Raccons, I'm betting. They will leave one bird alone and kill another, then rip them apart or leave them untouched; they sometimes, will eat the entrails, but nothing else. We've had problems with possums before, and they almost always eat the whole bird right where they kill it. I think any predator will sometimes kill a bird and then leave it uneaten, or just partially eaten; maybe they get scared away, or whatever. Minks , weaels, and skunks, will all kill the bird, then just eat the entrails, and we've had minks kill them and leave them mostly untouched. During the day. You could try putting a thin layer of flour on the ground around the coop; see what kind of tracks you find in the morning.