I had that happen once to a hen, was a combination of molting and an over zealous young cockerel with raging hormones, so she was staying down in the hay more and had gotten mites, not a fun combination to deal with all at once as was still pretty hot during the day so where she was missing feathers was getting sunburned, felt so sorry for her, she was in there for about a week before she could be put back with the flock, used Vetrycin spray, and some neosporin on it as a preventative besides complete coop clean out and dusting with sevin, I didn't see them but after dusting she started to do better. and of course week in hospital ward she wasn't out in the sun