I don't know about the rooster who's missing feathers, I haven't had that happen, but I do know that your two hens without feathers on their backs are probably being over-mated by the dominant rooster. (The one without missing feathers.) The reason their feathers are missing is when he mates with the hens, his Spurs will rub on her back feathers. When there are more hens and he mates with each hen less, it is much, much less common to see missing back feathers unless the roo has a favorite hen or two. What you could do is either take one of the roosters out, that should lessen the stress on the hens, or you could also get chicken saddles, I think that My Pet Chicken has some. It's basicaly a piece of fabric you put on your hen's back to protect her back from losing any more feathers. Since you have a 2:1 ratio of hens and roosters, if you don't remove a roo, I'd go ahead and get saddles for all your girls, just in case.
