Losing feathers and inflamed butt

[amaksimova]

My 4 year old barred rock has had an inflamed red butt for about a month. This week she has lost many of her feathers. She is one of 7 chickens. The others are not affected. They free range a few times a week otherwise get organic bar ale pellets as well as compost. Any ideas?
Pics are attached.

 

[sourland]

That's a complete and total molt with feather pecking/plucking on her butt. Although it would require the head ache of reintroduction, I would be tempted to remove her from the flock until her feathers regrew.