Nest site selection by bald eagles in Montana
by Kent Charles Jensen
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Fish
and Wildlife Management
Montana State University
© Copyright by Kent Charles Jensen (1988)
Abstract:
To maintain growth and stability in nesting populations of bald eagles (Haliaeetus leucocephalus) in
Montana it is necessary to ensure the continued availability of suitable nesting habitat. The objectives
of this study were threefold: first, to provide detailed information of nest site vegetative and structural
characteristics, secondly, to determine prey use and human activity factors relative to habitat use by
bald eagles, and third, to develop basic recommendations for nest site management. This study was
conducted during the summers of 1985 and 1986. Nest sites were studied in four areas of Montana. A
total of 35 variables was measured and recorded at each nest site and the same variables were collected
at selected non-use points for statistical comparison. Prey remains were collected from around nest
trees to determine prey use. Bald eagles nested in seven different tree species of which the major nest
tree species varied between population units. Average nest tree, nest, and nest stand heights were 30.4,
22.8, and 20.2 m respectively. There was no difference (P > 0.05) in height, nest height, or nest stand
height among the different nest tree species. Used nest trees were significantly (P < 0.05) taller than
non-use nest trees. Bald eagles tended to select nest sites away from areas of heavy human recreational
use and busy roadways. Agricultural activities did not appear to affect use of an area by nesting eagles.
Discriminant function analysis selected nest tree diameter at breast height (DBH), nest tree height,
canopy closure of the nest stand, human recreational activity, and nest tree decadence as the 5 variables
that most separated used from non-used sites. Prey use by bald eagles varied among population units
and appeared to be governed by the type of water body the eagles were nesting near. A major factor in
bald eagle nest site selection appears to be the structure of the nest tree in combination with an open
stand in an area of relatively little human recreational disturbance. Management practices should
encourage production of future nest trees, reduced and low levels of human activity and maintence of
present or higher levels of prey populations. Potential bald eagle nest sites should be evaluated
according to the variables selected by discriminant function analysis as being the most predictive of use
by nesting eagles. 
NEST SITE SELECTION BY BALD EAGLES 
IN MONTANA
by
Kent Charles Jensen
A thesis submitted in partial fulfillment 
of the requirements for the degree
of
Master of Science 
in
Fish and Wildlife Management
MONTANA STATE UNIVERSITY 
Bozeman, Montana
May 1988
/
APPROVAL
of a thesis submitted by
Kent Charles Jensen
This thesis has been read by each member of the 
thesis committee and has been found to be satisfactory 
regarding content, English usage, format, citations, 
bibliographic style, and consistency, and is ready for 
submission to the College of Graduate Studies.
5 ZicI h x
Date
Approved for
O
Date
Approved for the i
(Lr
Chairperson, ate Committee
Head, Major Department
Date Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of 
the requirements for a master's degree at Montana State 
University, I agree that the Library shall make it 
available to borrowers under rules of the Library. Brief
source is made.
Permission for extensive quotation from or 
reproduction of this thesis may be granted by my major 
professor, or in his absence, by the Director of 
Libraries when, in the opinion of either, the proposed use 
of the material is for scholarly purposes. Any copying or 
use of the material in this thesis for financial gain 
shall not be allowed without my written permission.
quotations from this thesis are allowable without special
permission, provided that accurate acknowledgement of
Date May 19, LVVtS
Signature
VACKNOWLEDGMENTS
I express my deepest gratitude to Drs. Robert L. Eng 
and Alan R. Harmata for their support, guidance, 
encouragement, and friendship during this project. I also 
thank my graduate committee, Drs. Wm. R . Gould, R. J . 
Mackie, and J. E. Taylor for their help.
I thank K. Dubois, P. Mullen, D. Becker, L. Thornton, 
Wm. Moritz, J. Toepfer, and D. Hinckley for assistance 
with data collection. For logistical support I thank D. 
Flath, F. King, K . Alt, K. Walchek, H. Youmans, D. 
Hinckley, D. Bricco, R. Escano, R. Meyers, C . Coffin, T. 
Grotzinger, T. Wittinger and L. Hicks. I thank Dr. R.
Lund and C . Bittinger for statistical consultation.
Special thanks go to Marilyn Wood and Mr. and Mrs. 
Dale Becker who treated me like family and allowed me to 
stay in their homes during field work.
My thanks, appreciation, and love go to Sandy who has 
supported us these many years, and to Chris and Rachel who 
were patient and loving. I also express my deepest 
gratitude to my parents, Chris and Myrna Jensen.
This project was supported under a cooperative 
agreement with the U. S. D. A., Forest Service; U . S. D. 
I., Bur. of Land Mgt; Mt. Dept, of Fish, Wildl., and 
Parks; Plum Creek Timber Co.; Mt. State Univ.; Mt. Agric. 
Exp. Sta.; and the U. S. Fish and Wildl. Service.
vi
TABLE OF CONTENTS
Page
VITA...........................   iv
ACKNOWLEDGMENTS.........   v
LIST OF TABLES................................... . . vii
LIST OF FIGURES.........   viii
ABSTRACT...............................   ix
INTRODUCTION............     I
DESCRIPTION OF STUDY AREAS........  3
Northwest Montana..............................  3
West-central Montana..........................  7
Greater Yellowstone Ecosystem....................  7
Eastern Prairie.............    8
METHODS............................................  9
Nest Site Characteristics........ ............. . 9
Forest Stand Characteristics.....................  12
Human Activity Characteristics...................  13
Prey Use..................................... . . . 15
Statistical Methodology..................  16
RESULTS.....................   18
Nest and Nest Tree Characteristics..............   18
Nest Site Characteristics..... V.................  24
Human Activity Characteristics..........   24
Discriminant Function Analysis...................  28
Food Habits . . .....................     28
DISCUSSION.....................   34
Nest Tree and Nest Site......................  34
Human Activities.................    38
MANAGEMENT RECOMMENDATIONS.........................  49
REFERENCES CITED...............  52
vii
LIST OF TABLES
Table Page
1. Climatic data for 4 regions of Montana.......  6
2. Characteristics measured at and around 
bald eagle used and non-used nest
sites in Montana, 1985-86.....................  10
3. Ratings of human activities within 1.6 
km of bald eagle nests in Montana,
1985-86..........................    14
4. Nest tree species used by bald eagles 
in the 4 population units of Montana,
1985-86..................................    18
5. Species, average tree heights, nest 
heights, and diameter at breast height 
of bald eagle use and non-use trees in
Montana. 1985-86.............................  19
6. Mean nest and nest tree characteristics 
of bald eagles in 4 population units
of Montana, 1985-86..................  21
7. Characteristics of use and non-use bald
eagle nest sites in Montana, 1985-86..........  26
8. Scaled scores for human activities at 
used and non-used bald eagle nest
sites in Montana, 1985-86....................  27
9. Ranked discriminant function analysis 
coefficients from 18 characteristics 
measured at used and non-used bald eagle 
nest territories and corresponding points
of non-use in Montana, 1985-86...............  29
10. Prey remains from bald eagle nests
in Montana, 1985-86........  30
11. Prey utilization of bald eagles in
various regions and associated 
water body types............. 46
viii
LIST OF FIGURES
Figure Page
1. Distribution of bald eagle nest
sites in Montana................ ............. 4
2. Percentage of dominant and codominant 
trees used for nesting by bald.
