Bald eagle body description

Migration

Not all eagles migrate. Those that do, have complex migratory patterns. While migrating, eagles ride columns of rising air called thermals and can average speeds of 30 mph (50 kilometers). Effortlessly, an eagle can circle in a strong thermal to a high altitude, then glide long distances in the direction of its migration until it finds the next column of rising air. Generally, the eagles follow seasonal food supplies. As lakes and streams freeze over, bald eagles must go south to find open fresh water or head to the coast.
   Adult bald eagles, such as this one, do not migrate with juveniles. Newly fledged eagles migrate before their parents. No one knows how the young birds know when and where to travel.
   Some fledgling eagles wander in a wide range their first few years. Some return to their origin, while others do not. Only the young eagle knows if this is a conscious decision, or if it simply loses its way.

   Adult bald eagles begin fall migration when the northern lakes and rivers freeze over. Depending on location, they usually migrate to the coast or large rivers near dams, where the water remains open. Wind currents play a large roll in determining their flight pattern.
   Some eagles are migratory while others are not. Many eagles in Florida do not migrate, but remain year-round. Most bald eagles migrate south in the fall to areas with sufficient food, and return north in the spring to nest. In the spring, the birds migrate quickly, but during the fall they migrate rather slow. They may remain in an area for a week or so before continuing on.
   Migrating eagles fly during the day at speeds averaging 30 miles per hour. To help them soar, eagles use thermals, which are rising currents of warm air, and updrafts generated by terrain, such as valley edges or mountain slopes. Soaring is accomplished with very little wing-flapping, enabling them to conserve energy. Long-distance migration flights are accomplished by climbing high in a thermal, then gliding downward to catch the next thermal, where the process is repeated.
   Bald eagles tend to migrate in groups. A "stream" of migrating bald eagles can be twenty to thirty miles long, with birds spread out about a half mile apart.

                    Diet and Feeding Habits

The bald eagle (Haliaeetus leucocephalus) is a member of the sea and fish eagle group. Its closest relatives, similar in appearance and habit, are found in Africa and Asia. Even though they are fish eaters, they will take whatever prey is available and easiest to obtain. Bald eagles which live along the coast and on major lakes and rivers feed mainly on fish. Bald eagles fish in both fresh and salt water.

   Eagles sit at the top of the food chain, making them more vulnerable to toxic chemicals in the environment, since each link in the food chain tends to concentrate chemicals from the lower link. Because of their size, they have few enemies and require a large hunting area.

   A bald eagle's lifting power is about 4 pounds. They do not generally feed on chickens or other domestic livestock, but they will make use of available food sources. Bald eagles will take advantage of carrion (dead and decaying flesh). Because of its scavanger image, some people dislike the bald eagle. Other people do not care for powerful and aggressive birds. Still other people object merely on the grounds that it is a bird of prey, which kills other animals for food.

   Once an eagle spots a fish swimming or floating near the surface of the water, it approaches its prey in a shallow glide and snatches the fish out of the water with a quick swipe of its talons. Eagles can open and close their talons at will. If an eagle is dragged into the water by a fish too large for the eagle to lift, it is because the eagle refuses to release it. In some cases this is due to hunger. An eagle might drown during the encounter with the fish or if it's unable to swim far enough to reach shore. The eagle can not fly again until it's out of the water, so it uses its large wings to swim. The eagle is a strong swimmer, but if the water is very cold, it may be overcome by hypothermia.

   The hunting area or home range patrolled by a bald eagle varies from 1,700 to 10,000 acres. Home ranges are smaller where food is present in great quantity.

   Because of the energy expended during hunting, an eagle has to spend a lot of time resting quietly. It's estimated that only one out of eighteen attacks are successful.

   Though not as fast as falcons, bald eagles are fast fliers. When diving, where lift is less important than reaching drag, the eagle pulls in its wings to minimize their surface area.

    Bald eagles have been seen hunting in pairs.

   An eagle protects its food by partially opening its wings, or tenting.

   An eagle can consume one pound of fish in about four minutes. The eagle holds its prey with one talon, holds onto its perch with the other, then tears off each bite with its beak.

   The bald eagle steals food from other bald eagles as well as other species. Chasing another raptor is usually enough to persuade it to drop its kill, but occasionally bald eagles will attack.

   Bald eagles do not have to eat every day. But if the bird goes too long without food, it may not be able to hunt effectively in order to survive.

   Eagles have an out pouching of the esophagus, called a crop, where they can store food when the stomach is full. The crop also separates indigestible substances, such as feathers, fur, and scales from the meat. The indigestible substance is mixed with mucus and formed into a mass. After the meal, the eagle eventually regurgitates the mass as a casting.

   For a scavenger like the bald eagle, the carcass of a seal is an unexpected large food supply. Rich with fat and protein, the seal's body will feed a group of eagles for days. Though many calories will be obtained, they will be lost in fighting over the food.

