People often talk about owls and hawks 'casting' pellets, but many birds cough up pellets. Songbirds and other insect-eating birds (Say's Phoebe above - click on photo to enlarge) often cast the exoskeleton remains of insects they have eaten, or regurgitate pellets made up of seed husks if they are seed eaters. Gulls and other fish-eating birds will 'spit-up' pellets as well. There are other examples, but the point is that not only raptors exhibit this behavior.