eagles in Montana, 1985-86..................... 22
3. Maturity classes of bald eagle nest
trees in Montana, 1985-86....................  22
4. Slope position of bald eagle nest
trees in Montana, 1985-86...*............. . . . 23
5. Percent by species of axial and 
non-axial bald eagle nests in Montana,
1985-86..........................      25
6. Prey use by nesting bald eagles by
population unit in Montana, 1985-86.............  32
7. Food use of nesting bald eagles in 
relation to associated water body
type in Montana, 1985-86...................   33
8. Height relationships among nest trees, 
nests, and nest stands for bald eagle
nest sites in Montana, 1985-86................. 36
9. Conceptual framework for bald eagle
nest site selection............................ 48
ix
ABSTRACT
To maintain growth and stability in nesting 
populations of bald eagles (Haliaeetus leucocephalus) in 
Montana it is necessary to ensure the continued 
availability of suitable nesting habitat. The objectives 
of this study were threefold: first, to provide detailed 
information of nest site vegetative and structural 
characteristics, secondly, to determine prey use and human 
activity factors relative to habitat use by bald eagles, 
and third, to develop basic recommendations for nest site 
management. This study was conducted during the summers 
of 1985 and 1986. Nest sites were studied in four areas of 
Montana. A total of 35 variables was measured and recorded 
at each nest site and the same variables were collected at 
selected non-use points for statistical comparison. Prey 
remains were collected from around nest trees to determine 
prey use. Bald eagles nested in seven different tree 
species of which the major nest tree species varied 
between population units. Average nest tree, nest, and 
nest stand heights were 30.4, 22.8, and 20.2 m 
respectively. There was no difference (P > 0.05) in 
height, nest height, or nest stand height among the 
different nest tree species. Used nest trees were 
significantly (P < 0.05) taller than non-use nest trees. 
Bald eagles tended to select nest sites away from areas of 
heavy human recreational use and busy roadways. 
Agricultural activities did not appear to affect use of an 
area by nesting eagles. Discriminant function analysis 
selected nest tree diameter at breast height (DBH), nest 
tree height, canopy closure of the nest stand, human 
recreational activity, and nest tree decadence as the 5 
variables that most separated used from non-used sites. 
Prey use by bald eagles varied among population units and 
appeared to be governed by the type of water body the 
eagles were nesting near. A major factor in bald eagle 
nest site selection appears to be the structure of the 
nest tree in combination with an open stand in an area of 
relatively little human recreational disturbance. 
Management practices should encourage production of future 
nest trees, reduced and low levels of human activity and 
maintence of present or higher levels of prey populations. 
Potential bald eagle nest sites should be evaluated 
according to the variables selected by discriminant 
function analysis as being the most predictive of use by 
nesting eagles.
IINTRODUCTION
The bald eagle (Hallaeetus leucocephalus) was 
classified as an endangered species in Montana on 14 Feb. 
1978, when only 12 occupied nesting territories were known 
to exist. Increased survey emphasis and a real increase in 
population has resulted in a much larger number of 
occupied bald eagle nesting territories. There are 
currently 54 known occupied nesting territories in 
Montana. Even though the bald eagle population has been 
increasing in recent years, the carrying capacity may be 
decreasing due to a long term trend of habitat alteration 
and degradation. This trend will probably continue, 
consequently reducing the availability of suitable nesting 
habitat for bald eagles in Montana.
This study was initiated in the summer of 1985 with 
the purpose of evaluating nest site characteristics of 
bald eagles in Montana. Objectives were:
1. To provide a detailed analysis of onsite vegetative and 
structural features of nest trees and nest stands used 
by bald eagles.
2. Determine prey use and human activity factors relative 
to habitat use by bald eagles.
3. Develop nest site management recommendations.
Results combined with macrohabitat data
2currently being compiled and analyzed by the U.S. Forest 
Service will provide a habitat suitability/potential 
index for the identification of potential bald eagle 
nesting habitat, and management prescriptions for existing 
sites.
3DESCRIPTION OF STUDY AREAS
Locations of bald eagle nesting territories were 
plotted on a 1:1,000,000 scale map of Montana to determine 
nest distribution (Fig. I). Nest site locations were 
obtained from the Montana Department of Fish, Wildlife, 
and Parks (Dennis Flath pers. comm.). Four areas of 
nesting concentration immediately became apparent: (I) the 
Northwest Population (NWP), (2) the West-Central 
Population (WCP), (3) the Greater Yellowstone Ecosystem 
Population (GYEP), and (4) the Eastern Prairie Population 
(EPP). These 4 areas were categorized on a hydrologic and 
physiographic basis.
Northwest Montana
The NWP includes the Flathead, Kootenai, and lower 
Clark Fork River (upstream to Fish Creek and Ninemile 
Valley) drainages. The region is bounded on the east by 
the Continental Divide, on the north by the Canadian 
border, on the west by Idaho, and on the south by the 
crest of the Bitterroot Mountains. The southeastern 
border of the region is marked by the Rattlesnake Creek 4
and Blackfoot River divides.
I l  I . I  I l  I I l  I 1 7
■GREATER YELLOWSTONE ECOSYSTEM POPULATION 
O  STUDY TERRITORIES
IQ . Il I i !3 . 14 * I ‘6 .
Figure I. Distribution of bald eagle nest sites in Montana
5In all but the drier valleys, the NWP region has an 
abundance of Pacific Coast forest tree species that are 
less common or absent elsewhere in the state (Arno 1979). 
Tree species largely restricted to this region include 
western hemlock CTsuaa heterophvlla), mountain hemlock (T. 
mertensiana), western redcedar (Thuia plicaba), Pacific 
yew (Taxus brevifolia), grand fir (Abies grandis), and 
western white pine CPinus monticola).
Other abundant tree species in the NWP that are 
common throughout the intermountain western United States 
are Douglas fir CPseudotsuaa menziesii), subalpine fir 
(Abies Iasiocarpa^. western larch CLarix occidentalism, 
Engelmann spruce CPicea enaelmannii), whitebark pine 
CPinus albicaulis), ponderosa pine NEST ponderosa), 
lodgepole pine (Pj. cpntprta) , Rocky Mountain juniper 
CJuniperus scopulorum), quaking aspen (Populus 
trernuloidesm. narrowleaf cottonwood (P. auaustifoliam, and 
black cottonwood (Pj. trichocaroam .
Northwestern Montana is strongly influenced by moist 
maritime air masses from the Pacific Coast. As shown in 
Table I7 the air masses provide abundant rain and snowfall 
and frequent humid, cloudy conditions, and relatively mild 
winter temperatures that are necessary for the survival of 
many of the coastal forest species (Arno 1979).
Table I. Climatic data for 4 regions of Montana, from Arno (1979).
Region .
Elevation of 
weather station (m)
Mean monthly temp. (C) 
Jan. July
Mean annual 
precipitation (cm)
Mean annual 
snowfall (cm) .
NWP 840 -6 18 61.0 216
WCP 1190 -6 18 43.2 216
GYE 1995 -9 16 51.4 361
EPP 1000 -5 22 33.0 HO
O l
7West-Central Montana
The WCP region includes the Clark Fork River drainage 
from the Missoula-Frenchtown Valley upstream. This region 
is bound on the west by the high Bitterroot Range. The 
Bitterroot Range forms the Montana-Idaho Divide and is a 
significant barrier to Pacific Coast moisture and thus to 
coastal plants (Arno 1979).
Although it has a relatively mild. Pacific-influenced 
climate, this region is generally drier than northwestern 
Montana (Table I). As a result. Pacific Coast forest 
species such as western redcedar, Pacific yew, western 
white pine, and grand fir are less common. West-central 
Montana is characterized by an abundance of the 
intermountain forest species western larch, ponderosa 
pine, and douglas fir (Arno 1979).
Greater Yellowstone Ecosystem
The GYEP region of southwestern Montana includes the 
Jefferson, Madison, Gallatin, and upper Yellowstone 
(downstream to Big Timber) River drainages. This 
region encompasses the western and northern part of an 
extensive, high elevation forested upland surrounding 
Yellowstone National Park. This upland is capped by large 
alpine plateaus in Montana and Wyoming.
