; 1988: 6 male, 4
female; 1989: 11 male, 1 female; 1990: 5 male, 4 female) (Table 2-2). While
on the study area, radio-tagged eagles were tracked approximately once per
week from September through June of each year. Radio-tracking procedures
and transmitters are described in Chapter 2. Because eagles are highly mobile,
we were not able to locate all surviving individuals during each aerial survey.
Some birds regularly found on the study area left for an extended period of
time, while others appeared on the study area only occasionally. Whenever
possible, we visually located each bird. Locations during the migratory period
(Chapter 3) were obtained from radio-tracking reports of other researchers,
sightings of patagial-marked birds, and recoveries of dead birds.

We used the Kaplan-Meier estimator (Kaplan and Meier 1958) modified
to allow for staggered entry of animals into the study (Pollock et al. 1989a,
1989b) to estimate survival rates for bald eagles to 4% years-of-age. Estimated
survival S(n) was calculated as

S(n) = [1-d(n)/r(n)] S(n-1)

where
d(n) = number of deaths in time period n
r(n) = number of individuals at risk during that time period
S(n-1) = estimated probability of survival until the end of the previous
time period

Approximate 95% confidence intervals were computed using

S(n) 1.96 -var[S(n)]

These survival rates are the probability that a bird survives from the start of
the study until the end of the nth study period. We used the log-rank test
(Pollock et al. 1989a) to compare survival by sex, timing of nesting, number
of chicks, and hatch order. We used logistic regression (White and Garrott
1990) to examine effects of dispersal age on survival. In each of the 4 years
(1987-1990) that young were radio-tagged, eggs in 75% of all nests hatched
by February 5. These were classed as normal. A nesting attempt was
considered late if eggs hatched after this date. We estimated hatch order from
the relative length of the eighth primary of siblings.





BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


All individuals that disappeared were considered to have died during the
month they were last located because we were unable to determine whether
these birds actually died or if radio failure occurred. Consequently, these
estimates represent minimum survival rates for this subadult population. We
did not calculate maximum survival rates because we had only 2 known deaths
during the study. We do not believe, however, that the results of the log-rank
tests and linear regressions were biased by assuming these missing individuals
had died. The probability of radio failure should be the same for all individuals.

We calculated overall survival for each year-long age class. Survival for
the first year age class was calculated from the time individuals fledged until
initiation of migration the following spring. At the end of the first year age
class, eagles were approaching 1\i years of age. Age class 2 included eagles
from 1% to 22 years of age. Each successive age class followed a similar
pattern. We ended survival analyses on 30 May 1991, although several
individuals were known to be alive in December 1991.

Results

Of the 44 8-week-old nestlings fitted with transmitters, 3 died prior to
fledging. Two died at approximately 9 weeks of age; 1 died at 10 weeks of age.
Survival from 8 weeks of age to fledging at approximately 11 weeks was 93.2%.

Only 2 radio-tagged eagles (both female) were known to have died during
the study. Both died during the northward migration period: 1 in North
Carolina 2 months after banding and 1 in New Brunswick 4 months after
banding. Four banded eagles from this study also were recovered dead: 2
males near the study area in Florida 2 years and 4 years after banding, 1 male
in Wisconsin 2% years after banding, and 1 female in Ontario 3 months after
banding. The sex ratio for 115 banded eagles was 1.1 males per 1 female; the
sex ratio of the 6 recovered eagles was 1:1. However, the sex ratio of eagles
recovered in northern states was 1:3, biased towards females.

Survival was similar for radio-tagged cohorts from 1987 to 1989 (Table 6-
1). The 1990 cohort had lower survival through 1 years of age. Survival was
lowest in the 1% year age class (63%) and increased considerably in the older
age classes beginning with eagles 2Y years old (Table 6-2). Cumulative
survival to 4% years of age was 50%.

We compared minimum survival curves of male and female bald eagles
through 1% years of age; the log-rank test showed no significant difference (Table
6-3). Survival through 4% years of life also did not differ significantly by sex (X2
= 1.53, P = 0.22); females had 47.1% survival while males had 54.2% (Fig. 6-1).







NONGAME WILDLIFE PROGRAM FINAL REPORT


Table 6-1. Survival rates through May 1991 for each cohort of radio-tagged bald eagles from 1987 to 1990,
north-central Florida.


