Why do some owl pellets have no bones?

I've read that some owls do not regurgitate bones in their pellets. Also, if the bones are really fine, rather than dense like the pelvis & many of the long bones of mammals, they may be digested. I just found what appears to be a densely-packed cylindrical owl pellet which is over 4" long which is comprised almost if not entirely of bird feathers - no bones. Birds have very light bones with air pockets in them to promote flight. Perhaps bird bones would be more easily digested & therefore not passed in the pellet. However, since most owls hunt at night, what birds would an owl have access to? I read the Great Horned Owls can have pellets that are 4"+ in length.