Bald eagle

Bald eagles are found throughout and only in North America, most often around water where they catch and scavenge fish. During nesting season, fish are a major part of their diet. They supplement their diet with waterfowl, small mammals, turtles and carrion. Bald eagles are notorious for their ability to pirate fish from other piscivorous (fish-eating) birds like ospreys (Pandion haliaetus) and common mergansers (Mergus merganser), chasing the other bird until it drops the fish. Bald eagles are very efficient foragers that are conservative in their energy expenditure. Although bald eagles will hunt in flight and by wading in water for prey, they generally perch on a tree or snag and wait for their prey to appear. They are opportunistic foragers and take whatever is available, but generally consume fish. Bald eagles also will scavenge dead fish, waterfowl and mammal carcasses, including large herbivores such as deer and livestock. Mammals, birds, and carcasses become a more important part of their diet in winter when fish can be more difficult to find and reach because of water levels or ice-cover.

Eagles don't reach adulthood and begin nesting until age four or five and can live a long life for a bird, up to about 30 years. At least two bald eagles in Pennsylvania, a nesting pair, are at least 25 years old. Bald eagles are known for their spectacular courtship, including acrobatic flight displays. The "cartwheel display" is perhaps the best known. In this courtship act, the pair flies to great altitude, lock their talons in flight, and tumble in cartwheels back toward the earth, breaking off their hold at last moment before colliding with the ground. These flight displays often occur in winter, giving support to the idea that many pairs remain bonded through the year. In Pennsylvania, some pairs seem to occupy the same areas all year long, while others leave their nesting area when ice forms on the water and decreases their ability to find fish.

Bald eagles build among the largest nests of all birds, a massive and often conspicuous structure that is reused and refurbished each year. Nests are almost always near water, including islands, riparian woods, hillsides and swamps. The only nests on cliffs in Pennsylvania are actually supported by trees that grow on the side of the cliff. Eagle nests, called an eyrie, are a huge pile of interconnecting sticks, rubbish, and cornstalks that support a cup of softer materials such as small twigs, grasses, mosses, weeds, sod and feathers. Sprigs of greenery, especially conifer branches, are often found in nests and can be delivered to the nest during the incubation or nestling periods. Typically, these stick nests are 5 to 6 feet (1.5 – 1.8 meters) in diameter, 2.5 to 4 feet (0.7 – 1.2 meters) deep and conform to the shape of the tree where they are built, the shape ranging from cylindrical to conical to flat. Some nests famously have reached huge dimensions including one in Ohio that was 9 feet in diameter and 12 feet high, weighting about two metric tons, that was used for 34 years. Adults will continue using and seasonally adding material to the same nest for years. With damage by storms and rambunctious eaglets, nests often need extensive repairs each year. Eagles will lay eggs in February through April, sometimes sitting on eggs when there is ice and snow on the ground. This is one of the reasons why it is not good to approach nests too closely. If you flush an adult off of a nest in the incubation period you can expose the eggs to cold air, causing nest failure.

Bald eagles normally produce one to three young per year. One pair in Northampton County produced a record four young in 2009, an extremely rare event. A good way to determine that eagles are incubating is to notice when an eagle sits quietly on the nest for long periods, sometimes hours, at a time. Hatching can be inferred when the sitting adult seems to sit higher on the nest contents.

Pairs bonds tend to last more than one year, but although bald eagles are generally believed to generally bond for life, this is poorly studied because of the difficulties in capturing and marking each bird. (Pennsylvania's population is largely unmarked.) The persistence of pairs at sites from year to year, sometimes for decades, suggests long-term pair bonds. However, it is possible that pair bonds can break up after nesting failures. When one of the pair dies, the remaining eagle often seems to find a mate and retains the same territory.

Nest-building generally begins one to three months before egg-laying. In some cases, it seems that pairs build or start to build a nest a year previous to egg-laying. Both sexes contribute to nest-building, but the female may place the sticks in place. Sticks are collected from the ground near the nest tree or broken off from nearby trees. Eagles sometimes use a previously built raptor nest as a base for building their own nest. Pairs sometimes build an alternative nest in their territory that they use

some years. The alternative nest may be in a location quite different from the original nest (on a hillside rather than an island, for instance). Bald eagles generally rebuild or refit their old nest each year. The normal time for this activity in this area is December through February, but they may begin nest repair earlier in the fall or when the nest is in use. In Pennsylvania, most egg sets are laid between mid-February and mid-March, with early March as the peak period. Eggs commonly hatch in April and the young fledge by the end of June or in July.