Cumulative percent survival per age class
Year
banded n 11/2 21/2 31/2 41/2

1987 10 70.0 50.0 50.0 50.0

1988 10 70.0 50.0 40.0

1989 12 66.7 66.7

1990 9 44.4

Total 41 63.4 52.9 50.0 50.0


Table 6-2. Survival rates of radio-tagged bald eagles by age class in north-central Florida, 1987 to 1991.


Lost
Age Known radio Percent
class n mortality signal survival 95% CI

l1 wks- I yrs 41 2 13 63.4 51.6 75.1

1X- 2Yyrs 26 0 4 83.5 67.3 99.7

2- 3Yyrs 18 0 1 94.1 83.3- 100.0

3 -1' r 6 0 0 100.0

Total 41 2 18 50.0 27.6- 72.4






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


Table 6-3. Survival rates through 1 years of age in relation to sex, timing of hatch, number of chicks in the
nest, and hatch order, of radio-tagged bald eagles in north-central Florida, 1987 to 1991.

Percent
Variable na survival 95% CI X2b p

Sex
Female 17 64.7 42.0 87.4
Male 24 62.5 43.1 -81.9 1.95 0.16

Timing of hatch
Normal 23 60.9 40.9- 80.8
Late 18 66.7 44.9- 88.4 1.37 0.24

Number of chicks
1 14 49.4 23.1 75.6
2 27 70.4 53.1 87.6 4.76 0.03

Hatch order
1 14 71.4 47.8 95.1
2 13 59.1 40.5 77.6 8.73 0.003

Total 41 63.4 36.6- 57.5

a n is the number at risk during the sampling period.
b Log-rank test (Pollock et al. 1989a).
c Normal = eggs laid during first 75% of all nesting attempts; Late = last 25%.


We also compared survival rates of young from normal and late nesting
attempts and found no significant differences through 1% years of life (Table
6-3). Age at migration from the nest area also did not affect survival through
1% years of age (X2 = 0.001, P = 0.94).

Survival rates through 1 years of age were affected significantly by the
number of young in a nest (Fig. 6-2) and the order of hatch (Table 6-3, Fig. 6-
3). Only 49% of eagles from 1-chick nests survived, while 70% from nests
with 2 chicks survived. Of eagles fledged from 2-chick nests, the older sibling
had significantly greater survival (71%) than the younger sibling (59%).

Discussion

First year minimum survival rates were 63.4%, much higher than those
reported by Brown and Amadon (1968), Gerrard et al. (1978), and Hodges et
al. (1987). All of the known mortality and most of the lost radio signals for











-H-6


I I 'I I I I
S D M

1 % Yr


J S D M J

2% Yr


S D M
3% Yr


J S D M
4Y Yr


Fig. 6-1. Survival curves for male and female bald eagles through 42 years of age, north-central Florida, 1987 to 1991.


Female Male






I-0 -0---- --G-







\ 8 a a-O-D-o-- o- BB E a O-O-o-q
'D- B BB -0-0--13-0- 0E B E0 -0- --0- 0


I't


I 'I I
M J

Banded













110
no~ ~ ~ ~ ~ ~~~~~I-- ----------------------------------_---_
1 chick 2 chicks

100 -----

\0 - - G


go\
90 \


13- -- -


80



70



60


Chi-square = 4.76

so P = 0.03


S- -4. ----4 .-- -4- -4. -*


40 II I I I I I I I I I I
M A M J J A S O N D J F M

Banded



Fig. 6-2. Survival curves for bald eagles fledged from nests with I versus 2 chicks through 1i years of age, north-central Florida, 1987 to 1991.


- - - -0- - -B - - - -









1st hatch 2nd hatch
-oo-- --- -
100 -- -




80--
5 ~5





C 0 70



Chi-square = 8.73
S N,












Fig. 6-3. Survival curves througChi-squyears of age for first- and second-hatched bald eagles fledged from nests with 2 chicks, north-central Florida, 1987 to 199
0 P P= 0.003 .8-- --- ---- -- ----





M A M J J A S O N D J p M
Banded 1%Yr




Fig. 6-3. Survival curves through 1 years of age for first- and second-hatched bald eagles fledged from nests with 2 chicks, north-central Florida, 1987 to 1991.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


radio-tagged eagles occurred from April to September, the migratory period,
and early in life. First year birds in Alaska migrated farther than older eagles
and had greater mortality (Hodges et al. 1987). In this study, first year birds
also had lower survival, but migration distance did not vary significantly by
age class (Chapter 3). Consequently, when attempting to increase survival in
a migratory population, management focus should be on young birds.