The GYEP region has a continental climate. The valleys 
have high base elevations, are too cold, and have too
8short of a growing season (60 days avg.) for any 
significant amounts of ponderosa pine. Most of the forest 
in the GYEP is dominated by lodgepole pine, Douglas fir, 
and subalpine fir. Engelmann spruce is locally abundant 
(Arno 1979) .
Eastern Prairie
The EPP region of Montana includes the Yellowstone 
River drainage upstream to Big Timber, including the Clark 
Fork, Bighorn, Tongue, and Powder River drainages of 
southeastern Montana.
Ponderosa pine is the only forest species of this 
region and occurs in diverse habitat types (Arno 1979).
Vast expanses of grassland in this region are dominated by 
wheatgrasses (Aaropyron spp.), gramas (Bgutelgua spp.), 
and needle grasses (Stipa spp.) (Kuchler 1964). Extensive 
riparian zones along the river drainages are dominated by 
plains cottonwood (Pgpulus deltoides) with interspersions 
of boxelder (Acer neoundo), green ash (Fraxinus 
pennsylvanica). and Russian olive (Elaeagnus anaustifolia).
The EPP region has a continental climate. Summers are 
longer, hotter, and drier than those in the mountainous 
forest regions. Most of the annual precipitation comes in 
spring and summer rains. Winters are generally dry and
cold.
9METHODS
The terminology used to describe bald eagle nests 
and nest sites follows that proposed by Postupalsky (1974) 
Field work was conducted July through September 1985 and . 
1986. By July, eagles had fledged or were near fledging, 
so an intrusion to the immediate nest site at this time 
caused little chance of abandonment.
Thirty five measurements and qualitative descriptions 
were recorded to characterize bald eagle nest sites (Table 
2). Nest tree and nest site parameters evaluated were 
similar to those discussed in Emlen (1956), Juenemann 
(1973), Grubb (1976), McEwan (1977), Lehman (1979), Todd 
(1979), and Anthony and Isaacs (1981).
The 35 measurements and descriptions fit into 3 major 
groups; (I) nest site characteristics, (2) forest stand 
characteristics, and (3) human disturbance characteristics 
Parameters measured and the methods used in data 
collection are listed in Table 2.
Nest Site Characteristics
Nest site characteristics were grouped into 4 
categories and included the first 23 items listed in 
Table 2. The nest tree was characterized by items 1-10,
Table 2. Characteristics measured at and around bald eagle used and 
non-used sites in Montana, 1985-86.
Parameter Method
A. Nest site characteristics
1. Nest tree species
2. Nest tree DBH^
3. Nest tree height
4. Nest tree crown class
5. Nest tree percent decadence
6. Nest tree maturity class
7. Nest tree position on slope
8. Nest tree distance from AWB^
9. Nest tree direction to AWB
10. Nest tree elevation above AWB
11. Nest height
12. Percent of nest covered from above
13. Distance of nest below tree top
14. Position of nest on tree
15. Direction of nest window
16. Size of nest window
17. Aspect at the nest tree
18. Percent slope at the nest tree
19. Type of AWB
20. Permanence of AWB
21. Elevation of the nest site
22. Length of shore within 1.6 km
23. Land ownership of nest site
Direct observation
Direct measurement with a DBH tape
Direct measurement with a Stratex Stratolevel
Direct observation (dominant, codominant, suppressed)
Direct observation (estimate to the nearest percent)
Direct observation (immature, mature, over-mature)
Direct observation
Direct measurement with rangefinder
Direct measurement with compass
Determmined from U.S.G.S. topographic map
Direct measurement with a Stratex Stratolevel
Direct observation (estimate to nearest percent)
Calculated from parameters 3 and 11
Direct measurement with a compass
Direct measurement with a compass
Direct measurement with a compass
Direct measurement with a compass
Direct measurement with a Stratex Stratolevel
Direct observation (lake, river, reservoir)
Direct observation (permanent or ephemeral)
Determined from U.S.G.S. topographic map 
Determined from U.S.G.S. topographic map 
Determined from S.C.S. county map 
and BLM Montana land status map
Table 2. Continued
Parameter Method
B. Forest stand characteristics
24. Canopy tree density
25. Average DBH
26. Understory density
27. Average understory height
28. Percent crown closure
29. Average stand height (canopy)
30. Height of nest above stand
31. Opening adjacent to nest tree
C. Human disturbance characteristics
Trees per hectare in the area sampled
Average of all trees measured
Trees per hectare in the area sampled
Average height of understory trees in the area sampled
Direct observation (estimated to the nearest percent)
Average of tree heights measured with inclinometer
Calculated from 11 and 30 above
Direct observation
32. Logging activities
33. Agricultural activities
34. Recreational activities
35. Other activities
32-35. Activity type was determined by direct observation. 
Disturbance rating was a scale score from 0-1 
depending upon the intensity and closeness of the 
activity to the nest tree.
1 DBH = Diameter at Breast Height
2 AWB = Associated Water Body
12
the nest by items 11-16, the topographic and geographic 
characteristics by items 17-22, and land ownership of the 
site by item 23.
Forest Stand Characteristics
Characteristics of forest stands around eagle nests 
were described by sampling the stand within 50 m of the 
nest tree. The point-centered quarter method (Cottam and 
Curtis 1956) was used to sample stands around each nest.
From the nest tree two 50 m transects were 
established, one directly upslope and one downslope.
Sample points were established systematically at 10 m 
intervals along the transects. For nest trees positioned 
on flat sites a random direction was selected for the 
first transect and the second was placed in the opposite 
direction. At each sample point the surrounding area was 
divided into 4 quadrants and the following data were 
recorded for the closest tree over 10 cm (4 in.) diameter 
at breast height (DBH): (I) species, (2) DBH, and (3) 
distance to the sample point. Data recorded for the 
closest tree under 10 cm DBH in each quadrant (I) species, 
(2) height, and (3) distance to the sample point. The 
heights of 5 "characteristic" trees within each sample 
area were then measured to determine the average height of 
the forest stand. Percent canopy coverage in each stand
13
was visually estimated. Characteristics of forest stands 
calculated from the above data are described in.items 24- 
31 (Table 2).
Human Activity Characteristics
Human activities and sources of potential human 
disturbance to nesting eagles within 1.6 km of the nest 
trde were recorded during visits to the nest and from 
topographic maps. Human activity variables and 
methodology follow those of Anthony and Isaacs (1981) with 
a few modifications.
All activities (items 32-35, Table 2) were scored as 
to intensity (Table 3). Intensity scores were estimated 
subjectively on a scale of 0 to 5, from no potential to 
highest potential for disturbance to nesting bald eagles. 
Disturbance is defined as a behavior pattern caused by the 
presence of human activity. A second score was given to 
each activity based on its distance from the nest, the 
closer the activity the higher the score. A score of I to 
4 was assigned as follows: 4 = activity was 0.0-0.4 km 
from nest, 3 = activity was 0.4-0.8 km from nest, 2 = 
activity was 0.8-1.2. km from nest, I = activity was 1.2- 
1.6 km from nest. Each human activity was rated by 
multiplying the intensity score by the distance score.
The lowest possible score for an activity was 0 and the 
highest was 20. The activity factors were recorded
14
Table 3. Ratings of human activities within 1.6 km of 
bald eagle nests' in Montana, 1985-86.
Types of activity Range of intensity ratings
Loaaina
Roads:
Main 4 to 5
Spur 3 to 4
Gated I to 3
Skid trail 
Practices:
I to 2
Clearcut 4 to 5
Partial cut 2 to 4
Tree planting 
Aariculture
I to 3
Ranching
. Grazing I to 3
Fencing I to 2
Haying I to 2
Farming
Dry cultivated field I to 2
Irrigated cultivated field I to 4
Recreation
Off-road vehicles 3 to 5
Commercial establishments 2 to 5
Fishing I to 3
Access points 2 to 5
Other recreational activities I to 5
Other Activities
Paved highway 4 to 5
Paved road 3 to 5
Gravel road 2 to 4
Dirt road I to 4
Railroad 2 to 4
Private homesites 2 to 5
Public facilities 2 to 5
Other activities I to 5
15
separately and then totaled for each of the 4 major 
categories. Each category was then given a scaled score 
determined by dividing the total possible score for that 
category into the attained score. The scaled score 
allowed for statistical comparison between activity 
categories.