Bald eagles generally have a clutch of one to three eggs with two the most common clutch size. One egg is laid per day, but not always in successive days, with the clutch completed in three to six days. The eggs are large (averaging 130 grams) and dull white in color with no markings. Incubation begins with the first egg, so the young hatch out over a series of days. Both adults have brood patches, but that of the female is better developed than the male presumably because she does more brooding. The incubation period is generally 35 days in length, but there is some variation. Hatching, like egg-laying, occurs over several days, with one to four days between hatchings. This leads to differences in size between the nestlings and consequential advantage in competition for food, a source of mortality of young nestlings if food is not readily available. Flight feathers emerge in two to three weeks and body contour feathers emerge along the upper wing in three to four weeks. Eaglets gain a lot of weight daily with a maximum average gain of 102 grams per day and 130 grams per day by males and females, respectively. They achieve maximum growth in three to four weeks. Competition between nestlings may lead to starvation or violent death of younger, smaller eaglets caused by their larger nest mates. Nest success can be affected by bad weather and water conditions that make it difficult for the adults to find and capture fish.

Some nest departures are unsuccessful, so eaglets are sometimes grounded near the nest for weeks before gaining flight ability, making them vulnerable to predators or accidents. Adults feed them, but not always successfully if the eaglets are caught in vegetation outside the nest. Young leave the nest about 8 to 14 weeks after hatching, depending on many factors. The adults may encourage fledging by circling the nest with food items. Humans may cause pre-mature and unsuccessful fledging by climbing to the nest or advancing to the bottom or the supporting tree. The juveniles continue to grow and develop after fledging and are cared for by the adults for four to 10 weeks after leaving the nest. The fledglings often follow the adults after leaving the nest site, but often stay fairly close to the nest area (less than 1,000 feet) during the post-fledging period.

Although we tend to focus on the nesting population of bald eagles, many eagles migrate through the state or spend the winter here. The migration population can be divided into two parts: eagles that nest in the southern United States that migrate north after nesting season in winter to spend time in the northern part of their range, including Pennsylvania; and northern eagles that migrate from Canada and northern states through Pennsylvania to the south and return north each spring to their nesting grounds. Southern eagles generally account for the eagles observed between August and September, their migration continuing into November. This migration peaks in mid-September in our state. Many Florida sub-adult bald eagles migrate north through Pennsylvania and other northeastern states each spring and then migrate south in late summer and fall. Northern eagles that migrate South in autumn generally migrate later in the season, accounting for most bald eagles observed in Pennsylvania in November and December Raptors, including eagles, migrating late in the fall and early winter tend to use terrain-derived uplifts and migrate close to topographic features that generate that lift, such as the ridge-tops and escarpment edges. Raptors that migrate in such conditions are probably more at risk from collisions with wind turbines and other structures built in these areas. Bald eagles tend to follow deflection currents along ridges and escarpments and our larger rivers where they can hunt for fish. By contrast, the eastern population of golden eagle follows leading lines and diversion lines along ridges and edges of the plateaus in the Appalachian Mountains. Bald eagles are attracted to open water even during migration.

Bald eagles are widely distributed in North America, especially where there is expansive aquatic habitat. Bald eagles thrive around bodies of water where adequate food exists and human disturbance is limited. They need a large tree or other supporting structure for their large, heavy stick nests. Bald eagles tend to select quality riparian forest and wetlands for their nesting habitat. As such, it can be considered an indicator species for this habitat and an "umbrella species" for protecting the valuable riverside forests and wetlands of the state, because they get more public recognition and support for conservation than many of the other inhabitants of these habitats. These habitats are important for a variety of wildlife from small cerulean warblers (Dendroica cerulea) that nest in tall trees to wood ducks (Aix sponsa) that nest in hollow trees along the river bank and American bitterns (Botaurus lentiginosus) that nest in large wetlands. Our perception of conditions that bald eagles will tolerate or with which they can be successful continues to change. Indeed, bald eagles are teaching us about eagle habitat as they continue to colonize parts of the state where they have been absent for many decades. More pairs are nesting closer to civilization, so long as their basic nest site and foraging requirements are met and their nest area is not disturbed.

Today, thanks to recovery efforts, bald eagles are nesting across the state and in places where they have not nested in decades, if not centuries. Bald eagles are increasing not only in number, but also in geographical coverage of the state. As of the summer of 2009, bald eagles nested in 46 of the state's 67 counties, and those nests have fledged more than 1,000 eaglets since the mid 1980s. Counties with the most nests were Crawford, Pike, Lancaster and York. Non-breeding adults and sub-adults may be found throughout the state at any time of year. In winter, dozens of eagles are typically found along the Delaware River between Matamoras and Hancock, N.Y., and along the Lackawaxen River in Pike and Wayne counties. Other concentration points include the lower Susquehanna River – south of Harrisburg – and Pymatuning Reservoir in Crawford County. Ice cover often is the limiting factor for bald eagle winter distribution. There are some winter roosting sites in Pennsylvania now that the population has increased to the point where eagles can congregate near good feeding areas. Some of these roosts are in agricultural areas near large bodies of water.