                         Nesting & Young

Nests - The shape of the eagle nest or aerie is determined mainly by the branch point it is built in. Sticks placed in deep, near vertical forks result in cylindrical or conical nests. Nests built on the ground or nearly level branches are disk shaped. Bowl-shaped nests my occur where the tree trunk branches suddenly into several smaller, upright branches.
   Bald eagles build their nests in large trees near rivers or coasts. A typical nest is around 5 feet in diameter. Eagles often use the same nest year after year. Over the years, some nests become enormous, as much as 9 feet in diameter, weighing two tons. Even when a nest tree falls or a strong wind blows a nest down, the established pair usually rebuilds at or near the site within a few weeks if it is near the breeding season. The nest may be built in a tree, on a cliff, or even on the ground if there are no other options available.

    Eagles are territorial during nesting season. They will keep other eagles out of their own nesting area. Their nesting territory is usually one to two square miles.

Sexual maturity - An eagle reaches sexual maturity at around four or five years of age. At that time, the eagle's energies become concentrated on the effort of finding a mate and raising offspring. Bald eagles mate for life, but when one dies, the survivor will not hesitate to accept a new mate.
   During breeding season, both birds protect the nest territory from other eagles and predators. Mating season - varies greatly by region. In the South it may last from late September through November, while in the Great Plains and Mountain West, it may last from January through March. In Alaska it lasts from late March to early April.

   Pairs of bald eagles have been seen whirling through the air with talons locked together. This could be a form of courtship or a ritualized battle between an intruding eagle and one defending its territory. Whichever it is, eagles do not actually copulate in the air, as some people believe.

   One way to determine the sex of an eagle is to examine its beak. Females have deeper (distance from top to chin) beaks than males.

   Some eagles do not breed every year. Bald eagles are capable of breeding annually from the age of four, but some of the adults, though paired, seem to choose not to breed. It might be an instinctive decision, based on the weather; availability of nesting sites, or food.

   Because an eagle lives up to 30 years in the wild, it has many years in which to produce offspring.

Eggs - In the Vancouver area eggs are laid in late March and early April, while in northern Canada and Alaska eggs are laid in May. In Florida, eggs are laid from November through January.

   Eagles lay from one to three eggs. Five to ten days after a successful copulation, the female lays a speckled off-white or buff colored egg about the size of a goose's. The second egg is laid a few days later, followed by a possible third.

    The 35 days of incubation duties are shared by both male and female, but it is the female who spends most of her time on the nest. Trading places on the nest can be a tense time. The brooding parent may have to call for relief, or may be reluctant to leave and have to be pushed off the eggs or young. During incubation, the male bald eagle regularly brings green sprigs of conifer branches to the nest. Why he does this, no one knows, but it could be for deodorizing the nest or possibly providing shade for the eaglets.
   During incubation, one parent is always on the nest, not only to keep the eggs warm but to protect them from squirrels, ravens, and gulls which will break open and eat the eggs.

   Human disturbance can have an impact on the bald eagle, as most of them need some privacy and quiet to breed. People wanting to observe or photograph the eagles can disturb them enough to cause them to abandon a nest. Use binoculars and spotting scopes for viewing, and keep at a reasonable distance.

   The eggs hatch in the order they were laid. Eaglets break through the shell by using their egg tooth, a pointed bump on the top of the beak. It can take from twelve to forty-eight hours to hatch after making the first break in the shell (pipping). Once the eggs begin to hatch, the female's vigilance becomes nearly constant. The male provides the majority of the food needed by his rapidly growing family. Eventually the female will take up her share of the hunting, but in the early days, all of her attention is given to the young eaglets in the nest.

Chicks - Sometimes two chicks will survive, but it is not uncommon for the older eaglet to kill the smaller one, especially if the older is a female, as females are consistently larger than males. Should one chick decide to kill its sibling, neither parent will make the slightest effort to stop the fratricide.

   Newly hatched, eaglets are soft, grayish-white down covers their small bodies, their wobbly legs are too weak to hold their weight, and their eyes are partially closed eyes, limiting vision. Their only protection is their parents.

   Eagles feed their young by shredding pieces of meat from their prey with their beaks. The female gently coaxes her tiny chick to take a morsel of meat from her beak. She will offer food again and again, eating rejected morsels herself, and then tearing off another piece for the eaglet.

    While on the nest with very young eaglets, parents move about with their talons balled into fists to avoid accidentally skewering their offspring.

Eaglet Growth - The young birds grow rapidly, they add one pound to their body weight every four or five days. At about two weeks, it is possible for them to hold their head up for feeding.

   By three weeks they are 1 foot high and their feet and beaks are very nearly adult size.

   Between four and five weeks, the birds are able to stand, at which time they can began tearing up their own food.

    At six weeks, the eaglets are very nearly as large as their parents.

   At eight weeks, the appetites of the young birds are at their greatest. While parents hunt almost continuous to feed them, back at the nest the eaglets are beginning to stretch their wings in response to gusts of wind and may even be lifted off their feet for short periods.

