Ornithologists may collect and analyze one species' pellets over a span of time to provide information on the seasonal variation in its eating habits. One advantage of collecting pellets is that it allows for the determination of diet without the killing and dissection of the bird. Pellets are found in different locations, depending on the species. In general, roosting and nesting sites are good places to look: for most hawks and owls, under coniferous trees; for Barn Owls, at the bases of cliffs or in barns and silos; for yet other species of owls, at their burrows or in marsh and field grasses.
Hawk and owl pellets are grey or brown, and range in shape from spherical to oblong or plug-shaped. In large birds, they are one to two inches long, and in songbirds, about half an inch. They are found in many other species, including grebes, herons, cormorants, gulls, terns , kingfishers, crows, jays , dippers, shrikes, swallows, and most shorebirds.
Ornithologists examining pellets have discovered unusual items in them-even bird bands that were once attached to a smaller species that was consumed by the predator bird. In the United States, screech owl pellets have contained bands from a Tufted Titmouse, Black-capped Chickadee, and American Goldfinch. In 1966, a Golden Eagle pellet in Oregon was found to contain a band placed on an American Wigeon four months earlier, and 1600 km (1000 miles) away in southern California.
(compliments of Wikipedia) 