Nest site, forest stand, and human activity data were 
collected at 32 nest sites and an equal number of points 
of non-use associated with each nest site. Non-use 
points were determined by selecting a point 2 km upstream 
or downstream from the nest tree for riverine nests, and 
by a point directly across the body of water for lake and 
reservoir nests. The first tree at this point that looked 
as if it could support an eagle nest (determined 
subjectively) was chosen as the "nest tree" of the non-use 
area, hereafter referred to as non-use nest trees.
Prey Use
Prey utilization by bald eagles was determined by 
direct observation, prey item remnants (hair, feathers, 
bones, scales, etc.), and examination of regurgitated 
pellets. Food remnants and pellets were gathered beneath 
nests and observed perches.
Avian and mammalian prey remains were identified by 
comparison with specimens in the Montana State University
16
Vertebrate Museum. Fish remains were identified by 
comparison with known skeletal and scale specimens.
The minimum number of individual prey animals from 
each nest was determined using the following criteria:
(I) I remnant representing an identified species and I 
pellet representing the same species or an unidentified 
species of that class equalled I individual of that 
species, or (2).I remnant representing an unidentified 
species of a given class (Aves7 Pisces, or Mammmalia) and 
I pellet containing an unidentified species of the same 
class equalled I individual of an unidentified species of 
that class (Swenson et al. 1986).
.Statistical Methodology
All statistical analyses were conducted using the 
Statistical Package for Social Sciences (SPSS) (SPSS Inc. 
1983). Analyses were performed on a Honeywell Level-66 
mainframe computer using Honeywell's CP6 operating 
system located at Montana State University.
Basic descriptive statistics (mean, range, standard 
deviation) were calculated for all continuous variables 
and relative frequencies were calculated.for discrete and 
coded variables. These calculations provided information 
on basic characteristics of bald eagle nest sites in the 4 
geographic areas..
17
Discriminant function analysis was used to determine 
if differences existed between the measured 
characteristics of nest sites and sites of non-use. 
Discriminate analysis constructs a linear function 
containing coefficients derived for each variable found to 
be significant (significance level set by user). This 
function can be used as a predictive model to assess the 
potential of non-used areas to be used as nesting sites by 
bald eagles. This function can also be used to identify 
habitat parameters deficient for use by nesting bald 
eagles, thereby indicating the need for specific 
management tasks to improve habitat. In all statistical 
analyses, differences were tested at the 0.05 level of 
significance.
18
RESULTS
Nest and Nest Tree Characteristics
Bald eagles nested in 7 species of trees in Montana. 
Nest tree species use varied throughout the state (Table 
4). Nest tree height and DBH characteristics compared to 
non-use trees are described in Table 5. Mean nest tree 
height was 30.4 m (SD = 7.3) and was significantly greater 
than the height of non-use trees (P < 0.01). There were no 
statistical differences among heights of different nest 
tree species.
Table 4. Nest tree species used by bald eagles in the 4 
population units of Montana, 1985-86.
Species NWP1 WCP2 GYEP3 EPP4
N(%) N(%) N(%) N(%)
Ponderosa pine 
Black cottonwood
5(42).
6(50)
5(100)
Plains cottonwood 
Narrowleaf cottonwood 
Western larch
1(12)
6(100)
1(8)
Douglas fir 6(75)
Lodgepole pine 1(12)
= Northwest population 
= West-central population
= Greater Yellowstone ecosystem population 
= Eastern prairie population
Table 5. Species, average tree heights, nest heights, and diameter at breast height 
(DBH) of bald eagle use and non-use trees in Montana, 1985-86.
Species Status1 N . Tree height(m) Nest height(m) Tree DBH(cm)
Ponderosa pine U 10 32.8* 24.3 97.0*
N 12 26.5 68.3
Black cottonwood U 6 29.4 21.9 90.1
N 4 26.9 71.3
Plains cottonwood U 6 30.0 22.9 98.1*
N 7 23.6 74.1
Narrowleaf cottonwood U I 33.0 23.6 75.8
N I 27.0 71.4
Western larch U I 34.7 26.0 86.5
N 0
Douglas fir U 6 26.9* 21.1 110.5*
N 5 23.6 90.0
Lodgepole pine U I 30.3 19.6 48.5
N 2 23.4 34.4
Russian olive U 0
N I 9.9 39.6
I
*
U = used nest trees, N = non-use trees 
indicates significance level at P < 0.05
20
Mean nest tree DBH was 95.9 cm (SD = 25.9) which was 
significantly greater (P < 0.01) than the mean DBH of 
70.4 cm (SD = 19.9) for non-use trees. Nest tree height 
and DBH was not significantly different (P > 0.05) among 
the four population units.
Data from continuous nest tree and nest variables are 
presented in Table 6. All nest trees sampled were either 
dominant or codominant in relation to other trees in the 
stand (Figure 2 ), as were all of the non-use trees 
sampled. Thirty of 31 nest trees sampled were either 
mature or over-mature (Figure 3), the one exception being 
an immature black cottonwood on the north shore of 
Flathead Lake. However, this tree was the dominant tree 
,in a stand of young cottonwoods and willows growing on a 
flood control dike. All of the non-use trees sampled were 
mature or overmature.
Fourteen (45.2%) of the nest trees on a statewide 
basis were in areas with zero slope. These were mainly 
along the. Flathead and Yellowstone Rivers in the NWP and 
EPP respectively. Of the remaining nest trees which were 
associated with slopes, 82.3% were positioned mid-slope or 
lower (Figure 4).
Nineteen (61.3%) of 31 nests sampled were positioned 
axially on the nest tree. The position of the remaining 
nests is described in Table 6. Axial nests are ones that 
are built with the base of the nest supported by the main
Table 6. Mean nest and nest tree characteristics of bald eagles in 4 population 
units of Montana, 1985-86.
Population unit—
Variable NWP WCP GYE EPP
NT1 2 DBH(cm) 90.0 101.2 98.4 98.1
NT height(m) 30.8 33.8 28.1 30.0
NT % decadence 31.7 44.0 51.2 10.8
NT distance to AWB3 (m) 125.0 261.2 466.0 78.3
NT direction to AWB (deg) 167 147 209 183
NT elevation above AWB (m) 16.4 25.2 47.9 3.2
Nest height (m) 23.8 23.1 21.3 22.9
Nest distance below top (m) 7.1 10.9 6.8 7.1
Percent canopy cover 9.6 5.0 16.3 10.8
Nest position on NT (deg) 245 245 113 47
Nest window direction (deg) 206 229 94 184
Nest window size (deg) 157 254 224 165
1 NWP = Northwest population 
WCP = West-central population
GYEP = Greater Yellowstone ecosystem population 
EPP = Eastern prairie population
2 NT = nest tree
3 AWB = associated water body
22
Figure 2. Percentage of dominant and codominant 
trees used for nesting by bald eagles 
in Montana, I 9 8 5 -8 6 .
Figure 3. Maturity classes of bald eagle 
nest trees in Montana, 1985 — 86.
Figure 4. Slope 
nest trees in
NO SLOPE
g g g  BOTTOM
LOWER THIRD 
g g g  M ID -S LO P E  
I  UPPER THIRD
TOP
position of bold eagle 
Montana, I 98 5 -86 .
