How Is the Owl's Digestive System Based Upon Pellets?

More than 200 species of owls exist in the world. The smallest, the elf owl (Micrathene whitneyi), is the size of a sparrow. The largest, Blakiston's fish owl (Bubo blakistoni), is 30 inches tall with a 6-foot wingspan and weighing 10 pounds. Owls have a two-part digestive system consisting of a proventriculus and a ventriculus -- also called a gizzard. Birds who don't regurgitate pellets have a three-part system: a crop for storage, a proventriculus and gizzard.

Depending on the size of the owl, some dine on insects; many dine on small rodents like mice and voles; the largest owls eat rabbits, other birds or even fox. Owls eat smaller prey whole. The food goes to the proventriculus where digestive enzymes break down the meal. Owl stomach acid is weak (pH 2.2 to 2.5) so they only digest soft tissue of prey, not bones or fur. The food moves to the gizzard where usable substances get used by the body and indigestible parts remain in the gizzard.

Gizzard remains include bits of undigested prey. Owl gizzards work like trash compactors. The muscles in the gizzard compress the leftovers into an oval pellet. That pellet remains in the owl's gizzard until the bird is ready to eat again, a time period of 10 to 12 hours. Pellets inhibit eating, so prior to the next meal, owls regurgitate pellets from the gizzard. The pellet is forced out by spasms in the esophagus, a process lasting a few seconds to several minutes.

Think of owl pellets as potential archaeological digs or crime scene evidence. Pellets are found under trees, in barns or around other owl roosting spots. Pellet contents reveal what an owl dined on before regurgitating the pellet. Intact skulls, pieces of vertebrae, hip bones, femurs, even teeth are found inside along with fur. In some science classes, students working with bone charts reconstruct skeletons in order to make proper identification of prey remains.