HI, I have 5 chickens and they all seem to be pretty healthy and producing a good amount of eggs but one of them has a featherless bum just below the vent, it also seems to be quite sore. And another has a dirty bum and i'm worried about what it is. The featherless one doesn't seem to have lice/mites, and to be sure i gave them a big clean out last week. I was just wondering what may be causing this soreness and irritation? thank you.