Walking on the rocks along the shoreline of Lake Superior near Lutsen recently, I found the gull pellet shown below. Owl pellets are commonly known, and not that hard to find -- if you know of a reliable owl-roosting site. This, however, is the first gull pellet I've seen. It's shown atop a patch of lichen. It's easier to see on an enlargement of the photo, but within the tangle you can find mammal bones, a mammal tooth, and mussel shells, all wrapped in a net of mammal hair. The gulls common to the area are Herring and Ring-billed. I assume one of them coughed this up. Birds expel pellets in a manner similar to a retch. The pellets normally contain the undigestable portions of a food item.
Surprising to me was the presence of mammal parts. I've never thought of gulls as mammal predators, although they certainly are opportunists when it comes to food, eating a wide variety of animal and vegetable matter. A study done on a wildlife refuge in Montana during the 1990s found that small mammals were the most numerous prey items found in Ring-billed Gull pellets. All foods eatern by the gulls, however, would not leave undigested evidence. Voles, by the way, were the most common mammal prey, based on pellet evidence.
Many species of birds produce pellets. In addition to owls and gulls, the list includes herons, hawks, warblers, and other songbirds.