I have a flock of 15 chickens. I am 99.9% sure that all of them are hens (they came sexed and I find 15 eggs every 12 hours). They were born in June 2012 so they are not that old. Just got up to everybody laying eggs daily. My brood breakdown is 6 Plymouths, 5 Rhode Islands and 4 Golden Comets. Recently I noticed that a few of the Plymouths were missing some feathers just at the tail base (where the tail meets the back of the bird). At first I ignored the issue, because it wasn't extensive. Now the issue has gotten worse, more feathers are missing and more birds are involved. BUT ONLY THE PLYMOUTHS ARE DOING IT! Almost all of my 6 Plymouths are missing feathers at their tail bases. NONE OF THE OTHER CHICKENS ARE MISSING ANY FEATHERS! I don't get it. If they were protein deficient, wouldn't all the chickens be feather eating regardless of breed? And if I had a rooster, (which I believe is highly unlikely) wouldn't he be pulling feathers on all the chickens regardless of breed? If the chickens were stressed wouldn't they all being eating feathers?The only thing I can figure is that the Plymouths are more sensitive to a lack of protein??? The skin where the feathers are missing looks pretty normal and pinkish. There isn't any bleeding/wounds/scale that would indicate any infectious or parasitic process. I believe there is feather pulling going on, but from what? and why are only my black and white girls doing it? HELP!