BALD EAGLE (Haliaeetus leucocephalus)

Quick facts

• apex predators at the top of the bird food chain
• second in size only to California condors and about the same size as golden eagles
• wingspans range between 1.8 and 2.3 m (5.9 and 7.5 ft)
• body length varies between 70–102 centimeters (28–40 in)
• mass is usually between 3 and 6.3 kilograms (6.6 and 14 lb). Females are about 25 percent larger than males, averaging 5.8 kg (13 lb), and against the males’ average weight of 4.1 kg (9.0 lb)
• have lived up to 48 years in zoos, although their life span in the wild is likely an average of between 21 and 25 years
• they mate for life
• have incredibly acute eyesight – a 3 km (2 mile) visual radius
• the largest nest in the Pacific Northwest measured 5 m (15 ft) across by 7 m (21 ft) deep weighing approximately 1500 kgs (5000 lbs)
• recently removed from the endangered species list in the USA

Description

The adult bald eagle is a striking dark brownish-black bird with a white head and tail. Juvenile birds are a mottled brown with white blotches. They do not obtain the full distinctive plumage of the adults until they are four or five years old. Bills, legs, and feet are a deep yellow. Bald eagles are not bald. They got the name from an old English word “balde” meaning white (white-headed).

Size

Second in size only to California condors (Gymnogyps californianus) and about the same size as golden eagles (Aquila chrysaetos), bald eagles dwarf most other North American raptors. Their wingspans range from 6.5 feet to 7.5 feet, while body length varies from about 3 to 3.5 feet. Bald eagles weigh between 6 and 8 pounds. Females are larger (9–11 lb) than males and have a slightly longer wings.

Length: 27–30+ inches (the farther north they are found, the larger)

Wingspan: Male – about 6.5 feet
Female – about 7 feet

Weight: Male – 9 pounds
Female – 12.8 pounds

Life Span: Bald eagles have lived up to 48 years in zoos, although their life span in the wild is likely an average of between 21–25 years.

Distribution

Bald eagles can be found anywhere from Florida north to Newfoundland; Baja California north to Alaska, and in particular the Pacific Northwest area. Within this area, they are nearly always found near water, along rivers, lakes, or the seacoast and coastal marshes, reservoirs, and large lakes. They also pass over mountains and plains during migration. The northern and interior populations may migrate to open water in the winter months. Bald eagles breed in much of Alaska (where they are most common), Canada, the Pacific Northwest, along the North American East Coast, the Mississippi River, and the Gulf Coast, around the Great Lakes, and in other areas with sufficient water and wildlife. The birds winter along the coasts and across the US. Some even reach northwestern Mexico. Although unique to North America, the bald eagle’s closest relatives live in other parts of the world. These include the African fish-eagle (Haliaeetus vocifer) of sub-Saharan Africa and the white-tailed sea-eagle (Haliaeetus albicilla) of Eurasia.

Raptor features

Bald eagles are apex predators and are top of the bird food chain. They have several adaptations that fit them for that role.

They have excellent eyesight and the frontal setting of their eyes gives them excellent binocular vision as well as peripheral vision. Eagles, and other hawks, have a two fovea in each eye; that’s two centres of focus on the retina. The “regular” one is for focusing on the horizontal plane. The other is higher on the retina and concentrates focus toward the ground. The birds not only see and process two images, one from each eye, as do most animals with eyes on the sides of their heads, but they also process two more images from below themselves. That explains why a perched bird will sometimes turn its head completely upside down when looking skyward. The ground-image fovea is then looking up.

The eagle has a strong, hooked beak with which it tears food (fishing eagles have some of the most powerful beaks in the world). Its talons are extra-large (3–4 inches) and grooved underneath, and the foot pads are rough, almost spiculate (needle-like), for increased grasping ability – vital when the prey is a large slippery fish. Bald eagles prefer fish, which they often capture by swooping down and snatching from the water’s surface. Another successful technique is to wade in the shallow water catching fish with the bill. They also take birds, especially waterfowl, and occasionallly mammals. They utilize feeding, day roost and night roost perches, and there is a definite dominance hierarchy for their use. Two adults sitting in close proximity will almost certainly be a pair. When hunting a raft of water birds, they will often fly as a pendulum does over the group. This works well to intimidate the members of the raft, sending one or more into panic, making them an easy mark. In addition to eating other animals such as ducks, muskrats, and sometimes turtles, they eat carrion (dead/decaying meat) willingly, and are notorious for robbing osprey of their catches.