At northern latitudes winter starvation is the most likely mortality factor
of young eagles and often occurs during migration (Sherrod et al. 1977,
Stalmaster and Gessaman 1984, McCollough 1986). In Florida eagles, 4 of 6
known mortalities in this study occurred during summer migration. Three of
the 4 occurred within 4 months after banding. This suggests that mortality
during summer migration, particularly at younger age classes, may
significantly affect overall survival of subadults, and points to a need for
habitat protection along migration routes and in summering areas (Chapter 3).

Two other recent studies in the eastern United States reported annual
survival rates for subadult bald eagles (Table 6-4). The estimated survival for
Florida eagles was the lowest, while Chesapeake Bay eagles had the greatest
survival, particularly at the youngest age classes (Buehler et al. 1991a). Maine
eagles showed increasing survival with age (McCollough 1986), as we found
in this study, but first year survival in Maine was slightly greater. By 3% years
of age, Maine and Florida populations had reached similar cumulative survival
rates; the Chesapeake Bay population reached a similar survival rate by 4%
years of age.

These survival estimates, particularly those for the 1% year age class,
correlate well with migratory tendencies of each population. Because a long-
distance migrant incurs greater energetic cost, one would predict different
survival rates for populations with different migration distances. In Florida,
all immature eagles migrate long distances (Chapter 3), while immature eagles
remain on the Chesapeake Bay year-round (Buehler et al. 1991b). In Maine,
McCollough (1986) estimated that the proportion of each cohort that dispersed
during their first winter ranged from 30-60%; many winter on the Chesapeake
Bay (Buehler et al. 1991b) resulting in a shorter migration distance than for
Florida eagles. Thus, in the Maine and Chesapeake populations fewer
individuals are exposed to the hazards of migration as young, inexperienced
fliers. Greater survival in Maine also is likely the result of supplemental
feeding at winter feeding areas enhancing survival (McCollough 1986),
particularly of the earliest age classes.

We found no significant differences in survival rates by sex, although
females had somewhat lower survival through 4Y years of age. Three of 4






NONGAME WILDLIFE PROGRAM FINAL REPORT


Table 6-4. Survival rates for 3 bald eagle populations in the eastern United States.


Annual minimum survival (%)

Age class Mainea Chesapeake Bayb This study
IX 73 100 63
2' 84 92 84
3' 91 75 94
4! 80 100
Cumulative to 3' 56 69 50
Cumulative to 4' 55 50

a McCollough 1986
b Buehler et al. 1991a


eagles recovered north of the study area were female, suggesting that female
mortality might be higher during migration than that for males; however,
sample size was small. Although no published data are available for migration
distance by sex in subadult bald eagles, results from this study indicate that
both males and females undergo long-distance movements and the mean
maximum distance traveled was not significantly different (Chapter 3). Thus,
survival rates for males and females would be expected to be similar and
migration distance does not appear to be a causative factor in mortality.

Eagles fledged from 1-chick nests had lower survival through 1% years of
age than those fledged from 2-chick nests. Female condition, which is
affected by food abundance, influences the number of eggs laid (Newton
1979). Perhaps pairs that produce only 1 young, either by laying only 1 egg
or by loss of a young chick, have a low quality nesting area. Young fledged
from a low quality area may receive inadequate amounts of food, which may
affect survival. In addition, fledglings with a sibling ranged significantly
farther from the natal nest prior to migration than single fledglings (Chapter
2). Consequently, young from 2-chick nests had more experience flying
which might have prepared them better for migration.

The first-hatched chick from 2-chick nests had greater survival through 1i
years of age than the second-hatched chick. The older sibling generally
dominated in food conflicts (Chapter 2) allowing it to energetically prepare
better or more quickly for migration. Thus, the older sibling generally left the
natal area first.





BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


The 1% year survival estimate for the 1990 cohort was over 20% less than
for the 3 earlier cohorts. There are 2 possible reasons for the lower survival
in 1990. Florida was in the second year of a severe drought in 1990 that
undoubtedly lowered prey availability and possibly reduced survival of 12 -
year-old eagles. Alternatively and more likely, survival may appear to be
lower because some eagles had not yet returned to the study area. Since a few
eagles do not return to Florida their first winter, the study may have been
terminated before all surviving 1990 birds had returned.

