Its possible that they're pecking each other or rats are getting in at night and biting them... I don't know though, I could be wrong. I'm not very knowledgeable on that sort of stuff.
Aw, I'm sorry. Do you think it could be from the weather? Its been freezing cold here for the past few days. I've put heat lamps in the coops and even have wrapped the entire chicken coop up with plastic just to keep it warmer in there.. I'm just hoping I don't lose another chicken.