   At three or four weeks, this eaglet is covered in its secondary coat of gray down. In another two weeks or so, black juvenile feathers will begin to grow in. While downy feathers are excellent insulators, they are useless as air foils, and must be replaced with juvenile feathers before an eaglet can take its first flight, some 10 to 13 weeks after hatching.

Quiet Please - Eaglets Growing by Carolyn Stearns / David Aiken (illustrator) describes the successful efforts of school children to save their eagle neighbors from developers near Chesapeake Bay. Amazon.com carries the book, as do the major book chains.

First Flights
   Down is gradually replaced by feathers, while the eaglets grow still stronger. Finally, an important moment arrives.

In An Eagle to the Sky (1970), Frances Hamerstrom, who spent many hours observing eagles, described the process for one young bird:

The.....EAGLET WAS now alone in the nest.
   Each time a parent came flying in to toward the nest he called for food eagerly; but over and over again, it came with empty feet, and the eaglet grew thinner. He pulled meat scraps from the old dry-up carcasses lying around the nest. he watched a sluggish carrion beetle, picked it up gingerly, and ate it. His first kill.
   Days passed, and as he lost body fat be became quicker in his movements and paddled ever more lightly when the wind blew, scarcely touching the nest edge; from time to time he was airborne for a moment or two.
   Parents often flew past and sometimes fed him. Beating his wings and teetering on the edge of the nest, he screamed for food whenever one flew by. And a parent often flew past just out of reach, carrying delectable meals: a half-grown jack rabbit or a plump rat raided from a dump. Although he was hungry almost all the time, he was becoming more playful as he lost his baby fat; sometimes, when no parent bird was in sight, he pounced ferociously on a scrap of prairie dog skin or on old bits of dried bone.
   The male eaglet stayed by himself for the most part. He was no longer brooded at night. Hunger and the cold mountain nights were having their effect, not only on his body but on his disposition. A late frost hit the valley, and a night wind ruffled his feathers and chilled his body. When the sunlight reached the eyrie's (the brood in a nest of a bird of prey) edge, he sought its warmth; and soon, again, he was bounding in the wind, now light and firm-muscled.
   A parent flew by, downwind, dangling a young marmot in its feet. The eaglet almost lost his balance in his eagerness for food. Then the parent swung by again, closer, upwind, and riding the updraft by the eyrie, as though daring him to fly. Lifted light by the wind, he was airborne, flying--or more gliding--for the first time in his life. He sailed across the valley to make a scrambling, almost tumbling landing on a bare knoll. As he turned to get his bearings the parent dropped the young marmot nearby. Half running, half flying he pounced on it, mantled, and ate his fill.

   Approximately 40% of young eagles do not survive their first flight.

   Once the young eagles have fledged (to acquire the feathers necessary for flight) they remain around the nest for four or five weeks, taking short flights while their primary feathers grow and strengthen. Their parents still provide all of their food.
   The young birds, with the exception of their color, resemble their parents, but are nothing like them in behavior. They have to learn how to hunt, and they only have the remainder of the summer to learn. After that, they're on their own. The first winter is the most dangerous and difficult part of an eagle's life.
   Higher predators are born with instincts that urge them to fly, to bite or to pounce, but precisely how to do these things is another matter. Through months of trial and error, eagles acquire basic skills such as lighting on perches or stooping on prey through practice. Eagles practice with almost fully developed bodies, and so sharpen their skills quickly.

   The immature bald eagle, such as seen here, is sometimes mistaken for a golden eagle. However, young bald eagles have more white mottled into their coloration overall, and they have the yellow beak of the adult. The golden eagle is more solid in color, and it has a beak that is more blue-black, with a nearly black tip.

   Eagles molt in patches, taking almost half a year to replace feathers, starting with the head and working downward. Not all feathers are replaced in a given molt. Until the bald eagle is mature, the replacement feathers are of different colors. As adults, the belly and back are dark, while the head is pure white. The distinct juvenile pattern, signaling that a bird is not ready to breed, may reduce aggression from territorial adults.

   As bald eagles age, their eyes and beak gradually turn yellow. The white hood and tail feathers grow in sometime in their fourth year.

The young eagle stays near the nest, practicing its abilities to fly and to hunt. The parents cannot tell juveniles how to hunt, they have to learn by watching the parents and by practicing. During this time they seem to spend more time looking at prey than they do actually attacking it.
   Over the next month, they meet with their parents to be fed, but have little other contact with them. They learn to soar and to spot prey.

   Until the first winter after their fledging, young eagles near the nest are often still fed by their parents. Although a young eagle has the instincts to hunt, it lacks the skills. If food is scarce during the winter, it may die.

   Six to nine weeks after fledging, juvenile eagles leave the nesting area.

   Nesting cycle - From the time the parents build the nest and the young are on their own, takes about 20 weeks. During the nesting cycle the parents remain within one to two miles of the nest.

   Communal gatherings of bald eagles offer many advantages to younger inexperienced eagles. Not only is food abundant on the salmon spawning grounds, but here the juveniles can watch their elders to learn how food is caught. They also learn very quickly how to steal food.