Eagles have a strong, and natural, instinct to breed annually. The urge is especially strong in the springtime. One of the reasons our migrating eagles head northward is to claim their territory and build a nest. Often they use the same territory and maybe the same nest for many years. Once they arrive, the work building or fixing their nest begins!
In most regions, a pair of eagles starts working on their nest from 1 to 3 months before the female lays the first egg. However, in the northern regions they can't delay this long. For example, according to Birds of North America, "In Saskatchewan, adults build or repair nests in September prior to migration and build or repair nests in April upon return from wintering grounds."
If building a nest from scratch, the birds carefully select a tree. They usually select a living tree. They look for a nest tree located where there dead trees ("snags") nearby. They use the snags as viewing posts, to keep an eye on the nest when they are off away from the nest finding food. If a nest tree remains standing, the eagles often use it for many years, as long as the nest is secure. The healthier the nest tree is to start with, the longer it will last. One nest, located in Ohio, was used for 34 years, until the tree finally blew down.
In forested areas, mates usually select one of the tallest trees in the area. A"super-canopy" tree (one sticking up above nearby trees) allows the eagles to see all around. Also, they can fly into the nest without bonking their huge wings into branches.
Eagles spend their first days dealing with their neighbors, if any. They will often position themselves at the edge of their territories as a signal to others to stay out. When an eagle inadvertently flies too close or through an occupied territory, the resident adults will give chase, and in extreme cases, engage in aerial tumbling and even talon-locking.
Throughout the season, and sometimes even during fall and winter, eagles keep adding sticks to the nest. Both the male and female bring nesting materials. They interweave the sticks, and fill in spaces with grasses, mosses, cornstalks, Spanish moss, and other fibers. To soften the sticks on the bottom, parents even line the nest with their own feathers.
One of the biggest nests in NY was measured at 7 feet across and 12 feet deep. Nests have to be big and strong enough to hold these large raptors. The average eagle nest is only 5-6 feet in diameter and 2.5-4 feet deep, and the first year a nest is built, it may be much smaller than that.
The one thing just about all eagle nests have in common, is the view. It is one of the most spectacular on earth! Eagles certainly know how to pick the prime lookout spots.