Owl pellets are compact wads of fur, feathers, bones and other indigestible animal parts formed within the owl's gizzard. Owls generally eat their prey whole. They cannot chew, so they cannot avoid or mechanically process any of these indigestible parts.
Owl pellets are valued by owl researchers because they can tell them a great deal about the owls' diets. Owls form the pellets gradually as the soft tissues of their prey are dissolved. The formation of a pellet is generally complete a few hours after an owl eats. An owl does not regurgitate the pellet immediately, often storing it up to 20 hours first. They have a special pouch near their gizzards to store pellets until regurgitation. When startled, however, owls often regurgitate their pellets immediately, sometimes before they are fully compacted.
Owls use their gizzards to process food items and deliver digestive enzymes and acids. The gizzards have small rocks and sand inside to aid in this process. Bones, teeth and feathers, however, cannot be processed in this way and are dangerous to the lower parts of the owls' digestive tracks. This is why owls must form pellets to be regurgitated, rather than simply passing the indigestible materials through the rest of their digestive tracts to be excreted.