We have 19 chicks that are almost 4 weeks old. The last week or so, we noticed that most of them have a bare spot on their back down by their tails. Some have it worse than others. We don't know if we have a bully chick that is picking on the others (we've never seen any picking) or if mites or something can cause this. We've never had this problem with chicks before. Any ideas on what's causing this and what we should do about it?