24
trunk or a fork in the main trunk of the nest tree. Axial 
nests are probably more secure than nests built off to the 
side of the main trunk where the support structure(s) 
are smaller and weaker. There appears to be a tendency 
for axial nests to be constructed more often in ponderosa 
pine and plains cottonwood nest trees than in other nest 
tree species (Figure 5). This is probably a result of the 
growth form for these two species. There was a direct 
line of sight from all nests to their associated water 
bodies (AWB).
Nest Site Characteristics
Nest site characteristics from used and non-used 
sites are presented in Table 7. As the data indicate, the 
only statistical differences between used and non-used 
nest stands were in the characteristics of percent crown 
closure and understory density of the stands. Crown 
closure was significantly less in used stands, as was the 
understory density. These differences indicate that 
nesting bald eagles prefer mature, open stands as nest 
sites.
Human Activity Characteristics
Human disturbance characteristics associated with 
used and non-used sites are presented in Table 8. These 
data suggest that bald eagles select nest sites away from
PE
RC
EN
T 
OF
 N
ES
T 
TR
EE
S
Figure 5. Percent by species of axial and non —axial 
bald eagle nests in Montana, 1985 — 86.
Ul
Table 7. Characteristics of use and non-use bald eagle nest stands 
in Montana, 1985-86.
Variable Status2 N NWP
Area
WCP
of State— 
GYEP EPP SW
Percent canopy U 31 15.4 13.0 13.8 6.8 12.9
cover N 32 16.6 27.0 22.0 9.3 18.0
Canopy density U 31 434.2 234.9 313.4 95.1 305.2
(trees/ha) N 32 439.8 438.7 295.5 139.9 337.9
Average canopy U 31 20.2 21.7 18.7 20.5 20.1
height (m) N 32 19.9 23.9 18.7 19.4 20.1
Understory U 30 1512.3 171.9 471.0 22.7 748.0
density (trees/ha) N 32 1074.1 2823.1 1144.1 363.3 1209.4
Understory U 30 2.2 2.8 1.9 3.6 2.5
height (m) N 32 2.0 1.4 1.6 4.5 2.4
Height of nest U 30 3.5 4.2 3.1 2.8 3.4
above canopy (m)
1 NWP = Northwest population 
WCP = West-central population
GYEP = Greater Yellowstone ecosystem population 
EPP = Eastern prairie population 
SW = Statewide
 ^ U =  Use, N = Non-use
* = Significant at P < 0.05
Table 8. Scaled scores for human activities at used and non-used bald eagle 
nest sites in Montana, 1985-86.
Variable Status1 2 N NWP
Area
WCP
of State— 
GYEP EPP SW
Logging U 31 0.0923 0.1176 0.0171 0.0000 0.0591
N 32 0.0768 0.1098 0.0025 0.0000 0.0466
Agriculture U 31 0.4022 0.1768 0.2501 0.5768 0.3604
N 32 0.2532 0.2232 0.2520 0.5633 0.3161
Recreation U 31 0.2472 0.3976 0.2744 0.1663 0.2628
N 32 0.3886 0.4518 0.4786 0.1924 0.3781*
Other (roads) U 31 0.2168 0.1728 0.1175 0.1498 0.1711
N 32 0.3186 0.2576 0.1812 0.2557 0.2610*
Total Disturbance U 31 0.9585 0.8648 0.6569 0.8929 0.8534
N 32 1.0372 1.0424 0.9144 1.0116 1.0017
1 NWP = Northwest population 
WCP = West-central population
GYEP = Greater Yellowstone ecosystem population 
EPP = Eastern prairie population 
SW = Statewide
2 U =  Use, N = Non-use 
* = Significant at P < 0.05
28
recreational and road/vehicular activities. The total 
disturbance figures show that on a state-wide basis bald 
eagle nest sites received less human disturbance than non­
use sites.
Discriminant Function Analysis
Table 9 presents the discriminant function analysis 
scores. These values were calculated using, all continuous 
variables measured at used and non-used sites.
The 5 highest discriminant function analysis scores 
(Table 9) suggest that bald eagles select tall, large nest 
trees that are somewhat decadent; and select open stands 
away from human recreational activities.
The Canonical coorelation (0.89) indicates a high 
degree of association between the discriminant scores and 
the 2 groups being tested. The Wilks' lambda score (0.20) 
suggests that there is a high degree of separation between 
the characteristics of used and non-used sites, and the 
eigenvalue (3.99) indicates that the discriminant function 
is a good predictor of use or non-use of an area by 
nesting bald eagles.
Food Habits
Forty six vertebrate species were identified 
from the prey remains collected (Table 10). All items 
were collected from debris accumulations at the base of
29
Table 9. Ranked discriminant function analysis
coefficients from 18 characteristics measured at 
used bald eagle nest territories and 
corresponding points of non-use in 
Montana, 1985-86.
Variable Discriminant Function Coefficient
Nest tree DBH1 0.5805
Nest tree decadence 0.5143
Recreational activity —0.5004
Nest tree height 0.3859
% Crown closure (nest-stand) 
Elevation above AWB2
-0.3831
0.3263
Understory height 0.3167
Understory density -0.3024
Amount of shoreline 0.2948
Logging activity 0.2678
Other activity (roads) -0.2041
Aspect -0.1742
Stand density (canopy) -0.1626
Average stand height (canopy) -0.1161
Average stand tree DBH 0.1113
Direction to AWB -0.1038
Slope -0.0376
Agricultural activity -0.0368
Eigenvalue = 3.99306
Canonical Coorelation = O .8942717 
Wilks' Lambda = 0.2002781 
Significance P = 0.1511
1 DBH = diameter at breast height
2 AWB = associated water body
30
Table 10. Prey remains from bald eagle nests 
in 4 areas of Montana, 1985-86.
