I'd guess fox or large bird of prey. I have hawks and eagles. They dont leave alot of feathers behind, sometimes just a head. So heartbreaking. It sometimes feels like I have a free chicken take-out restaurant for eagles. Othertimes, I won't lose any for months. When I suspect fox or raccoons, I let my huge dogs sleep in the pen for a few nights. My dogs like chickens as friends, not food. The dog barks and scares away ground predators. I think the dog urine smell helps scare away predators, as well.