Are you sure that one chicken isn't doing the pecking? Sometimes if there is a lack of room or protein, or boredom from not getting outside, there can be feather picking. Some bad tasting creams such as Nustock, Bag Balm, or Rooster Booster Anti-Pick Lotion may help if it is from pecking. I would check them for mites and lice (and the eggs) and sometimes it is necessary to do this at night with a flashlight for the mites that only come out in the dark, but hide in the crevices of the coop during the day. Sevin dust and permethrin dust or spray is good for use on the chickens to treat lice or mites, but the coop must also be cleaned out and treated.