Cicegory H-I
MCP 
N Z
GYE 
N Z
EPP 
N Z
Birds
Red-necked grebe flftxliceps gyiaegens) 
Great blue heron (Ardea herodiaaT 
Sandhill crane (.Qcm canadensis) 
California gill Ttasus californicua) 
Aaerican coot CFViIica americana)
Cansda gpose (Brmta canadensis)
Mallard (Anas platyrhynchoa)
Northern pintail (Anas acuta)
Fbrthern ahoveler (Anas clypeta) 
Blue-winged teal (Mas discors)
Cinnmon teal (Mas cyanoptera)
ReAead (Aythya meriic^ na)
Ibddy duck (Cteyura jamaicensia)
Banw's gpl#*sneye TBucephala islandica) 
Comon merganser (Merges merganser) 
Unidentified waterfowl 
Gkeat homed owl (Bubo virginianua)
Ruffed grouse (Bonasa vahcllus)
Fbrthem flicker (CoIaptes auratus) 
Blade-billed magpie "("Hca pica)
Clark's rut cracker (FbcIfraga ooluabiana) 
Mountain bluebird (Sialia currucoidea) 
Red-winged blackbird (Agelaiue phoenlceua) 
Unidentified ncrrvaterfowl birds
Birds subtotal
Fbm aquirrel (Sciurus niger)
Richardson's ground squirrel (Spcnaophilus richsrdsoni) 
Cohabian ground aquirrel (Spennophilua colimhianus) 
Blarkfail prairie dog (Cyncnyi Ludcvicianua)
Fbrthem pocket gopher uhcaasys talpoidcsT 
Striped skunk (Ftephitis mephitis)
Beaver (Castor csnsdenais)
Fiiskrat ^ dstra iibethica)
Uiitetail jackrabblt (lepua townaendi)
Snowehoe hare (Lepua — r Icarus)
Deer (Odoooileus app.T 
Pronghorn CMtilocaprs asericana)
Unidentified nmimil
Mmmal subtotal
Fish
Longpoee sucker (Catostonus catostonus)
Vhite sucker (Cstotceus oosnarsonD 
Largescale sucker (Catotosua mecrOcheilus)
IKsh dub (Gils traris)
Carp (Cypciius carpio)
Fbrthera sawfish (rPtychocheilus oregpnsnsis)
Channel catfish (Ictalurua punctatxa)
Largmsouth bass (Acropterus sahsoides)
Brown trout (Sahso trutts)
Cutthrot trout (Salsp clsrki)
Rainbow trout (Salao gsirdneri)
Bull trout (Sslvelitus oonfluentus)
Unidentified sahsonid 
Unidentified non-salmon id fish
Fish subtotal
Grand total
1.5
1.5
0.4
0.4
I 1.3 I 0.4
I 1.5 I 0.4
4 6.0 20 27.0 I 1.3 25 9.3
2 2.9 4 5.1 6 2.2
2 2.9 I 2.0 6 8.1 9 11.5 18 6.7
I 1.3 I 0.4
2 2.9 2 0.7
3 4.1 I 1.3 4 1.5
I 2.0 I 0.4
3 4.1 3 1.1
I 1.5 I 0.4
I 1.3 I 0.4
2 4.1 4 5.4 6 2.2
S 7.5 7 14.3 4 5.4 2 2.6 18 6.7
I 1.3 I 0.4
I 1.3 I 0.4
I 2.0 I 0.4
I 1.3 I 0.4
I 1.5 I 0.4
I 1.3 I 0.4
I 1.3 I 0.4
S 7.5 4 8.2 4 5.4 2 2.6 IS 5.6
25 37.3 17 34.7 49 66.2 22 28.2 113 42.2
3 3.8 3 1.1
I 1.3 I 0.4
I 2.0 I 0.4
3 3.8 3 1.1
I 1.3 I 0.4
I 2.0 I 1.3 2 0.7
2 4.1 2 2.6 4 1.5
3 6.1 3 1.1
9 11.5 9 3.4
I 2.0 I 0.4
2 4.1 2 2.6 4 1.5
I 1.3 I 0.4
2 2.9 3 6.1 I 1.3 6 2.2
2 2.9 13 26.5 I 1.3 23 29.5 39 14.6
I 2.0 5 6.8 6 2.2
4 5.4 4 1.5
I 1.5 I 0.4
5 6.8 5 1.9
4 6.0 29 37.2 33 12.3
4 6.0 12 24.5 16 6.0
5 7.5 5 1.9
I 1.5 I 0.4
I 2.0 3 4.1 4 1.5
I 1.5 I 0.4
I 1.3 I 0.4
I 1.5 I 0.4
4 6.0 4 1.5
19 28.3 5 10.5 6 8.1 4 5.1 34 12.7
40 59.7 19 38.8 24 32.5 33 42.3 116 43.3
67 49 74 78 268
31
nest trees or from below nearby perches; therefore they 
likely represent food use during the nesting season.
Fish accounted for 43.3% of total food remains on a 
statewide basis (Table 10). Twelve species of fish were 
identified from remains.
Birds accounted for 42.2% of the total prey remains 
with 22 species represented on a statewide basis (Table 
10). American coots were the most common bird species 
identified. They were found in remains from 3 of the 4 
population units and made up 9.3% of the total prey 
remains. Mallards were the only prey species to be 
identified from all 4 population units, an indication of 
the ubiquitous nature of the species.
Mammals represented 14.6% of the prey remnants 
statewide (Table 10). Twelve species of mammals were 
identified and represented 4 mammalian groups: rodents, 
lagomorphs, mustellids, and ungulates.
Prey use by population unit is illustrated in Figure 
6. The frequency of birds, fish, and mammals in prey 
remains was significantly different within the NWP and 
GYEP population units (X test, P < 0.001), the 
NWP and EPP (P < 0.001), the WCP and GYEP (P < 0.001), and 
the GYEP and EPP (P < 0.001).
Prey utilization of bald eagles associated with lakes 
and reservoirs was significantly different than prey 
utilization of bald eagles nesting in riverine habitats 
(X test, P < 0.001) (Fig. 7).
32
GYEP
BIRDS MAMMALS
Figure 6. Prey use by nesting bald eagles by 
population unit in Montana, 1985 — 86.
Figure 7. Food use of nest ing bald eagles in relat ion 
to asociated water body type in Montana, 1 9 8 5 - 8 6 .
34
DISCUSSION
Nest Tree and Nest Site .
Results of the multivariate discriminant analysis 
indicated that large, old trees, in terms of height, DBH, 
and percent decadence, located in open forest stands away 
from intense human recreation areas are preferred as nest 
sites by bald eagles. These findings are in accordance 
with previous studies of nest site selection by bald 
eagles. Juenemann (1973) and Mathisen (1983) found that 
bald eagles on the Chippewa National Forest in Minnesota 
selected dominant or codominant trees close to water and 
near edges or openings in the forest stand. All 31 nest 
trees sampled in my study were associated with openings. 
Snow (1973), Swenson (1975), Grubb (1976), McEwan and 
Hirth (1979), Todd (1979), Lehman (1980), and Fraser 
(1981), also reported nest trees being located near 
openings, and in open canopy stands. Explanations offered 
for this phenomenon are increased nest accessibility (Snow 
1973), increased maneuverability in open stands (Lehman et 
al. 1980), and a survival advantage to young that have 
fledged prematurely and fallen to the ground (Fraser 
1981).
All studies of nesting bald eagles have emphasized 
the importance of the nest being in proximity to
35
water. My data show that all nests provided unobstructed 
line-of-sight to their AWB, and that average distance to 
water was 225 m. These data do not differ drastically 
from the 329 m reported by Corr (1974) in Alaska, 201 m 
reported by Gerrard et al. (1975) in Saskatchewan and 
Manitoba, and 135 m in Maine (Todd 1979). Average 
distances to.the AWB of 1060 m and 654 m were reported by 
McEwan and Hirth (1979) and Andrew and Mosher (1982) for 
eagle nests in Florida and on the Chesapeake Bay, 
respectively. These authors felt the increase in distance 
that they observed was a reflection of the eagles' 
response to increased shoreline activity by humans in 
those areas.
Bald eagles utilized 7 different tree species as nest 
trees in Montana. Although the tree species used varied 
within and among population units, the structure (tree 
height, crown class, maturity) of the nest tree was 
constant among all tree species used. This suggests that 
bald eagles select nest trees on the basis of the 
structure alone. The tree must be large enough to allow 
the nest to be at or above the level of the surrounding 
canopy (Fig. 8), and support a large nest platform. There 
was also an indication that eagles preferred nest trees 
with an open growth form which allows unencumbered 
movement to and from the nest. The literature supports 
the contention that eagles choose nest trees on the basis
15
Nest Tree Nest Stand Nest Tree Stand
(use) (n o n -u s e ) (n o n -u s e )
Figure 8. Height relationships among nest trees, nests, 
and nest stands for bald eagle nest sites in Montana, 
1 9 8 5 -8 6 .
37
of structure and not on species preference. Lehman (1979) 
found that eagles in California nest in Ponderosa pine and 
sugar pine (P_i_ lambertiana) . In Oregon, the most often 
used nest tree species were Ponderosa pine and Douglas fir 
(Anthony et al. 1982), and in Alaska, Sitka spruce (Picea 
sitchensis) and Western hemlock were the most often use 
species (Hodges and Robards 1981). Different species of 
nest trees are used by bald eagles across the United 
States (Juenemann 1973, McEwan and Hirth 1979, Andrew and 
Mosher 1982). The one commonality is that all of these 
species, given the right conditions, can produce the 
necessary vegetative life-form to support bald eagle 
nests.
The selection of vegetative structure for 
nest sites has also been documented for other types of 
birds. Weller and Spatcher (1965) noted that passerine 
birds, waterfowl, and wading birds associated with prairie 
marshes all had specific vegetative structural 
requirements, regardless of species composition, for 
nest building and concealment.
