RIR with a bare bottom

[Jimmy]

I have 12 hens (no rooster). They all live in a 8'X12' unheated coop.and have not been out much because of the weather(Northern Pa.) Everyone seems to be healthy and productive (they are nine months old). I noticed yesterday that one of the RIR's had a round bare spot about 3" in diameter just below her vent. She is acting normal. I am clueless on this...can anyone help?

 

[katrinag]

Have you checked for mites/lice?

 

[FarmerDenise]

Could she have had a bad case of poopy butt and the lump of poo got pulled off along with the feathers? I would also inspect for insects. Have you found a bunch of feathers in the henhouse? Maybe something got a hold of her rear end and pulled the feathers out.
Those are my guesses