The shape of the eagle nest or aerie is determined mainly by the branch point where it's built. Sticks placed in tree forks result in cylindricalor conical shaped nests. Disk shaped nests are built on the ground or a tree branch which is nearly level.Bowl shaped nests occur where the tree trunk branches off into 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.
Eagles are territorial during nesting season. They will keep other eagles out of their own nesting territory, which 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.
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.