Keep us posted on if you ever solve this riddle. From reading all these posts, there are a bunch of head-only eaters out there (owl, hawk, coon, skunk, weasle, possum, etc). It dont sound like a fox or coyote, but a mink may also do that. A bunch of leg-hold traps set around a few chicken heads will tell you what is doing it. My money is still on the owl.