This article provides a short summary of the bald eagle's (Haliaeetus leucocephals) pertinent biological information. Included in this summary are the facts about the eagle's breeding behavior, habitat, diet, and geographic range in the United States.
The bald eagle, Haliaeetus leucocephalus, is well known as our Nation's symbol. Its large and powerful appearance is distinguished by its white head and tail contrasting against its dark brown body. Though once endangered, the bald eagle population in the lower 48 States has increased considerably in recent years. Regional bald eagle populations in the northwest, Great Lakes, Chesapeake Bay, and Florida have increased 5-fold in the past 20 years. Bald eagles are now repopulating areas throughout much of the species' historic range that were unoccupied only a few years ago.
The bald eagle ranges throughout much of North America, nesting on both coasts from Florida to Baja California, Mexico in the south, and from Labrador to the western Aleutian Islands, Alaska in the north. The earliest known record of a bald eagle comes from a cave in Colorado. Deposits from that cave are dated at 670,000 to 780,000 years old (Dr. Steve Emslie, University of North Carolina, pers. comm. 1998). An estimated quarter to a half million bald eagles lived on the North American continent before the first Europeans arrived.
Haliaeetus leucocephalus (literally, sea eagle with a white head) is the only species of sea eagle native to North America. It was first described in 1766 as Falco leucocephalus by Linnaeus. This South Carolina specimen was later renamed as the southern bald eagle, subspecies Haliaeetus leucocephalus leucocephalus (Linnaeus) when Townsend identified the northern bald eagle as Haliaeetus leucocephalus alascanus in 1897 (Peters 1979). By the time the bald eagle was listed throughout the lower 48 States under the Endangered Species Act in 1978, the subspecies were no longer recognized by ornithologists (American Ornithologists Union 1983).
The bald eagle is a bird of aquatic ecosystems. It frequents estuaries, large lakes, reservoirs, major rivers, and some seacoast habitats. Fish is the major component of its diet, but waterfowl, seagulls, and carrion are also eaten. The species may also use prairies if adequate food is available. Bald eagle habitats encompass both public and private lands.
Bald eagles usually nest in trees near water, but are known to nest on cliffs and (rarely) on the ground. Nest sites are usually in large trees along shorelines in relatively remote areas that are free of disturbance. The trees must be sturdy and open to support a nest that is often 5 feet wide and 3 feet deep. Adults tend to use the same breeding areas year after year, and often the same nest, though a breeding area may include one or more alternate nests. A 35-year old nest at Vermilion, Ohio, measured 8\1/2\ feet across at the top and 12 feet deep before it blew down in 1925 (Herrick 1932). In winter, bald eagles often congregate at specific wintering sites that are generally close to open water and offer good perch trees and night roosts.
Bald eagles are long-lived. The longest living bald eagle known in the wild was reported near Haines, Alaska as 28 years old (Schempf 1997). Bald eagles from Arizona are known to have exceeded 12 years of age (Hunt et al. 1992). In captivity, bald eagles may live 40 or more years.
It is presumed that once they mate, the bond is long-term, though documentation is limited. Variations in pair bonding are known to occur. If one mate dies or disappears, the other will accept a new partner. The female bald eagle usually weighs 10 to 14 pounds in the northern sections of the continent and is larger than the male, which weighs 8 to 10 pounds. The wings span 6 to 7 feet. The northern birds are larger and heavier than southern birds, with the largest birds in Alaska and Canada, and the smallest in Arizona or Florida.
Bald eagle pairs begin courtship about a month before egg-laying. In the south, courtship occurs as early as September, and in the north, as late as May. The nesting season lasts about 6 months. Incubation lasts approximately 35 days and fledging takes place at 11 to 12 weeks of age. Parental care may extend 4 to 11 weeks after fledging (Wood, Collopy, and Sekerak 1998). The fledgling bald eagle is generally dark brown except the underwing linings which are primarily white. Between fledging and adulthood, the bald eagle's appearance changes with feather replacement each summer. Young dark bald eagles may be confused with the golden eagle, Aquila chrysaetos. The bald eagle's distinctive white head and tail are not apparent until the bird fully matures, at 4 to 5 years of age.
As they leave their breeding areas, some bald eagles stay in the general vicinity while most migrate for several months and hundreds of miles to their wintering grounds. Young eagles may wander randomly for years before returning to nest in natal areas.
Northern bald eagles winter in areas such as the Upper Mississippi River, Great Lakes shorelines and river mouths in the Great Lakes area. For mid-continent bald eagles, wintering grounds may be the southern States, and for southern bald eagles, whose nesting occurs during the winter months, the non-breeding season foraging areas may be Chesapeake Bay or Yellowstone National Park during the summer. Eagles seek wintering (non-nesting) areas offering an abundant and readily available food supply with suitable night roosts. Night roosts typically offer isolation and thermal protection from winds. Carrion and easily scavenged prey provide important sources of winter food in terrestrial habitats far from open water.
The first major decline in the bald eagle population probably began in the mid to late 1800s. Widespread shooting for feathers and trophies led to extirpation of eagles in some areas. Shooting also reduced part of the bald eagle's prey base. Big game animals like bison, which were seasonally important to eagles as carrion, were decimated. Waterfowl, shorebirds and small mammals were also reduced in numbers. Carrion treated with strychnine, thallium sulfate and other poisons were used as bait to kill livestock predators and ultimately killed many eagles as well. These were the major factors, in addition to loss of nesting habitat from forest clearing and development, that contributed to a reduction in bald eagle numbers through the 1940s.