It isn’t only owls that produce pellets, Corvids, Cormorants, Herons, Kingfishers, Gulls, Swallows and lots of other species including Robins also produce them. Although you are more likely to find an owl pellet, as they tend to be more compact and therefore survive on the ground longer than a pellet from a bird such as a Robin or Blackbird, which is softer and more easily washed away by the rain.
A pellet is a mass of the undigested parts of what the bird has eaten. This can include bones, fur, feathers, claws, beaks, exoskeletons of insects or plant matter. Pellets from birds that eat worms often contain bits of sand.
The pellet can take 6 to 10 hours to be produced in the bird’s muscular stomach. This is then regurgitated up through the beak and dropped on to the ground. These pellets can often be found under roosting sites.
Whilst out walking I found a pellet which was 3cm by 1.5cm. Full of beetle elytra (the hardened outer wings), sand or small stones. It wasn’t under a tree but in the middle of a field.
I am guessing, because of where I found this and the content, that the pellet is from a Corvid such as a Rook.
I’ll be grateful if someone can confirm.