The bald eagle (Haliaeetus leucocephalus) is a majestic bird of prey endemic to North America. As a sea eagle, it is closely related to the white-tailed eagle (Haliaeetus albicilla) of Eurasia, sharing a similar ecological niche. Its range spans from Canada and Alaska to the contiguous United States and northern Mexico. Revered as the national bird and symbol of the United States, the bald eagle faced near extinction in the late 20th century but has since made a remarkable recovery.
Physical Characteristics and Identification
Bald eagles are not actually bald; their name comes from an old English word meaning "white-headed." Adult bald eagles possess a distinctive plumage of evenly dark brown feathers with a contrasting white head and tail. Their beak, feet, and irises are bright yellow, and their legs are feather-free. The beak is large, hooked, and features a yellow cere. While males and females share identical plumage coloration, females are notably larger, exceeding males in size by approximately 25%.
The size of bald eagles varies geographically, adhering to Bergmann's rule, with larger birds found further from the equator. Alaskan eagles are among the largest, with females weighing over 7 kg (15 lb) and boasting a wingspan of up to 2.44 m (8 ft 0 in).
Immature bald eagles exhibit a dark brown plumage overlaid with messy white streaking until they reach sexual maturity, typically around their fifth year. They can be distinguished from golden eagles (Aquila chrysaetos) by their larger, more protruding head, larger beak, straighter-edged wings held flat, stiffer wing beat, and feathers that do not completely cover the legs.
The bald eagle's call consists of weak, staccato, chirping whistles, "kleek kik ik ik ik," reminiscent of a gull's call.
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Habitat and Distribution
The bald eagle's natural range encompasses most of North America, including Canada, the continental United States, and northern Mexico. They occupy diverse habitats, from the bayous of Louisiana to the Sonoran Desert and the deciduous forests of Quebec and New England. Northern populations are migratory, while southern birds are typically resident, remaining on their breeding territory year-round.
Bald eagles thrive in wetland habitats near seacoasts, rivers, large lakes, marshes, and other extensive bodies of open water with plentiful fish. They favor old-growth and mature stands of coniferous or hardwood trees for perching, roosting, and nesting. The tree's height, composition, and location are more critical than the specific tree species. Nesting trees must have good visibility, be over 20 m (66 ft) tall, possess an open structure, and be located close to prey. Nests may be located as low as 6 m (20 ft) above the ground in standing water, such as mangrove swamps, or between 16 to 38 m (52 to 125 ft) in more typical trees on dry ground. Trees or forests used for nesting should have a canopy cover of 20% to 60% and be in close proximity to water, with most nests found within 200 m (660 ft) of open water.
During winter, bald eagles are less sensitive to habitat and disturbance and congregate at spots with ample perches and waters with abundant prey, particularly partially unfrozen waters in colder climates. Non-breeding or wintering eagles may also spend time in upland, terrestrial habitats, especially in areas with minimal human disturbance.
Behavior and Flight
The bald eagle is a powerful flier, soaring on thermal convection currents. It can reach speeds of 56-70 km/h (35-43 mph) when gliding and flapping, and about 48 km/h (30 mph) while carrying fish. Its dive speed ranges from 120-160 km/h (75-99 mph), although it seldom dives vertically. Despite being less morphologically adapted for faster flight than golden eagles, the bald eagle is surprisingly maneuverable in the air.
Bald eagles are partially migratory, depending on their location. They remain in their territory year-round if open water is accessible, but migrate south or to the coast if the water freezes, preventing them from obtaining food. They select migration routes that utilize thermals, updrafts, and food resources, ascending in thermals and gliding down or ascending in updrafts created by wind against cliffs or terrain.
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Diet and Feeding Habits
The bald eagle is an opportunistic carnivore with a diverse diet. While fish is a primary food source, they consume a wide variety of prey. In 20 food habit studies, fish constituted 56% of the diet of nesting eagles, birds 28%, mammals 14%, and other prey 2%. Bald eagles are known to consume over 400 different species.
To hunt fish, bald eagles swoop down over the water and snatch their prey with their talons. They hold the fish in one claw and tear the flesh with the other. Spicules on their toes enable them to grasp fish securely, an adaptation also found in ospreys. Bird prey may be attacked in flight, with eagles capable of killing prey up to the size of Canada geese mid-air. The bald eagle's grip is estimated to be ten times stronger than that of a human.
Bald eagles can fly with fish equal to their own weight, but if the fish is too heavy, they may be dragged into the water. While they can swim, some eagles drown or succumb to hypothermia. It is often claimed that bald eagles cannot take flight carrying prey more than half their own weight unless aided by favorable wind conditions. When attacking large prey, such as mature salmon or geese, eagles have been observed dragging the prey in a labored, low flight over the water to a bank, where they finish off and dismember it.