An eagle will wait on a favourite perch for an osprey to return to its nest with a fish in its talons for its own young, then harass the smaller raptor until it is forced to drop its prey for the eagle to retrieve. Bald eagles spend hours perched in trees overlooking water, their keen eyes alert for feeding opportunities. When not fishing, they sometimes steal food from ospreys, pursue injured or healthy waterfowl, or settle in for a meal of road-kill or fish chopped up in turbines at dams.

Reproduction

A pair of bald eagles will remain together for each nesting season as long as both are alive. They engage in various greeting and courtship flights, the most spectacular of which consists of locking talons in midair and descending for several hundred feet in a series of spiralling somersaults. Copulation occurs on branches or other secure perches and is preceded by tail pumping and wing flapping displays by the male. Eagles construct their nests near water in tall trees or on cliffs using large sticks. The nest is lined with twigs, grasses and other soft materials. Each year, a pair works together to build a large stick nest high in a tree or on a cliff. Sometimes a pair reuses the same nest for years. In Florida, bald eagles nest almost year round; they begin nesting from March to May in other areas (Pacific Northwest). Since these nests are used year after year, they may become very large.  A part of their courtship ritual is adding material to the nest every year (January–March) so they can increase in size over a period of time. The largest nest in the Pacific Northwest measured 15 feet (5 metres) across by 21 feet (7 metres) deep weighing approximately 5000 pounds (1500 kilograms). One could easily place a small  compact car in the nest and it would be in proportion (relative size). Bald eagles lay two, occasionally three, eggs that are incubated by both parents. The parents take turns for 34 to 36 days sitting upon and protecting the eggs. Often only one chick survives, but if food is plentiful they may rear two or occasionally three. Young birds fledge (fly out of the nest) after 12 weeks and remain with the parents for 3–6 months afterward.

Conservation

When the bald eagle was adopted as the United States national symbol in 1782, there were between 25,000 and 75,000 birds nesting in the lower 48 states. Illegal shooting, habitat destruction, lead poisoning, and the catastrophic effects of DDT contamination (a chemical pesticide) in their prey base reduced eagle numbers to a mere 417 pairs by 1963. Legal protection began with the Bald Eagle Protection Act of 1940 and continued with the Endangered Species Preservation Act of 1966 and the 1978 listing under the Endangered Species Act of 1973. The single-most important regulation affecting bald eagle recovery may have been the banning of DDT for most uses in the United States in 1972. In 1995 the U.S. Fish and Wildlife Service down-listed bald eagles from endangered to threatened in most of the United States. They were never listed in Alaska, and had already been listed as threatened in Michigan, Minnesota, Wisconsin, Oregon, and Washington. In the 17 years since they were declared endangered in most of the country, bald eagles have undergone a strong increase in numbers and an expansion in range. Private organizations, state, and federal agencies counted 4450 occupied nesting territories – a ten-fold increase from the 1963 low. Though the recovery has been spectacular, bald eagles remain threatened by illegal shooting and loss of habitat due to wetland drainage and human occupation of waterfront areas. Lead poisoning from shot ingested when feeding on carrion was a major problem prior to the phasing out of lead shot for waterfowl hunting in 1991. Large quantities of lead remain in the environment. Over the winter of 1994–95, 29 bald eagles died in Arkansas and nine died in Wisconsin from an unknown toxic agent. In the past 15 years, the National Wildlife Health Research Centre has diagnosed more than 100 cases of poisoning in bald eagles. Many of these cases are believed to be intentional poisonings through illegal use of pesticides and other restricted chemicals such as strychnine. DDT breakdown residues remain in the environment and continue to cause reproductive problems for eagles in many parts of the country. The DDT directly affects the bald eagle egg.  The toxins cause a thinning of the egg shells: as thin as paper.  So that when the parents take turns incubating the eggs, the eggs will actually crush and break under the weight of the adults. This therefore results in a very high mortality of unhatched eggs.