Survival rates for immature eagles are useful for refining the statewide
estimates of annual productivity. State of Florida estimates from aerial nest
surveys currently are undercounting fledglings by approximately 3.6% (J.
Hardesty, P. Wood, J. Smallwood, and M. Collopy, unpubl. data). Pre-fledging
mortality of radio-equipped eagles was 6.8%. Use of these 2 figures as
adjustments to the state productivity estimate results in slightly lower annual
production. Annual survival rates then can be applied to a particular cohort to
estimate the number of individuals expected to survive to adulthood.





NONGAME WILDLIFE PROGRAM FINAL REPORT


CHAPTER 7: SYNTHESIS AND CONCLUSIONS

The state of Florida supports over half of the breeding population of bald
eagles in the southeastern United States; 601 breeding pairs in 1991 (S. Nesbitt,
pers. commun.). This represents a significant resource for the Southeast and for
Florida. Currently, primary management emphasis and protection is focused on
active 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 had not been quantified.
Because eagles do not breed until approximately 5 to 6 years of age (possibly
later in Florida), a large time gap occurs in management of eagle populations.
This situation is analogous to that which occurred in waterfowl research and
management in the past; emphasis was on nesting areas and subadult needs
were not recognized or considered (Weller and Batt 1988). This research was
initiated because of the paucity of information concerning subadult bald eagle
populations. We examined various aspects of subadult eagle biology that might
be pertinent to survival or management of the Florida subadult eagle population.

Fledgling eagles (birds prior to their initial migration) remained dependent
on adults for food and stayed at or near the natal nest until they initiated
migration at an average of 7 (4-11) weeks post-fledging. Habitat protection
within the 229 m (750 ft) primary protection zone used in Florida was not
sufficient to meet the habitat needs of fledgling eagles because they range
outside of the primary zone by 3 weeks post-fledging. Of greater importance
is the extent of the protection period. It should extend until fledglings initiate
migration away from the natal area, not just until fledging. Disturbance near
a nest while fledglings still are dependent on adults may cause premature
dispersal of young from the nesting area prior to their attaining adequate food
reserves. Fledglings in less than optimum physical condition when initiating
migration may be less likely to survive the energetic demands of migration.

Survival analyses in this study indicated that greatest mortality occurred
during the first summer of life after initiation of migration. Three of 4 eagles
that died during migration died within 4 months of banding. The fact that
young from 1-chick nests and the younger sibling from 2-chick nests had
lowest survival also indicates that food availability during the post-fledging
period can affect survival. Nests with only 1 chick probably are located in low
quality territories resulting in less food available for the nestling. Bortolotti
(1986) found that growth rate for 1-chick broods was slower than for 2-chick
broods because of food limitations. In 2-chick nests, the older sibling generally
has first access to prey delivered and will monopolize it until satiated. If prey
is not abundant, the younger sibling will more likely be deprived. This also
points to a need to avoid disturbance at nests during the post-fledging period to
ensure that adults do not decrease prey deliveries due to excessive disturbance.






BALD EAGLE POST-FLEDGING ECOLOGY Wood and Collopy


Timing of migration for fledgling eagles did not correlate well with
environmental factors, but did appear to relate to food availability. Fish
abundance on Newnans Lake was declining during the later stages of fledgling
dependence. We observed that the first-hatched chick often dominated in food
conflicts, often left the study area first, and tended to have greater survival. We
suggest that young with greater food availability might migrate at a younger age
because they can reach peak condition more quickly. However, additional prey
availability data and experimental food provisioning studies are needed to test
this hypothesis. Regardless of geographic origin, young required a minimum of
approximately 4-7 weeks post-fledging to initiate migration (Harper 1974,
Kussman 1976, McCollough 1986, this study) and presumably to reach a
physical condition capable of sustaining them during long-distance migration.

In contrast to the initial migration of fledglings, it is doubtful that subadult
(1- to 4-year-old) eagles in subsequent years 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 1 of the major lakes in the study area reached its peak during the
time most subadults left the study area (Fig. 3-4). Water levels are dropping at
this time (Fig. 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 (Fig. 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.

Thus, earliest migration by subadults occurred when fish abundance was at
an annual high, although waterfowl abundance had declined. These early
migrants doubtfully left in response to lack of prey; however, late migrants more
likely left the study area in response to decreasing prey availability. Most
subadults left the study area before summer air temperatures and humidity
reached the annual high.

Locations of radio-tagged eagles outside of Florida ranged from South
Carolina to Prince Edward Island, Canada. These data and the data of others
(Gerrard et al. 1978, Buehler et al. 1991b) 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 and primarily in the 1 A year age class. Minimum
survival cumulative through 4% years of age was 50% and did not vary by sex.