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.
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.
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.
An immature bald eagle is sometimes mistaken for a golden eagle. However, a young bald eagle has more white mottled into its coloration overall; a golden eagle is more solid in color, and its beak 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 juvenile bald eagles mature, their head and tail feathers gradually turn white; simultaneously the eyes and beak gradually turn yellow. Complete transformation to maturity is achieved sometime in the fifth year.
After fledging, young eagles stay near the nest for six to nine weeks practicing their ability to fly and hunt. The parents cannot tell juveniles how to hunt, they have to learn by watching the parents and practicing. During this time, they seem to spend more time looking at prey than they do actually attacking it.
Until the first winter after they fledge, young eagles near the nest are often still fed by their parents, but have little other contact with them. Although a young eagle has the instincts to hunt, it lacks the skills. Eventually, they learn to soar and spot prey. If food is scarce during the winter, it could die. Approximately 50% of young eagles do not survive their first year.
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.