My Chicken has plucked her bottom

My chicken has plucked her bottom, she is otherwise well but I'm not sure that she is laying very well. I am down to two birds and one is laying daily the other only occasionally - any ideas?

you probably have already looked, but check her for lice. Mite in the coop could also set her skin irritating. beware there is no sign of blood spots on her, as the other girls will attack any wound or sign of blood. Maybe a squirt of purple spray, or anti peck spray might help break the habit.