Bald Eagle

Haliaeetus leucocephalus

Description: The bald eagle is a large, powerful bird of prey; its wingspan may measure up to eight feet, and its length up to three and a half feet. Females are larger than males. The eagle has a hooked bill for tearing flesh, strong tarsi, and short, muscular toes, with long, sharp talons; the talon on the rear toe is designed to pierce the flesh and vital organs of its prey. The bald eagle's species name means 'white head'; it has been suggested that the bird's common name was given to it at a time when the word 'bald' meant white. In addition to its white head, the eagle has white tail feathers. Adults have bright yellow bills and feet. Birds under five years of age are mostly brown, with a varying amount of white spots and patches on their undersides. Their bills and feet are darkly colored. Young birds often appear larger than adults, perhaps due to the thickness of their juvenile plumage.

The bald eagle's call is a quick series of short whistles or chirps, said by some to seem out of character for such an impressive bird. Various other chirps and clucks are heard less often.

Habitat: Bald eagles are generally found near fresh or salt water; the majority live along the seacoast, however lakes and rivers with abundant fish also attract eagles, either along their migratory routes, as seasonal destinations, or as their year-round homes. Apart from proximity to water, bald eagle habitat is highly variable; they may be found in forest, desert, grassland, and tundra biomes.

Distribution: Bald eagles were once abundant near water throughout most of North America, from the subtropics to the Arctic. Today, their largest population is found along the coast of Alaska, numbering around 35,000. The highest number of breeding pairs outside of Alaska and Canada is found in Florida, where over 2,000 eagles reside. Washington hosts the second highest population in the contiguous states. At least one pair of bald eagles now breeds in every US state but New Hampshire and Hawaii; the birds do winter in New Hampshire, however. Hawaii has never been home to eagles.

Behavior: Bald eagles feed mostly on fish, which they may catch themselves, find washed up along beaches and lakeshores, or steal from other fishing birds like the osprey (Pandion haliaetus). To catch a fish, an eagle flies over the water and uses its superior eyesight to locate it; the bird then swoops over the surface of the water, grasping the fish with its sharp talons. This is made more difficult due to counter-shading, a coloration possessed by most fish in which they are darker on the top and lighter on the bottom, blending in with the dark depths of the water below and the bright sky above. Once captured, most fish are easily lifted from the water and carried away, but an especially large fish may be too heavy for the eagle to carry, and it must kill or immobilize the fish in the water and swim to shore with it.

Once on dry land, the eagle holds the fish down with its talons and tears into it with its hooked bill; a large fish may provide a meal for several eagles, as well as gulls, crows, and other scavengers. Eagles also feed on carrion and may kill the occasional duck or other waterfowl. The author once observed a bald eagle in Park County, Colorado, walking behind an emaciated cow, presumably waiting for its impending demise; the local lakes had frozen over for the winter, leaving little in the way of fishing opportunities.

In most of North America, bald eagles breed in the spring, but in Florida, breeding occurs in the winter. Breeding may occur every year for eagles four years old or more; pairs typically mate for life, but they may skip a year if food availability or weather conditions are unsuitable.

In most areas, a nest site is selected in the top of a sturdy tree, but where trees are unavailable, eagles may nest on rocks, cliffs, or on the ground. Nests are constructed of sticks and branches, and lined with softer material like leaves and feathers; tree nests are often funnel-shaped, while ground nests are typically disc-shaped. Bald eagles return to the same nests every year, adding more materials to them. As the birds may live thirty years, their nests can become enormous, weighing as much as two tons. Nest trees sometimes blow down in winter storms, or simply break under the weight of their nests; the mating pair usually builds a new nest nearby, rather than searching out a new territory.

Females lay one to three eggs, usually a few days apart; both parents take turns incubating the eggs, which takes five or six weeks; the chick that hatches first has an advantage over its siblings, which it may kill or push out of the nest to eliminate its competition for food. Female chicks are especially successful in this sibling rivalry, thanks to their larger size. The adult eagles bring fish and other food to the nest, which they tear into smaller chunks and offer to their chicks. Food rejected by the young eagles is eaten by the adults.

Eaglets are ready to take their first flight at around nine or ten weeks of age, having dedicated much effort to the development of their flight muscles by flapping their wings in the nest. They may begin by facing into the wind, wings spread, allowing it to lift them from the nest, or by flapping their wings and flying from the nest to another branch on the nest tree. Gradually, they master the use of air currents and thermals to elevate and propel themselves with less wing flapping.

While some eagles remain in their breeding locations all year, many migrate; migration routes may vary from year to year, corresponding with air currents and waterways, allowing the birds to travel with a minimum expenditure of energy and a food supply en route. Some eagles also migrate locally; for example, bald eagles on the west coast of Washington's Olympic Peninsula tend to stay near the Pacific coast in the summer, and spend the winter months about twenty miles away, along the Strait of Juan de Fuca on the Peninsula's north coast.

Human Impact: Bald eagle populations were severely diminished in the first three quarters of the twentieth century. The birds were hunted for sport and because they were viewed as a threat to livestock and commercial fisheries. The state of Alaska offered a bounty for dead eagles as recently as the 1950s. While lambs have been preyed upon by golden eagles (Aquila chrysaetos), bald eagles are not known for killing domestic animals, and their impact on fish populations is negligible.

A more efficient killer came in the form of commercial pesticides like DDT (dichloro-diphenyl-trichloroethane), which breaks down very slowly and tends to accumulate in the fatty tissues of living organisms. In the early 1970s, a study of human fat tissue in the general US population revealed that DDT was present in every sample taken. Since then, the frequency of DDT occurrence in human tissue has dropped, but it can still be found in some human samples in the United States, and more in areas of the tropics where it continues to be used to control mosquitoes that carry malaria. When DDT washes into watersheds, it finds its way into rivers and lakes, and ultimately into the ocean. It works its way through the food chain, reaching higher concentrations in animals occupying each successive link. Predators and scavengers tend to exhibit the highest pesticide concentrations in their tissues, as they consume many contaminated animals. DDT is chemically similar to estrogen, causing a hormonal reaction in eagles and some other birds that causes a lower absorption of calcium, in turn resulting in the thinning of their eggshells. In the 1950s and 1960s, eagle numbers declined rapidly as they laid thin-shelled eggs, unable to support their growing embryos.

When the use of DDT in the United States was banned in 1972, the viability of bald eagle eggs began to gradually improve; the present levels of DDT in the eagles' ecosystem do not seem to have a significant impact on their health, and the species continues to recover. Alaska remains the birds' stronghold; in some areas of the southeastern panhandle, bald eagles may outnumber crows.

Illegal hunting continues to threaten bald eagles, but it is increasingly rare; wildlife rehabilitation facilities treat and release birds injured by hunters or in collisions with airplanes and automobiles, or those that have been raised illegally by humans and lack the skills to survive in the wild. Perhaps the best-known bald eagle rehabilitation facility is the Alaska Raptor Center in Sitka, where as many as 200 eagles are treated each year. Birds undergo physical therapy and flight training prior to being released.

After being decimated throughout most of North America, bald eagles are gradually regaining lost ground; they may soon become nearly as common a site in the interior of the continent as they are in the Pacific Northwest and coastal Florida. As their numbers stabilize, it is crucial that they continue to receive protection from hunting and exploitation, lest the mistakes of the past return to threaten bald eagles yet again.

The bald eagle was once abundant throughout North America, but hunting, habitat loss, and pesticides have taken a heavy toll.

Eagle populations are recovering, and numbers are on the rise around inland waterways; © Ted Steinke

Coming in for a landing; © Ted Steinke

A young eagle displaying its juvenile plumage; © Kent Keller

Juvenile bald eagles may be confused with golden eagles (Aquila chrysaetos), but their beaks are typically more robust; Glenn and Martha Vargas; © California Academy of Sciences