There are more than 60 different kinds of eagles in the world with behaviours and habitats so varied that some may not even be of the same species. The life cycle of an eagle varies with its breed as well. The golden eagle, found in North America, Europe, North Africa and Asia, is the most widespread variety and a good one to consider when studying the life cycle of an eagle.
Eagles build their nests, or eyries, atop tall trees, high cliffs and bluffs. The female usually lays a clutch of two eggs, though she can lay as many as four. She incubates the eggs for about 40 days by sitting on the nest to keep them warm. Depending upon the climate, incubation can range from 30 to 50 days. Peter Nye, writing about bald eagles in The Journey North, says that males may incubate the eggs too. More commonly, the male takes part in this life cycle stage by catching small mammals to feed to the nesting female.
The newly hatched eaglet's survival depends on its place in the pecking order. After emerging from its egg covered in white fluff, the helpless hatchling is completely dependent upon its mother for food. It weighs about 85 g (3 oz). The first hatchling to emerge from its egg has an age and size advantage over the others in the nest. It grows stronger faster and can compete more successfully for food. Later hatching eaglets may starve if they are not feisty enough to compete.
Before they "fledge" or leave the nest for the first time, young eagles remain as "nestlings" for 10 to 12 weeks. That is how long it takes for them to become fully feathered enough to fly and large enough to begin hunting for prey. The fledgling eaglet continues to return to the nest and stay around its parents for another month or more, learning how to hunt and refining its flying techniques. It may beg for food as long as the adult birds are willing to feed it. In all, it will be at least 120 days after birth before the young eagle is totally independent.
Even after it has left the nest, the juvenile eaglet faces survival challenges. The British Forestry Commission reports that between 60 and 70 per cent of them do not survive the first winter. Once independent, juvenile eagles migrate to find a winter territory. Where prey is plentiful, they don't have to migrate at all, but they do have to disperse to find a big enough territory to support them. Expert Hope Rutledge, writing about golden eagles on her American Bald Eagle Information website, says adult golden eagles can roam a territory of up to 427 square km (165 square miles). In four to five years, the juvenile will reach maturity. Until then, it may return to its birth nest from time to time.
When golden eagles reach sexual maturity, at between four and five years, they develop golden plumage on their heads and necks and reach a wing span of almost seven feet (two meters). Until then, it is possible to age the birds by their plumage. Eagles form mating pairs for life and build enormous nests up to 3 m (10 feet) in diameter, weighing up to 907 kg (2,000 lb). The adult pairs have no natural predators except man and can live up to 30 years.