My chickens after 6 months of living a peaceful existence are now pecking each others feathers out of their tails. I think I know which chicken is the bully but not positive. The only thing I changed recently is I took their water dish out and put in a water system with nipples, and I went to a deeper bedding for winter. I tried to introduce four juveniles to the flock but had to remove them after two days because one got pecked pretty bad ever since then they are picking on eachother the coup is big enough for 16 birds. And the run is adequate there are only 5 birds in the coup.