Any indigestible parts are compressed into pellets and regurgitated. Because the prey was eaten whole, pellets can contain entire skeletons. Pellets are clean and safe to handle. Dissect one and discover what the owl has eaten. Often the skull and teeth can be keyed out to determine the species of prey.
Pellets also contain feathers and fur of the former critter.
Contents of an owl pellet. This pellet contained 3 vole skulls and other various parts. Usually two pellets a day are regurgitated. One at the day roost and one at the hunting grounds. It takes between six and seven hours after a meal for the pellet to come up.