The only characteristic of forest stand structure 
that appeared to be selected for was less crown closure, 
or open stands. Lehman et al. (1980) also found bald 
eagles preferred to nest in open stands (20-40% crown 
closure), but could not demonstrate selection for any 
other characteristics of forest stands. Andrew and Mosher 
(1982) found eagles nesting in denser stands than occurred
38
randomly, apparently in response to increased human 
disturbance.
Figure 8 illustrates the relationships between nest 
tree height, nest height, and nest stand height for use 
and non-use sites in Montana. A clear relationship is 
apparent showing that nest trees are taller than the 
surrounding stand, and that they offer a structure for 
eagles to build their nest slightly above the surrounding 
canopy.
Human Activities
There are many discussions on the effects of human 
activities on nesting bald eagles (Broley 1947, Hensel 
and Troyer 1964, Gerrard et al. 1975) but there are few 
specific studies that addressing this topic. Juenemann 
(1973), Newton (1979), Stalmaster et al. (1983), and 
Fraser et al. (1985) have indicated that disturbance due 
to human activities was, or had the potential to be a 
limiting factor for nesting bald eagles.
McEwan and Hirth (1979) found that production of 
young was independent of habitat alteration and road use. 
near active nest sites in Florida. Mathisen (1968) and 
Fraser et al. (1985), determined that there was no great 
human impact on nesting bald eagles in the Chippewa 
National Forest in Minnesota. However, Thelander (1973), 
Andrew and Mosher (1982), and Anthony and Isaacs (1983),
39
found that bald eagles nested farther from water in 
response to .-waterfront activity at nesting areas in 
California, Chesapeake Bay, and Oregon, respectively.
Data from this study suggest that bald eagles selected 
nest sites away from human activities, especially those 
that were recreational or road-related.
At the Red Rocks nest site on the Blackfoot River 
near Missoula there was a significant amount of 
recreational activity within 200 - 400 m of the nest 
tree. During I, 1-hour observation period I counted 36 
inflatable rafts (not fishing), 21 fishing boats or rafts 
and 12 individual float-tubes passing below the nest tree 
During this I-hour period there was at least I adult bald 
eagle in the air in the vicinity of the. nest site at all 
times. The Red Rocks eagles did not nest successfully 
that year (1986). Admittedly, I visited this nest site 
on a hot, clear, Sunday afternoon in mid-July, and the 
amount of recreational activity I recorded was no doubt 
more intense than usual. Nevertheless, the observation 
did demonstrate that intense recreational activity can, 
and did disturb nesting adult bald eagles.
At present levels, agricultural activities appear to 
pose no serious threat to nesting bald eagles. 
Discriminant analysis showed that eagles did not select 
nest sites in areas with lower agricultural activities. 
These data indicate that nesting eagles are tolerant of 
present standard agricultural practices in Montana.
40
Furthermore, the presence of agricultural-type activities 
at a particular nest site probably reduces the possibility 
of intense recreational or road-related activities at that 
same site.
My observations of nest sites suggested 
that humans on foot (fishing, camping, backpacking) 
within the nesting territory elicited stronger responses 
from adult bald eagles than boat fishing or occasional 
vehicular activities. In all instances, when I approched 
an eagle nest site on foot during the fledging or 
immediate post-fledging period, I was met with prolonged 
defense responses from the nesting adults. However, 
several times during observations of nesting adults 
during the same time periods, I observed canoes, small- 
motored fishing boats, and inflatable rafts approach and 
pass near active nest trees with no outward signs of 
disturbance to the resident adult bald eagles. In one 
instance, I gained access to a road with locked gate that 
passed within 60 m of an active nest tree. I drove a 
small pickup truck on the road past the tree 6 times over 
a 2-day period. There was no response from the resident 
adults. However, travel by foot on the same road elicited 
a strong defense response from the adult birds as soon as 
I was within sight of the nest tree (250 m). Harmata 
(1984) observed that wintering bald eagles did not respond 
to vehicular traffic in the vicinity of roost trees, if 
the vehicles did not stop.
41
Disturbances due to logging practices appeared to be 
negligible in this study. However7 Anthony and Isaacs 
(1981) found that bald eagles in Oregon tended to select 
nest sites away from areas that were disturbed due to 
logging practices. Anderson (1985) stressed that logging, 
when done at the appropriate time of the year (non­
breeding season), had no adverse effects on bald eagle 
nesting activities or productivity. This is supported by 
I case in Montana (Hubbard Reservoir), where the stand 
around a bald eagle nest tree was clearcut. This nest 
site has been active and productive since, the logging 
activity. However, clearcutting adjacent to nest trees 
does make them more susceptable to wind damage and blow­
downs, and should not be practiced. Hodges (1985) 
demonstrated that the probability of nest destruction was 
significantly increased when there was clearcut logging 
within 45 m of the nest tree. Hodges (1985) further 
concluded that the 100 m protective zone around bald eagle 
nest trees in Alaska may be inadequate for.wind protection 
and for providing future nesting and perching habitat even 
though the buffer zone was effective at reducing the 
levels of disturbance from logging practices around the 
site.
I agree with Anderson (1985), that forest 
management programs can provide nesting habitat concurrent 
with the production of forest products through
42
manipulation of forest stand structure, i.e. providing 
dominant or codominant nest trees with adequate nest- 
support limbs within an uneven aged, open forest stand.
Prey Utilization
Fish are likely to be underrepresented in collections 
of prey remains and pellets from near bald eagle nests 
because of the high rate of digestability and 
decomposition of fish bones and scales (Imler and Kalmbach 
1955, Dunstan and Harper 1975, Ofelt 1975). This is 
especially true in fish species with fine bone structure 
such as salmonids. Todd et al. (1982) found that even 
though American eel (Anguilla rostrata) and tomcod 
(Microoadus tomcod), both fine-boned fish, were commonly 
observed prey of bald eagles in Maine they were nearly 
absent from food debris collections. The common 
occurrence of bottom-dwelling fish in eagle diets implies 
frequent foraging in shallow waters, scavenging on dead 
fish, and/or taking advantage of the greater vulnerability 
of spawning or benthic-feeding fish (Swenson 1979). . Data 
collected, during this study suggests that bald eagles prey 
to a greater degree on catostomid and cyprinid fishes than 
on salmonids. The data also indicate that bald eagles are 
opportunistic predators, capitalizing on locally abundant 
or vulnerable fish species. They further show that 
although fish made up the.largest percentage of the prey 
remnants on a statewide basis, no single species of fish
43
was identified in more than 2 of the 4 population units.
Birds associated with aquatic habitats made up 33.6% 
of the total prey remains and 79.6% of the bird remains.
The wj.de variety of avian species found in remains is also 
an indication of the opportunistic nature of the bald 
eagle.
Mammals ranged widely in importance among population 
units (Table 10) from a high of 29.5% of the prey remains 
from the EPP to a low of 1.3% of the prey remnants from 
the GYEP. No mammal species was present in more than 2 of 
the population units. Whitetail jackrabbits had the 
highest representation among mammal species with 9 
individuals accounting for 23.1% of the total mammals and 
3.4% of the total prey items statewide. A bias may 
present in the mammal use data because large mammals fed 
upon as carrion would not be represented. However, the 
use of ungulates for food during the nesting season 
is probably small. The variety of mammalian prey-types 
present indicates an opportunistic predator preying upon 
animals from a wide variety of habitat types.
In the WCP and the EPP prey use was distributed rather 
evenly among fish, mammals, and birds. In the NWP and 
GYEP mammals accounted for only a trace of the total prey 
use. Birds accounted for 37.3% and 66.2% of the prey 
remnants for the NWP and GYEP respectively while fish made 
up the remaining 59.7% and 32.5%, respectively. Bird use
44
was 33.7% higher -in the GYEP than the NWP. Similar prey 
utilization has been reported for the GYEP by Swenson et 
al. (1986).