Nesting eagles are particularly sensitive to human intrusions or disturbances, but more eagles are nesting near communities and activity areas than ever before. Eagles may forage a mile or two from a nest, but tend to be very efficient hunters that do not wander far from good foraging opportunities where they e a nest, exposing young to harsh weather or to predators, causing nest failure. Eaglets sometimes fledge prematurely from human interference and subsequently suffer injury or mortality. These and other human activities can cause eagles to abandon a nest.

• Bald eagles build the largest nest of any North American bird.
• A pair of bald eagles generally mates for life.
• The bald eagle is exclusively a bird of North America.
• The wingspan of a bald eagle ranges from five and a half feet to eight feet.
• Bald eagles are 28 to 40 inches in length.
• An eagle may weigh eight to 12 pounds.
• Female eagles are about 25 percent larger than male eagles.
• Both sexes will sit on the nest and protect the eggs or young, exchanging places while the other forages for food.
• The bald eagle is one of ten sea- or fish-eagles in the world (genus Haliaeetus). Eagles belong to the family called Accipitridae, which also includes kites, hawks and old world vultures.

Most Americans can identify an adult bald eagle at close range without difficulty. The striking white head, neck and tail are unmistakable field marks. At a distance, however, or silhouetted against a clouded sky, bald eagles may be more challenging to positively identify. Immature eagles, like the one above, lack the definitive color pattern of adults. Still they carry telltale traits that help positively identify them.

Bald eagle traits to look for:

Perching Bald Eagle
Bald eagles perch and roost in an upright square-shouldered stance grasping a branch with strong, yellow talons. They are 28 to 38 inches tall. Females are larger than males. The bald eagle's beak is large and heavily curved. The color of this hooked beak ranges from bright orange-yellow in adults to dark gray in first year birds. Use the Age Identification Chart if you need guidance. The bald eagle's upper legs are feathered, but its lower legs are bare to the talons.

On the Wing
Bald eagles soar on broad wings held in a flat plane. Their wingspan ranges from five and a half to eight feet. In flight, the bald eagle pumps its wings in slow, powerful wing beats. The front or leading edge of the wings runs fairly straight across, especially when viewed from a distance. The bald eagle's head is prominent and when compared visually to its tail, it extends forward more than half the length of its long, wedge-shaped tail. Immature bald eagles have white showing in the wing pit area, unlike golden eagles.

Comparing other large raptors

Golden Eagle Golden eagles are sometimes confused with immature bald eagles, especially first-year bald eagles. Golden eagles, however, have no white markings in their wing pit area, unlike immature bald eagles. When soaring, golden eagles hold their wings slightly raised above the horizontal plane. The front or leading edge of the golden eagle's wings is curved. The trailing edge has deeper curves as compared to the bald eagle as well. The golden eagle's head is not as prominent as in the profile of a bald eagle. It protrudes in front of the wings less than half the length of its tail. The golden eagle is well-named and its golden hackles can be seen in good light. The golden eagle's legs are fully feathered down to the tarsus, but it has yellow feet like the bald eagle.

Osprey Ospreys are generally smaller than bald eagles, 21 to 26 inches in length with a wing-span of four and a half feet to six feet. The osprey soars with a distinct crook in its wings. Its profile is more like a gull's than an eagle's. From a distance, its wings appear to cup downward. The underside of an osprey's body and its head are white and a dark cheek line extends across its eye. Dark wrist marks show when flying.

Turkey Vulture In flight, the turkey vultures hold their wings in a sharp V-shape called a dihedral. This large bird rocks back and forth on large wings in an unstable flight. Turkey vultures have a rather small head compared to a bald eagle.

Black Vulture Black vultures hold their wings in a slight dihedral, similar to golden eagles. Their head and neck appear small and its tail very short. When soaring, black vultures show distinct white wing patches on their wingtips.

Red-tailed Hawk Red-tailed hawks are smaller than bald eagles with a four- to five-foot wingspan. This buteo is 18 to 25 inches long, about twice the size of a crow. The typical eastern red-tailed hawk has a white throat, a darker belly band, and a light red tail.

Rough-legged Hawk Rough-legged hawks are considerably smaller than bald eagles with only a four- to four-and-a-half-foot wingspan. They have small beaks and feet for a raptor of their size. This large winter buteo is very lanky in appearance and comes in either a light or dark morph (plumage). An open field raptor, it characteristically flies in a dihedral and hovers over its small mammal prey. It frequently perches in the open on small branches on top of trees and shrubs. Light morphs have contrasting dark carpal patch.