Barn Owls are renowned for making nests in manmade structures, but most other species of owl seek out shelter away from the influence of man. The cavities of rotten trees, or trees hollowed out by woodpeckers often host owl nests. Owls also readily seek out the abandoned nests of other species of birds such as eagles, hawks, or crows. Some owl species also use the cover provided by caves and niches in rocks to provide them with protection from the elements. Great Horned Owls run squirrels out of their own nest, squash it flat, and take it as their own. Once a suitable nesting place has been found, it may be used for many years.
When owls do construct their own nests, the quality of workmanship is shoddy at best. Many nests are hastily constructed out of discarded owl feathers and the feathers and fur of their prey.
Barn Owls typically don't even make an attempt to build a nest. The only comfort and protection that Barn Owl chicks get is from the regurgitated owl pellets that are strewn around the nesting area. Owl nests become coated with excrement and uneaten animal parts, making them very filthy compared to the nests of other species of birds.
Burrowing Owls do make an attempt at providing a good nest for their young. These birds nest in holes dug in the ground insulated with grass, plant stalks, and other material, which provides protection for the young.