Breeding Biology Bald eagles are sexually mature when four or five years old. During the breeding season, bald eagles establish and defend territories. Mated adults usually return to the same breeding territory each year. Thus, a newly mated pair must find a territory that is unoccupied. If one member of a pair dies, the other will remate.
Eagles also require isolated areas, for they are easily disturbed by human activities.
In February or March, Wisconsin bald eagles begin building a nest or repairing one they built a pervious year. They usually build in a tall tree, often a live white pine. Nests are made of large sticks and other vegetation, the sticks forming the shell and more flexible greens lining the inside platform. The average nest is four feet in diameter and three feet deep. Eagles will build more than one nest in their territory, although they use only one each breeding season.