Sometimes they build two nests, so they can alternate sites annually, which may reduce lice infestation. Some bald eagle nests have been used for up to 30 years, but often by this time they have grown so heavy from their annual accumulation of material that they crash to the earth under their own weight. Most birds of prey build platform nests. Sometimes they build on cliff ledges, rather than in trees. American kestrels, typically cavity nesters, sometimes build their nests in the underparts of golden eagle nests.
Grackles sometimes do the same in osprey nests.
Great blue herons are platform builders and generally nest in colonies. The male brings the sticks to the female and she arranges them on the nest, usually in the highest part of the tree. Mourning doves also build platform nests, sometimes on top of abandoned robin nests. Mourning dove nests are often insubstantial, and the eggs in them may be visible from below. Woodpeckers meet the nesting challenge by gouging cavities in the trunks of dead trees.