The NWP and GYEP are similar in that bald eagle nest 
sites are predominately associated with lakes or 
reservoirs rather than rivers. The major difference 
between lakes in .the 2 units may be recreational use; that 
may cause such a wide difference in prey utilization. NWP 
lakes are decidedly more populated with summer homes and 
recreational resorts than lakes in the GYEP. Resulting 
heavier use by recreational boaters and fishermen may 
result in effectively reducing the numbers of waterfowl 
available for predation by eagles. Davenport (1974) 
determined that piscivorous waterfowl on Yellowstone Lake, 
Wyoming, avoided sections of the lake that had heavy human 
use. A study at Ruby Lake, Nevada (Bouffard 1982), also 
showed a marked decrease in waterfowl use of the lake with 
increasing levels of recreational use. Data collected in 
the NWP supports this conclusion. The 2 eagle pairs 
nesting on the north shore of Flathead Lake showed a 
higher use of avian prey species than eagles from 
Wildhorse Island and East Channel Island on the southern 
portion of the lake and eagles nesting on Lake Mary Ronan 
and Ashley Lake. The north shore of Flathead Lake was 
primarily agricultural whereas the southern portion of the 
lake as well as both the east and west sides was mainly in 
residential and recreational use. Lake Mary Ronan
45
and Ashley Lake also had a considerable number of summer 
homes built around them. Further, both of the lakes are 
popular and well known fisheries with easy public access. 
Lakes with bald eagle territories in the GYEP also support 
well known and popular fisheries but in some cases access 
is limited due to remoteness, and in all cases the number 
of summer homes on the lakes is few or none. This 
certainly results in reduced recreational boating use on 
these lakes and may result in higher densities of 
waterfowl available as prey.
The increased use of mammals in riverine habitats is 
probably a result of 2 factors. First, rivers and streams 
support higher numbers of aquatic mammals such as beaver 
and muskrat than do lakes. Secondly, the rivers in the 
WGP and EPP are associated with open areas such as 
intermountain basins in the WCP and sagebrush/grassland 
prairies in the EPP. Open habitats provide a greater 
opportunity for eagles to forage on terresterial mammals 
than do the closed timber stands of the GYEP and NWP.
My findings generally concur with results of several 
other studies on bald eagle food habits with respect to 
prey utilization with associated water body (AWB) type 
(Table 11).
My data suggest that bald eagle prey utilization 
is highly reflective of prey abundance and availability 
within the home range of individual pairs, and that eagles
46
Table 11. Prey utilization of bald eagles in various 
regions and associated water body types.
Reference and 
water types Fish-% Birds-% Mammal s-%.
This study 
Montana - Lakes 51 45 4
McEwan and Hirth 1980 
Florida - Lakes 90 8 I
Dunstan and Harper 1975 
Minnesota - Lakes 90 8 I
Swenson et al. 1986 
Montana/Wyoming - Lakes 31 53 16.
Todd et al. 1982 
Maine - Coastal 17 76 7
Wright 1953
New Brunswick - Estuary 90 9 I
Cash et al. 1985 
Nova Scotia - Coastal 66 24 10
Dugoni et al. 1986 
Louisiana - Swamp 41 42 16
This study 
Montana - Rivers 37 40 23
Haywood and Ohmart 1981 
Arizona - Rivers 71 . 16 13
Haywood and Ohmart 1981 
Arizona - Rivers 50 17 33
Swenson et al. 1986 
Wyoming/Idaho - Rivers 56 32 12
47
opportunistically take advantage of local and seasonal 
abundances of various prey species. These findings are in 
agreement with the results of other studies (Table 11).
Figure 9 illustrates a conceptual framework for the 
assessment of bald eagle nesting habitat. It stresses an 
interrelationship between prey availability, human 
activities, and the vegetational and physical structure 
of the nest site. These relationships are poorly 
understood, but are nevertheless important, even if only 
heuristically.
As an example, in this framework a given 
amount of human activity may be compensated for by 
increased prey availability, or an increase in the 
security offered by the biological and physical 
characteristics of the nest site. However, there is a 
more important question that should be considered within 
with this framework. It is the determination of the 
threshold levels in each part of the framework that cannot 
be compensated for by increased suitability of the other 2 
components. For example, what is the level of human 
activity that cannot be compensated for by an increase in 
nest site security, or an increase in prey availability.
*
Nest Site
C h a rac te r is t ics
(VEGETATIVE)
(GEO MOR P HO LOGIC)
Prey
Avai labi l i ty
Human
Activ it ies
Figure 9. Conceptual f ram ework  fo r  bald 
eagle nest site selection.
49
MANAGEMENT RECOMMENDATIONS
Management of nesting habitat for bald eagles must be 
directed towards the entire potential nesting site, rather 
than at individual nest trees for maintence of successful 
eagle nesting habitat. Nest trees should be, and are 
protected. However, management of nest stands on a site- 
specific basis should have, as a main objective, the 
promotion of growth of future nest trees that offer the 
desired structure preferred by nesting eagles. This can 
be accomplished through silvicultural techniques (Anderson 
1985) in forested areas. For cottonwood river bottoms, 
the management thrust should be maintence of the free- 
flowing stream or river system that constantly erodes and 
deposits sediments, thereby providing the appropriate 
conditions for continuous germination, growth, and 
maturation of cottonwood trees.
The evaluation of potential bald eagle nest sites 
should also consider both the structure of the nest tree 
and associated stand. Results of the discriminant 
analysis indicate that a large, old nest tree, a 
relatively open nest stand, and an area away from human 
activities are selected as nest sites by bald eagles.
50
This relationship can be described by the following 
equation:
(NTHT)(0.3859) + (REAC)(-0.5004) + (CRCL)(-0.3831) +
(NTDE)(0.5143) >15.1 for use, or
< 15.1 for non-use,
where NTHT = nest tree height, REAC = recreational 
activity, CRCL = percent crown closure of the nest stand, and 
NTDE = percent decadence of the nest tree. However, these 
criteria must be viewed in light of certain macrohabitat 
variables found to be essential to use of a site for 
nesting by bald eagles (Wright and Escano 1986). They 
found all bald eagle nests in Montana to: (I) be within 
1.6 km (I mi) of water, (2) have a direct line of sight to 
the AWB, (3) receive < 760 cm (300 in) of snowfall 
annually. These variables must also be considered when 
evaluating potential nest sites.
In eastern Montana there appears to be a danger of 
some nest trees being lost prematurely to firewood cutting 
practices. In some cases, land owners were not aware of 
bald eagles nesting on their property, perhaps some type 
of educational material on the basic life history and 
habitat requirements of bald eagles could be prepared for 
distribution to land owners. Educational material along
with contact from the designated resource agency
.
representative should go far in alleviating problems of 
this type.
51
Prey base management may be the most important part 
of a comprehensive bald eagle management scheme. However, 
it is also the most difficult to assess. Management 
should strive to maintain present prey populations within 
eagle nesting territories. Furthermore, bald eagle prey 
needs should be assessed prior to any planned rough-fish 
removal in bald eagle territories. Future research 
concerned with bald eagle nesting habitat should focus on 
the specific influences of prey availability and abundance 
on habitat selection and use by nesting bald eagles.
Human activities in bald eagle territories should 
be minimal from the time of clutch initiation through the 
post-fledging period. The U. S. Forest Service, National 
Park Service, and Bureau of Land Management all have 
implemented access closure policies to protect bald eagles 
from disturbance during this period. These management 
policies should continue. Furthermore, site-specific 
observations should be conducted on all occupied nesting 
territories to determine if the nesting eagles are 
resident or migratory. If bald eagles are determined to 
be resident, year-long protection of the sites is
necessary.
52
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