I use moth balls, have never had it bother the chickens, they seem to leave them alone. I had a few snakes get in with the rabbits last year and eat some babies, talk about a horrifiing find for a 9 year old girl to walk up on. I put them under the shed.
I might be wrong, but if you read the ingredients on snake away or the other repellents, they are the same thing that moth balls or crystals are made out of, I don't use the crystals more afraid the birds would eat them.
Years ago I saw a video of crows in a garden picking up moth balls and rubbing them all over their feathers, they said they figured out they killed mites/lice, now how a crow could figure this out I have no idea, but they did pick them up and rub them all over their body, it was amazing.