We have 20 hens and 1 rooster. Our Rhode Island reds have all kinds of feathers missing on their backs. We've checked for mites or lice but don't see any. Maybe we are missing them? Pur white rocks look healthy, the rooster looks deathly, but some of the barred rock are missing feathers as well. However they don' look as bad as our rhode island reds. Advice? Here are some pictires.