Species–Habitat Model for Bald Eagle
7.7
Species – Habitat Model for Bald Eagle
Common Name:
Bald Eagle
Scientific Name:
Haliaeetus leucocephalus
Species Code:
B-BAEA
B.C. Status:
Yellow-listed
Identified Wildlife Status:
None
COSEWIC Status:
Has not been examined.
7.7.1
Introduction
General habitat ratings for the Bald Eagle are predicted to have a low reliability as no model verification has
been done, and there is very little documented of the wildlife and habitat associations for this area of British
Columbia. Trees used for Bald Eagle nesting and important food sources in this area of northeastern B.C. are
not known (Blood and Anweiler, 1994). The information presented in the species-habitat model has largely been
extrapolated from other regions and has not been validated for the study area. As information is very limited on
this species within northeastern British Columbia, relevant literature from B.C. and western north America has
been included where applicable.
7.7.2
7.7.2.1
Distribution
Provincial Range
An estimated 21,000 Bald Eagles nest within British Columbia occurring over the entire province with the
exception of the alpine and sub-alpine zones (Blood and Anweiler, 1994). An estimated 20,000 to 30,000 Bald
Eagles winter within the province with over 90% of these birds wintering on the coast (Blood and Anweiler,
1994).
The relative provincial importance of the Taiga Plains eco-province has been estimated in terms of providing
Bald Eagle habitat. Within this eco-province, the BWBS biogeoclimatic zone received a rank of low to no
importance for nesting habitat and a rank of no importance for winter habitat value (Blood and Anweiler, 1994).
The Bald Eagle is a summer visitor to the area and scattered breeding may occur on the larger rivers within the
study area. During field work, no nests were observed but a few individual birds were seen on the Liard and
Beaver rivers on different occasions, generally perching in large balsam poplars. M. Labine (personal
communication) in the Fort Liard area of the Northwest Territories has seen Bald Eagles mainly in the spring
and the fall feeding on winter-killed ungulates in the spring. No winter use is expected.
7.7.2.2 Elevational Range
Bald Eagles may nest up to 1370 m elevation (Campbell et al., 1990), although, they generally nest at low
elevations (Blood and Anweiler, 1994).
Species–Habitat Model for Bald Eagle
7.7.3
Habitat Use (Life Requisites and Seasons)
7.7.3.1 Living
Bald Eagles are primarily associated with aquatic habitats and large trees. In the British Columbia Interior,
important nesting habitat occurs along medium to large rivers with a low-gradient and extensive
floodplain/wetland development, around wetland complexes (Blood and Anweiler, 1994), on lake shores, and on
islands in rivers and lakes (Campbell et al., 1990). Bald Eagles will usually nest within 200 m of water (Gerrard
et al., 1975) and rarely nest farther than a few hundred metres from shore in wilderness areas; although, in areas
of human disturbance, eagles will nest farther away from water; few nests are located more than 1.6 km from
permanent water (Blood and Anweiler, 1994; Anthony et al., 1982; Anthony and Isaacs, 1989). Bald Eagles are
not known to occupy areas with less than 0.8 square km of open water (Peterson, 1986). Bald Eagles may nest
up to 1370 m elevation (Campbell et al., 1990), although, they generally nest at low elevations (Blood and
Anweiler, 1994). Generally, Bald Eagles are selecting areas for nesting with suitable forest structure, low human
disturbance, and high diversity or accessibility to prey species (Livingstone et al., 1990).
7.7.3.2 Feeding
Bald Eagles are opportunistic feeders having a diverse diet including mainly birds (mostly water birds), fish, and
carrion (Blood and Anweiler, 1994). Bald Eagles will congregate in large numbers to take advantage of locally
or seasonally abundant foods (Blood and Anweiler, 1994). Important food sources within the Taiga Plains ecoprovince are not known (Blood and Anweiler, 1994). Bald Eagles will likely hunt in the recent clearcuts,
wetlands, floodplains, and main rivers of the study areas.
7.7.3.3 Reproduction (Nesting)
Nesting may occur in a variety of stand types including coniferous, deciduous, or mixed stands (Blood and
Anweiler, 1994). Tree structure may be more important than tree species in determining Bald Eagle nest site
selection (Gerrard et al., 1975; Anthony et al., 1982). Large, usually old-growth, prominent trees, live or dead,
are needed to support their massive nests. In the B.C. Interior, most eagles nesting along rivers use cottonwoods
while those in upland areas around lakes tend to use Douglas-firs or aspens (Blood and Anweiler, 1994).
