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Bald Eagle
Haliaeetus leucocephalus
The recovery of the bald eagle is a North American
conservation success story. The eagle has emerged from an era
of reproductive failure mostly from exposure to DDT that soon
will occupy its former range. But even though the bald eagle
is one of the most identifiable birds in the world, there is
much we do not know about its ecology. Along the north Pacific
Coast, the eagle is one of the most striking and widespread
birds. And we think it plays an important ecological role as a
predator and scavenger. Its life cycle is closely intertwined
with salmon that form a mainstay of its diet. Our research at
PWLF is focussing on the behaviour and ecology of the eagle.
The bald eagle resides year round on the north Pacific Coast
from southern Alaska to northern California. It is a
particularly abundant species along the shore of British
Columbia and Alaska. Bald eagles breeds across North America
from Alaska to Labrador south to northern California, Wyoming,
Minnesota and central Ontario (Buehler 2000). Moreover,
isolated populations are scattered across most of the USA and
populations are now more or less continuous along the Gulf and
southeast States. In winter, the bald eagle migrates south of
much of Canada and interior of Alaska. The Pacific coast
population resides year-round in the region although they
disperse in search of food, especially spawning salmon. Two
subspecies are recognized but not without controversy (Buehler
2000); the northern subspecies H. l. alascanus is
larger than the southern subspecies H. l. leucocephalus.
The breeding season is characterized by strong territorial
behaviour toward other eagles near the nest. The large stick
nests are located along the shore and waterways. Most nests
are bulky piles of branches with a lining of moss or grass.
Many are built in crowns of tall trees and on snags and a few
are built on the ground on treeless islands. Typically two egg
clutches are laid in February or March in Pacific Coast eagle
nests. Eggs hatch 35-40 days later in March or April. Both
parents incubate the eggs. Young eagles are covered in a white
down when they hatch from eggs. Their parents provision the
eaglets with flesh from birds, fish, mammals and marine
invertebrates, the diet reflecting local abundance of prey.
Food is scavenged and prey are killed by eagles. Young eaglets
are capable of extended flight in July and by August most
eagles depart for salmon spawning streams. They spend the
winter scavenging fish from spawning streams, hunting ducks
along beaches, and searching for dead animals that come
ashore.
Northern Bald
Eagles are slightly larger on average than the southern
eagles, and females are about 25% larger than males throughout
their range. For example, Alaskan male eagles average about
4.2 kg and females about 5.4 kg. Adult bald eagles have a
wingspan that ranges from 1.6 to 2.4 metres; only the
California condor is a larger bird pf prey in North America.
Buehler (2000) summarized the reproductive data for the bald
eagle: clutch size: 1-3 eggs with a mean of 1.87; age when
breeding begins: 4 or more years old; average nesting success
was 1 young/nest attempt in Alaska; one brood raised per year;
life span up to 28 years in the wild and 36 in captivity;
adult annual survival at least 80%.
References
Buehler, D. A. 2000. Bald eagle Haliaeetus
leucocephalus. In the Birds of North America, No. 506.
The Birds of North America, Inc.
Philadelphia, PA. USA
Butler, R.W. 1997. The great blue heron. UBC
Press, Vancouver.
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. Volume 2.
Royal British Columbia Museum, Victoria.
Stalmaster, M. V.and J. A. Gessaman 1982.
Food consumption and energy requirements of
captive
bald eagles. Journal of Wildlife
management 46:646-654.
Stalmaster, M. V. and J. A. Gessaman. 1984.
Ecological energetics and foraging behaviour of
overwintering bald eagles.
Ecological Monographs 54:407-428.
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