Oh wow! Ok, I see this quite a bit around my house. I draw a lot of wildlife in even though I am in the city. I put food out for cats, deer, squirrels, birds, etc. and I noticed things like this around my yard. Mainly under trees and I couldn't figure it out. I always take the remains and bury them the best I can. Good to know.
Yup, those sound like owl pellets!