Important nest trees within the Taiga Plains eco-province have not yet been identified (Blood and Anweiler,
1994). M. Labine (personal communication) has seen a few birds nesting in big balsam poplars along the Liard
River, Northwest Territories.
7.7.3.4 Spatial Requirements for Nesting
Nests are rarely farther than a few hundred metres from shore in wilderness areas; although, in areas of human
disturbance, eagles will nest farther away from water; few nests are located more than 1.6 km from permanent
water (Blood and Anweiler, 1994; Anthony et al., 1982; Anthony and Isaacs, 1989).
7.7.3.5 Breeding Season
Within British Columbia, the Bald Eagle breeding season extends from mid-February to the end of August (Campbell
et al., 1990). In the southern Interior, egg-laying begins in late-March to early April and begins even later in the
northern Interior (Blood and Anweiler, 1994). One to three chicks hatch (average clutch size of 2) after an estimated
33 to 40 day incubation period and remain in the nest for an average of 10 to 11 weeks (Blood and Anweiler, 1994).
Young may fledge from late June to late August depending on location (Blood and Anweiler, 1994). For this model,
the breeding season is estimated as late March to August.
Table 28: Seasons of Use With Rated Life Requisites for Bald Eagle in the La Biche and Sandy Creek
Study Areas.
Month
January
February
Season*
W
W
Rated Life Requisites
NA
NA
Species–Habitat Model for Bald Eagle
March
April
May
June
July
August
September
October
November
December
* as defined in RIC (1997a).
W
W
G
G
G
G
G
W
W
W
RP-SH
RP-SH
RP-SH
RP-SH
RP-SH
RP-SH
NA
NA
NA
NA
The breeding season as defined in Table 28 is rated for security habitat (SH) values required for reproducing by
eggs. Ecosystem units will not be rated for the winter season as no winter use is expected in the study areas (NA
- not applicable).
7.7.4
Ratings
7.7.4.1 Rating Scheme – 4 Class, 1 Season
A 4-Class rating scheme of high (H), moderate (M), low (L), and nil (N) is employed due to the intermediate
level of knowledge on habitat use of Bald Eagles (Resources Inventory Committee, 1997a). The used ratings
scheme is defined in Table 29.
Table 29: Relative Quality Classes (from RIC, 1997a and RIC, 1997g).
Code
Quality relative to the best in B.C.
Suitability/Capability
H
M
L
N
Equivalent (75%-100% of best)
Slightly-moderately less (25%-75% of best)
Substantially less (5%-25% of best)
Much less (0%-5% of best)
High
Moderate
Low
Nil
One season (Breeding) will be rated for the Bald Eagle.
7.7.4.2 Provincial Benchmark
A Provincial Benchmark has not yet been established for the Bald Eagle. Blood and Anweiler (1994) gave habitats
within the Georgia Depression and Coast and Mountains Eco-provinces the highest ratings for habitat importance for both
the nesting and winter seasons within British Columbia. The Taiga Plains eco-province received a rank of low to no
importance for nesting habitat and a rank of no importance for winter habitat value (Blood and Anweiler, 1994).
Therefore, Class 1 habitat is not expected within the La Biche and Sandy Creek areas for Bald Eagles.
7.7.4.3 Assumptions
Habitat ratings for the Bald Eagle is presented in sections 7.7.4.4 and 7.7.4.5. Nesting habitat ratings for the breeding
season are Bald Eagle nesting habitat may not be strongly linked to ecosystem units, yet some site series may more
reliably provide suitable trees and stand characteristics favourable for nesting. Further study is needed to validate and
refine these ratings. The following assumptions have been made:
•
Only forested units are rated due to the rarity of Bald Eagles nesting on the ground or on cliffs. Non-forested and
non-and-sparsely vegetated units are assumed to have no nesting value and are therefore given nil value in the ratings
table.
•
Structural stages 1-5 have no nesting value as trees are not large enough to support the weight and structure of Bald
Eagle nests. Second-growth forest with a remnant old-growth component can have some value for nesting if suitable
old-growth nest trees remain.