When food is plentiful, bald eagles can gorge themselves, storing up to 1 kg (2.2 lb) of food in a pouch in the throat called a crop, allowing them to fast for several days if food becomes scarce. They may also hunt cooperatively when confronting relatively large prey, such as jackrabbits or herons, with one bird distracting the prey while the other ambushes it. When hunting waterfowl, bald eagles repeatedly fly at a target, causing it to dive repeatedly, hoping to exhaust it for capture.
Bald eagles also obtain food as carrion or through kleptoparasitism, stealing prey from other predators. Due to their dietary habits, they are sometimes viewed negatively by humans. Adult eagles are generally more adept at hunting live prey than immature eagles, which often rely on scavenging. They are not selective about the condition or origin of a carcass, whether provided by humans, other animals, auto accidents, or natural causes, but avoid carrion where human disturbances are frequent.
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In the Pacific Northwest, spawning trout and salmon form the bulk of the bald eagles' diet from late summer through fall. While they occasionally catch live salmon, they typically scavenge spawned salmon carcasses. Southeast Alaskan eagles primarily prey on pink salmon (Oncorhynchus gorbuscha), coho salmon (O. kisutch), and, more locally, sockeye salmon (O. nerka), with Chinook salmon (O. tschawytscha) also consumed. Benthic fishes, such as catfish, are usually consumed after they die and float to the surface, though they may be vulnerable to predation while temporarily swimming in the open. Bald eagles also exploit water turbines, which produce battered, stunned, or dead fish that are easily consumed. They may habitually follow predators, such as brown bears (Ursus arctos), gray wolves (Canis lupus), and red foxes (Vulpes vulpes), to scavenge the kills secondarily. After North Pacific salmon die off after spawning, local bald eagles consume salmon carcasses almost exclusively.
Waterbirds are the next most significant prey base for bald eagles, with their contribution to the diet varying depending on the availability of fish near the water's surface. Bald eagles can prey on seabirds of all ages, from eggs to mature adults, and can effectively cull large portions of a colony due to easy accessibility and lack of formidable nest defense. Along some portions of the North Pacific coastline, bald eagles that historically preyed mainly on kelp-dwelling fish and supplementally sea otter (Enhydra lutris) pups are now preying mainly on seabird colonies due to declines in both fish (possibly due to overfishing) and otters (cause unknown), raising concerns for seabird conservation.
ND-LEEF Study
A study at the University of Notre Dame Linked Experimental Ecosystem Facility (ND-LEEF) provided valuable insights into the diet of bald eagles. Researchers at ND-LEEF sought to have as light of a footprint as possible, and therefore mimic nature. The quiet, natural setting and proximity to the St. Joseph River are key factors that likely attracted the bald eagles to take over an existing red-tailed hawk nest at ND-LEEF in 2015. Given the arrival of the bald eagles, ND-LEEF morphed its site map to reposition where two additional watersheds are being placed in order to minimize impact on the nesting pair. The location of the nest at ND-LEEF is unusual in that there is only one nearby body of water, the St. Joseph River. The adult eagles tear food and feed the eaglets until they have acquired the ability to self-feed. Researchers programmed an in-nest cam to take five separate still images, capturing all areas of the nest every 20 minutes. The team combed through over 26,000 images to identify prey items brought into the nest. The study identified 135 prey items and at least 26 prey “taxa” over 75 days. Fish comprised the majority of recorded prey items (73 percent), with redhorse suckers being the most common.
Conservation Status and Threats
Bald eagles (Haliaeetus leucocephalus) are currently classified as Least Concern by the International Union for Conservation of Nature (IUCN), with a large range, large population, and an increasing population trend. However, during the mid-1900s, they were close to extinction due to habitat destruction, habitat degradation, illegal shooting, and contamination of their food source from the insecticide DDT. Legal protection through the Bald Eagle Protection Act of 1940, the Endangered Species Preservation Act of 1966, and the Endangered Species Act of 1973, along with the banning of DDT in 1972, played a crucial role in their recovery.
Despite their successful recovery, bald eagles continue to face threats, including illegal shooting, loss of habitat due to wetland drainage and human occupation of waterfront areas, and lead poisoning. Lead poisoning from shot ingested when feeding on carrion was a major problem before the phasing out of lead shot for waterfowl hunting in 1991, but large quantities of lead remain in the environment. Intentional poisonings through the illegal use of pesticides and other restricted chemicals, such as strychnine, also pose a risk. DDT breakdown residues persist in the environment and continue to cause reproductive problems for eagles in some areas. Furthermore, competition with humans for waterfront property remains a challenge.
The Peregrine Fund's Role
The Peregrine Fund has supported Bald Eagle conservation for decades.