They line their nests with spider silk-satin sheets for the crib. But not all parents make a soft and fancy nest for their young. Killdeer scrape together a few pebbles or may lay their eggs on a bare patch of sand, or atop a gravel roof. Kestrels, though their young are more dependent than killdeer, use cavities but lay their eggs on the bare surface. Mammals are not as dependent on holes in trees as are the many cavity-nesting birds, which include bluebirds, chickadees, house sparrows, nuthatches, tree swallows, even wood ducks.
Some animals return year after year to reuse old nests.
It you watch such nests over several seasons, you can see them grow. It weighed about two tons and finally collapsed the tree which held it. Bald eagles have nested at Barr Lake State Park near Brighton since 1986, though they have had to rebuild when their nest was lost to storms and a fallen tree. The natural world is dynamic, and we see that when we're able to watch an animal family through a season. As the snows of late winter fall upon her back, she stays steadfast upon her eggs, fed by her mate.