Depluming mites are like scaly leg mites in that they are invisible to the human eye. These mites infest the follicles that the feathers grow from. The discomfort these mites create will drive a chicken to pluck her own feathers out.
Edited by chickengeorgeto - 4/1/16 at 10:29pm
Event though you have no roosters, the other hens may be picking her feathers. Have you seen any evidence of mites or lice on her? The barred rock also has some missing feathers under her neck. Chickens will pick feathers out of boredom, for some extra protein, and if they have too little room, especially if they don't get out to free range daily. Give your girls some extra protein treats such as some chopped egg, tuna, or 24% gamebird starter. BluKote or Nustock are good to cover bare spots, but here is some info on mites, lice, and how to recognize and treat them: