Most owls regurgitate pellets of undigested hair/fur and bones. As would be expected, the ones left by Great horned owls are large, those of the small owls such as Screech owls, much smaller. Finding a stash of pellets under a tree is a very indicator that an owl has been roosting there for some time. This was found under a tree where a Great horned owl had been roosting for a week or two.