Thanks everyone. I think it must be the heat. I mixed some oyster shell in with their food to make sure everyone is getting some. I have it in a seperate feeder in the coop, but it never seems to dissappear. They have a really large run with lots of shade. In fact it will be raining wild grapes on them later in the year! I haven't let them free range in quite a while. Lots of new dogs in the neighborhood and I'm afraid I'd lose chickens and friends over that one.
We do have snakes around here, but I can't imagine a snake getting past the chickens. They'd eat a small one, and wouldn't a big one just eat the chicken?
Come to think of it......I had a bluebird nest where the eggs and then the babies dissapeared one a day. I moved the nest box, and we haven't had that problem since.....That kind of sounds like snake too, huh?