I have a Naked Butt Barred Rock - she's defeathering herself to line the nest box (her favored nest box is full of her distinctive feathers) and she's 2nd of 5 in the pecking order, so I'm pretty sure it's not the other chickens. I've tried adding material to her favorite nest box (straw, shavings, timothy hay, rice hulls, shredded newspaper) but she just pushes it away. The bare area extends all the way around her tail in a circle. What I'm afraid of is that the other chickens will start to peck at that bare area. I checked her very carefully for mites or bugs and didn't find any, but I did find a small scab, where she - or one of the other chickens - pecked hard enough to draw blood.