Bald Eagle Nesting Information
Sexual maturity and breeding generally coincide with attaining adult plumage (head and tail becoming all white, beak yellow, and the eyes white), from 3 to 5 years of age.
Nest site selection and building can begin as early as late September in Oklahoma. Several nests may be constructed by a single pair in the same territory.
Nests are typically located in one of the taller trees in an area, just below the crown, about 3/4 the way up the tree, against the trunk or in a sturdy fork of the tree. Assorted interwoven sticks up to 1 inch or more in diameter make up the bulk of the nest. The nest is lined with soft grass, moss, or other fine materials. Nests can take months to complete, are added to each year, and in exceptional cases can end up weighing several tons.
One clutch of 1-3 eggs is laid per year (rarely 4 eggs). However, a replacement clutch may be laid in the event of a loss during laying or early incubation. Incubation begins sometime from December or January in Oklahoma and lasts for 33.5 days, while the hatching process can take up to 2 days. If more than one egg is present, the others hatch at intervals of 2-3 days. This results in eaglets of different sizes with the first hatched eaglet being the largest, a difference that is maintained until growth is completed before fledging. After fledging eagles may gain weight, but they do not grow very much larger dimensionally. Female Bald Eagles, as with many birds of prey, average larger in size than males; however, there can be an overlap between the sizes of the sexes. Size is almost the only visible difference between them. Eagles nesting in the northern part of their range also average larger in size than those from the southern parts of their range.
About a third, on average, of Oklahoma Bald Eagle nests fail, either during incubation or brooding, for mostly unknown reasons. Causes of nest failure could be due to food shortage, inclement weather, human disturbance, predators, and environmental contaminants.
Development of Bald Eagle Young
Week One: At hatching, the young are covered with a light gray down and have limited mobility. Their eyes, dark brown in color, are closed, but open after a few hours. The female parent does the majority of the brooding while the male parent provides most of the food for the family. Aggressive antagonistic behavior can appear shortly after hatching wherein the oldest, largest eaglet tries to dominate or even kill its sibling(s).
Week Two: The second down plumage, darker in color, begins to replace the first. At the end of this period thermoregulation is attained; that is, the eaglet can maintain its own body temperature under normal weather conditions without brooding from an adult.
Week Three: Black contour feathers on back, shoulder, breast and wings begin to emerge.
Week Four: Maximum body growth nearing completion; flight feather development underway.
Week Five: Male and female parents bring relatively equal amounts of food. Parents begin spending more time away from the young and often perch in nearby trees.
Week Six: Young are able to tear pieces of food off and feed themselves, and begin to stand and walk.
Weeks Eight – Twelve: Nestlings begin “branching.” They flap their wings while perched on the nest and hop onto nearby branches, practicing and building up flight muscles, coordination, and landing skills. Most of Oklahoma’s Bald Eagles fledge between 11 and 12 weeks after hatching. After the first flight the eaglets may return to the nest a few times to spend the night (roosting) or to get food brought there by the adults.
Some nestlings fall to the ground and remain there before gaining flight ability. Parents usually continue to feed the young on the ground; but in this location, the chicks are highly vulnerable to predation.
After fledging, the young are still dependent on the adults to feed them for a period of up to a couple of months until they gain the experience and skills to find and catch their own food. Satellite telemetry studies of a few Bald Eagles reared in Oklahoma show that they typically migrate north during the hottest months of the summer to cooler climates such as Minnesota, the Great Lakes area or Canada.