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About Owl Pellets
Owls have a tendency to eat their prey whole. Now, there are plenty of things that are difficult to digest in a whole animal, including but lot limited to claws, beaks, bones, fur, and other such unsavory things. And an owl doesn't want that going through its system. So, it regurgitates all those indigestible bits once they have been collected in the gizzard, creating an owl pellet. The digestible bits continue to pass through their digestive system.
Pellets serve a secondary use to owls as well: scouring the digestive tract of the owl, particularly the gizzard. As such, they are necessary to the health of the owl.
A pellet is formed between six and ten hours of a meal, and regurgitated sometime within a few hours of that. Regurgitation typically happens somewhere where the owl is comfortable, such as its roosting and nesting sites. The size of the pellet is proportional to the size of the owl, anywhere from a several centimeters to just one centimeter along.
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Value of Owl Pellet to Science
Different species of owls often have very different diets, and the diets of individual species may vary considerably from place to place and season to season. Understanding any changes to the diet is critical for conservation efforts.
So, as you might imagine, owl pellets have proven to be an enormous boon to the scientists that study owls. Precisely because they contain such complete parts of the things that owls eat, they are easy to interpret, rendering a complete picture of the diet of the owls, as well as what prey species live in the area. There have even been recorded cases of the tags on smaller birds being found in owl pellets, providing information regarding their migrational patterns. As mentioned previously, because they are found beneath owl nesting and roosting sites, they're a good way to study their habits. They're also easy to come by, as an owl creates one after every meal, they don't degrade quickly, and they're a way to study owls that is noninvasive to their normal living habits.
This ease of interpretation also makes them an ideal introduction to science for young students, once the barf jokes have stopped being funny. For this reason, they are commonly sold to schools anywhere from elementary to college level to teach basic biology and ecology. (I was one of those third grade students, once upon a time!)
A warning, however: owl pellets are not sterile, and often carry the viruses and bacteria of the organism ingested, including some that are contagious to humans. If you find one and wish to dissect it yourself, sterilize it in a microwave oven beforehand, and use latex gloves while handling. There have been salmonella outbreaks in schools related to the improper handling of owl pellets!
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Pellets in Other Species
Such pellets are not entirely unique to owls, either. Many birds, predators or not, create similar features, anything from hawks to herons, grebes to gulls. For instance, in falconry, the pellet is instead referred to as a “casting". All the same scientific benefits of studying owl pellets also apply to the pellets of other birds.