I have 10 hens and a roo in a coop/run. Hatched this spring. All hens are laying good. 7 to 10 eggs everyday. 3 barred rock,3 marans,2 white rock,2 orpington and a barred rock roo. The barred and maran hens have lost their feathers around their vent/butt. The other 4 and roo are fine. Just noticed yesterday. I always look them over good and have seen no signs of lice or mites. I have older hens that have done that in the past, but got better. I keep wood ash in there and change the pine shavings every weekend in the coop/nesting boxes. I have sand in the run and rake out the poop a few times a week. They also are let outside a few times a week when I am off from work. I've got them off the roost at night and looked them over good with a flashlight. Can't see anything.I wonder if it might be self pecking or something in the pecking order? I have 36 all together. Another coop/run has 12 hens an a white crested black polish roo. His crest or head looks the same. Bald in the back an red. Any ideas? Thanks for your advice. JDR.