The Bald Eagle (Haliaeetus leucocephalus
) is a bird of prey found in North America that is most recognizable as the national bird and symbol of the United States. This sea eagle has two known sub-species and forms a species pair with the White-tailed Eagle. Its range includes most of Canada and Alaska, all of the contiguous United States, and northern Mexico. It is found near large bodies of open water with an abundant food supply and old-growth trees for nesting.
The Bald Eagle is a large bird, with a body length of 71-96 cm (28-38 in), a wingspan of 168–244 cm (66–88 in), and a weight of 3–6.3 kg (6.6–14 lb); females are about 25 percent larger than males. The adult Bald Eagle has a brown body with a white head and tail, and bright yellow irises, taloned feet, and a hooked beak; juveniles are completely brown except for the yellow feet. Males and females are identical in plumage coloration. Its diet consists mainly of fish, but it is an opportunistic feeder. It hunts fish by swooping down and snatching the fish out of the water with its talons. It is sexually mature at four years or five years of age. The Bald Eagle builds the largest nest of any North American bird, up to 4 m (13 ft) deep, 2.5 m (8 ft) wide, and one tonne (1.1 tons) in weight.
The species was on the brink of extirpation in the continental United States (while flourishing in much of Alaska and Canada) late in the 20th century, but now has a stable population and has been officially removed from the U.S. federal government's list of endangered species. The Bald Eagle was officially reclassified from "Endangered" to "Threatened" on July 12, 1995 by the United States Fish and Wildlife Service. On July 6, 1999, a proposal was initiated "To Remove the Bald Eagle in the Lower 48 States From the List of Endangered and Threatened Wildlife." It was delisted on June 28, 2007.
The plumage of an adult Bald Eagle is evenly brown with a white head and tail. The tail is moderately long and slightly wedge-shaped. Males and females are identical in plumage coloration, however females display reverse sexual dimorphism and are 25 percent larger than males. The beak, feet, and irises are bright yellow. The legs are unfeathered, and the toes are short and powerful with long talons. The highly developed talon of the hind toe is used to pierce the vital areas of prey while it is held immobile by the front toes. The beak is large and hooked, with a yellow cere.
The plumage of the immature is brown, speckled with white until the fifth (rarely fourth, very rarely third) year, when it reaches sexual maturity. Immature Bald Eagles are distinguishable from the Golden Eagle in that the former has a more protruding head with a larger bill, straighter edged wings which are held flat (not slightly raised) and with a stiffer wing beat, and feathers which do not completely cover the legs. Also, the immature Bald Eagle has more light feathers in the upper arm area, especially around the very top of the arm.
Body length ranges from 71 to 96 cm (28–38 in). Adult females have a wingspan of up to 2.44 m (88 in), while adult males may be as small as 1.68 m (66 in). Adult females weigh approximately 5.8 kg (12.8 lb), males weigh 4.1 kg (9 lb). The size of the bird varies by location; the smallest specimens are those from Florida, where an adult male may barely exceed 2.3 kg (5 lb) and a wingspan of 1.8 m (6 ft). The largest are Alaskan birds, where large females may exceed 7.5 kg (16.5 lb) and have a wingspan of over 2.4 m (8 ft).
Habitat and Range
The Bald Eagle prefers habitats near seacoasts, rivers, large lakes, and other large bodies of open water with an abundance of fish. Studies have shown a preference for bodies of water with a circumference greater than 11 km (7 miles), and lakes with an area greater than 10 km² (3.8 square miles) are optimal for breeding bald eagles.
The Bald Eagle requires old-growth and mature stands of coniferous or hardwood trees for perching, roosting, and nesting. Selected trees must have good visibility, an open structure, and proximity to prey, but the height or species of tree is not as important as an abundance of comparatively large trees surrounding the body of water. Forests used for nesting should have a canopy cover of less than 60 percent, and as low as 20 percent, and be in close proximity to water.
The Bald Eagle is extremely sensitive to human activity, and occurs most commonly in areas free of human disturbance. It chooses sites more than 1.2 km (0.75 miles) from low-density human disturbance and more than 1.8 km (1.2 miles) from medium- to high-density human disturbance.
The Bald Eagle's natural range covers most of North America, including most of Canada, all of the continental United States, and northern Mexico. It is the only Sea Eagle native to only North America. The bird itself is able to live in most of North America's varied habitats from the bayous of Louisiana to the Sonoran Desert and the eastern deciduous forests of Quebec and New England. Northern birds are migratory, while southern birds are resident, often remaining on their breeding territory all year. The Bald Eagle previously bred throughout much of its range but at its lowest population was largely restricted to Alaska, the Aleutian Islands, northern and eastern Canada, and Florida.
It has occurred as a vagrant twice in Ireland; a juvenile was shot illegally in Fermanagh on January 11, 1973 (misidentified at first as a White-tailed Eagle), and an exhausted juvenile was captured in Kerry on November 15, 1987. Bald Eagles will also congregate in certain locations in winter. From November until February, one to two thousand birds winter in Squamish, British Columbia, about halfway between Vancouver and Whistler. The birds primarily gather along the Squamish and Cheakamus Rivers, attracted by the salmon spawning in the area.
Population decline and recovery
Once a common sight in much of the continent, the Bald Eagle was severely affected in the mid-20th century by a variety of factors, among them thinning of egg shells, attributed to the use of the pesticide DDT. Bald Eagles, like many birds of prey, were especially affected by DDT due to biomagnification. DDT itself was not lethal to the adult bird, but it interfered with the bird's calcium metabolism, making the bird either sterile or unable to lay healthy eggs. Female eagles laid eggs that were too brittle to withstand the weight of a brooding adult, making it nearly impossible to produce young. By the 1950s there were only 412 nesting pairs in the 48 contiguous states of the US. Other factors in Bald Eagle population reductions were a widespread loss of suitable habitat, and illegal shooting, which was described as "the leading cause of direct mortality in both adult and immature bald eagles," according to a 1978 report in the Endangered Species Technical Bulletin. In 1984, the National Wildlife Federation listed hunting, power line electrocution, and collisions in flight as the leading causes of eagle deaths. Bald Eagle populations have also been negatively affected by oil, lead, and mercury pollution, and by human and predator intrusion.
The species was first protected in the U.S. and Canada by the 1918 Migratory Bird Treaty, later extended to all of North America. The 1940 Bald Eagle Protection Act in the U.S., which protected the Bald Eagle and the Golden Eagle, prohibited commercial trapping and killing of the birds. The Bald Eagle was declared an endangered species in the U.S. in 1967, and amendments to the 1940 act between 1962 and 1972 further restricted commercial uses and increased penalties for violators. Also in 1972, DDT was banned in the United States. DDT was completely banned in Canada in 1989, though its use had been highly restricted since the late 1970s.
With regulations in place and DDT banned, the eagle population rebounded. The Bald Eagle can be found in growing concentrations throughout the United States and Canada, particularly near large bodies of water. In the early 1980s, the estimated total population was 100,000 birds, with 110,000–115,000 by 1992; the U.S. state with the largest resident population is Alaska, with about 40,000–50,000 birds, with the next highest population being the Canadian province of British Columbia with 20,000–30,000 birds in 1992.
It was officially removed from the U.S. federal government's list of endangered species on July 12, 1995 by the U.S. Fish & Wildlife Service, when it was reclassified from "Endangered" to "Threatened." On July 6, 1999, a proposal was initiated "To Remove the Bald Eagle in the Lower 48 States From the List of Endangered and Threatened Wildlife." It was delisted on June 28, 2007. It has also been assigned a risk level of Least Concern category on the IUCN Red List.