After an owl swallows a mouse, strong acids in the owl's stomach begin to digest the mouse's muscle and other soft parts. The owl can't digest the bones and fur that come along with the meal, so the owl's stomach forms these indigestible materials into tight packages called pellets. Several hours after a meal, an owl will regurgitate one of these pellets. The pellets, along with feathers and other remains, can be found under owl roosts.