Bald Eagle

Appearance

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, but sexual dimorphism is evident in the species in that females are 25 percent larger than males. The beak, feet and irides are bright yellow. The legs are feather-free, and the toes are short and powerful with large 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 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 beak, straighter edged wings which are held flat and with a stiffer wing beat, and feathers which do not completely cover the legs.

The Bald Eagle has sometimes been considered the largest true raptor in North America, excluding the California Condor, which is a member of the New World vultures and may not be a true accipitrid. However, the Golden Eagle, in its American race, broadly overlaps in body weight and overall size with the Bald Eagle. Additionally, the Bald Eagle's close cousins, the longer-winged but shorter-tailed White-tailed Eagle and the overall larger Steller's Sea Eagle, may rarely vagrate to coastal Alaska from Asia. At any rate, the Bald Eagle is certainly a large bird, with a body length of 70–102 centimeters.

The wingspan is typically between 1.8 and 2.3 m and mass is usually between 3 and 6.3 kilograms.

Females are about 25 percent larger than males, averaging 5.8 kg, and against the males' average weight of 4.1 kg. The size of the bird varies by location and generally corresponds with Bergmann's rule, since the species increases in size further away from the Equator and the tropics. The smallest specimens are those from Florida, where mature males may weigh as little as 2.3 kg and have a wingspan of 1.68 m.

The largest eagles are from Alaska, where large females may weigh up to 7.5 kg and span 2.44 m across the wings.

Among standard linear measurements, the wing chord is 51.5–69 cm, the tail is 23–37 cm long, and the tarsus is 8 to 11 cm. The culmen reportedly ranges from 3 to 7.5 cm, while the measurement from the gape to the tip of the bill is 7–9 cm.

Distribution

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.

Behavior

The Bald Eagle is a powerful flier, and soars on thermal convection currents. It reaches speeds of 56–70 kilometers per hour when gliding and flapping, and about 48 kilometers per hour while carrying fish. Its dive speed is between 120–160 kilometers per hour, though it seldom dives vertically.

It is partially migratory, depending on location. If its territory has access to open water, it remains there year-round, but if the body of water freezes during the winter, making it impossible to obtain food, it migrates to the south or to the coast. The Bald Eagle selects migration routes which take advantage of thermals, updrafts, and food resources. During migration, it may ascend in a thermal and then glide down, or may ascend in updrafts created by the wind against a cliff or other terrain. Migration generally takes place during the daytime, when thermals are produced by the sun.

Habitat

The Bald Eagle prefers habitats near seacoasts, rivers, large lakes, oceans, 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, and lakes with an area greater than 10 square kilometers 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 no more than 60 percent, and no less than 20 percent, and be in close proximity to water.
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 endemic to North America. Occupying 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, remaining on their breeding territory all year. At minimum population, in the 1950s, it was largely restricted to Alaska, the Aleutian Islands, northern and eastern Canada, and Florida.

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.

Reproduction

Bald Eagles are sexually mature at four or five years of age. When they are old enough to breed, they often return to the area where they were born. It is thought that Bald Eagles mate for life. However, if one member of a pair dies or disappears, the other will choose a new mate. A pair which has repeatedly failed in breeding attempts may split and look for new mates. Bald Eagle courtship involves elaborate calls and flight displays. The flight includes swoops, chases, and cartwheels, in which they fly high, lock talons, and free fall, separating just before hitting the ground.

The nest is the largest of any bird in North America; it is used repeatedly over many years and with new material added each year may eventually be as large as 4 meters deep, 2.5 meters across and weigh 1 metric ton; one nest in Florida was found to be 6.1 meters deep, 2.9 meters across, and to weigh 3 short tons.

This nest is on record as the largest tree nest ever known for any animal. The nest is built out of branches, usually in large trees near water. When breeding where there are no trees, the Bald Eagle will nest on the ground. Eagles produce between one and three eggs per year, but it is rare for all three chicks to successfully fly. Both the male and female take turns incubating the eggs. The other parent will hunt for food or look for nesting material. The eggs average about 73 millimeters long and have a breadth of 55 millimeters.

The average lifespan of Bald Eagles in the wild is around 20 years, with the oldest confirmed one having been 28 years of age. In captivity, they often live somewhat longer. In one instance, a captive individual in New York lived for nearly 50 years. As with size, the average lifespan of an eagle population appears to be influenced by its location.

Food

The Bald Eagle's diet is opportunistic and varied, but in some areas they feed mainly on fish. In the Pacific Northwest, spawning trout and salmon provide most of the Bald Eagles' diet.