•
Mature and old-growth structural stages (6-7) provide the most suitable nest trees. Stage 7 has higher value than
stage 6 due to presence of more prominent trees, more trees with broken tops, and a more open forest structure. Due
Species–Habitat Model for Bald Eagle
to their large wing span, Bald Eagles are more or less restricted to using the largest trees along the forest edge or
dominant trees protruding above the canopy (Blood and Anweiler, 1994). Super-dominant trees are important to
provide not only a suitable structure for nests but also prominent perching sites from which to watch for prey or
intruders (Livingstone et al., 1990).
•
Within the study areas, balsam poplar and aspen are the most important nest tree species. They generally have
sturdier branches, attain greater size, and have more structure than spruces. Within the study areas, balsam poplar
and aspen generally reach the greatest size and are found in largely riparian habitats. Therefore, units with high
proportions of these tree species in the later structural stages are assigned higher nesting values. White spruce also
reach a large size along the major rivers and may provide some moderate nesting values.
•
Large diameter trees are generally required for nesting. Trees with a DBH of less than 76 cm are rarely used (Blood
and Anweiler, 1994). More productive, moist units therefore receive higher ratings as these units are more likely to
produce suitable large trees.
•
Proximity of nest trees to water and prey base is very important. Therefore, floodplain ecosystems are rated higher
due to their location along major water bodies. Nesting values of all ecosystems will decrease as move away from
water body. Ecosystems will likely have minimal nesting values unless they are within approximately 1.6 km of a
water body or wetland.
•
Habitats further than 1.6 km from a waterbody or wetland had reduced ratings.
•
The impacts of human disturbance on Bald Eagles are still being argued and are of major concern in many areas
(Blood and Anweiler, 1994). They were not considered in this species model due to the wilderness nature of the La
Biche and Sandy Creek areas.
Species–Habitat Model for Bald Eagle
7.7.5
References
Anthony, R. G., R. L. Knight, G. T. Allen, B. R. McClelland, and J. I. Hodges. 1982. Habitat Use by
Nesting and Roosting Bald Eagles in the Pacific Northwest. Trans North American Wildlife Natural
Resource Conference 47: 332-342.
Anthony, R. G. and F. B. Isaacs. 1989. Characteristics of Bald Eagle Nest Sites in Oregon. Journal of
Wildlife Management 52(1): 148-159.
Blood, D. A. and G. G. Anweiler. 1994. Status of the Bald Eagle in British Columbia. Wildlife Working
Report No. WR-62. Prepared for the Wildlife Branch of the Ministry of Environment, Lands and
Parks, Victoria, B. C.
Campbell, R. W., N. K. Dawe, I. McTaggart-Cowan, J. M. Cooper, G. W. Kaiser, and M. C. E. McNall.
1990. The Birds of British Columbia. Vol.II. Nonpasserines. Diurnal Birds of Prey Through
Woodpeckers. Royal B.C. Museum, Victoria, B.C.
Gerrard, J. M., P. N. Gerrard, W. J. Mather, and D. W. A. Whitfield. 1975. Factors Influencing Nest Site
Selection of Bald Eagles in Northern Saskatchewan and Manitoba. Blue Jay 33: 169-176.
Labine, M. Personal communication. 1997, 1998. Renewable Resource Officer III. Renewable Resources.
Fort Liard, Northwest Territories.
Livingstone, S. A., C. S. Todd, W. B. Krohn, and R. B. Owen. 1990. Habitat Models for Nesting Bald
Eagles in Maine. Journal of Wildlife Management 54(4):644-653.
Peterson, A. 1986. Habitat Suitability Index Models: Bald Eagle (Breeding Season). U.S. Fish Wildlife
Service Biology Report 82(10.126).
Resources Inventory Committee. 1997a. Standards for Wildlife Habitat Capability/Suitability Ratings in
British Columbia. Review Draft, May 18, 1997. Wildlife Interpretations Subcommittee, Wildlife
Branch, Ministry of Environment Lands and Parks, Victoria, B.C.
Resources Inventory Committee. 1997g. Wildlife Habitat Assessment Form and Coding Rules. Final
draft, May 1, 1997. Wildlife Interpretations Subcommittee, Wildlife Branch, Ministry of
Environment Lands and Parks, Victoria, B.C.