by Bob Thomas

Owls usually swallow mice, their principal prey, whole. The muscle and organs of the carcass are digested and the indigestible elements such as bones and hair are regurgitated. Called “owl pellets” or “castings”, they appear as elliptical packages of bones wrapped in hair. Pellets are formed in the gizzard and are ejected 6-12 hours after the meal. They are usually found on the ground beneath an owl’s roost or nest. It is not uncommon to find a couple of hundred pellets near an active nesting site.

All of the bones of the mice being eaten are in the little packages. Since mammal bones, especially skulls, are easy to identify to species, biologists studying the food habits of owls or the distribution of small mammals need only collect and dissect owl pellets. Though a bit more difficult to identify, the same technique yields data on other food items, including lizards, snakes and birds.

Eagles, hawks, falcons, and kites also produce castings. Many of them swallow their prey in bite-sized pieces, but some swallow their food items whole just as do owls, making their castings difficult to distinguish.

The next time you spy an owl frequenting the same roost, take a look on the ground below. Maybe you’ll find one of these treasures of nature.