I had a similar story with my small flock. All the chickens were about the same age and there had been no trouble at all until the dominant hen (our only barred rock) turned nasty one day. She relentlessly pecked at our olive egger (the lowest rank hen). At one point she tore a 1" hole in the OE's neck. (Minor surgery and a long separation followed) We tried everything. Separated the bully for several weeks from the others--which was SUPPOSED to make it such that she would re-enter the flock at a lower rank--didn't work. We ended up having to set up a second run. Every time we tried to reintegrate the flock as one, the bully BR would strip every feather off the olive egger's head and neck within 2 days. We also tried "pinless peepers", which are plastic blinder that don't completely blind the dominant hen, but block enough of her vision to make her aim bad when she tries to peck. That worked pretty well for a few weeks, but the pinless peepers seemed to be causing beak damage to the bully, so we took them off--which put us right back at square one. OE pecked clean within 2 days. HOWEVER we do now have a solution. We got a new rooster. More accurately, one of our hatchlings grew up. Before him, we had an a**hole rooster who got aggressive with humans, so he is in the freezer now. Our new rooster, who is now about 18 months old, is a sweetheart with humans and is lees rough on the hens than his predecessor. He also seems to be mediating the relationship between the bully and the low rank OE. He doesn't really interfere with the dominant hen, but what he seems to do is keep the OE out of her way. I watched one night as the birds were entering the coop to roost for the night. The dominant BR was inside already and making quite a production of getting herself settled. The OE wanted to go in too, but the rooster placed himself in her way and would not let her enter the coop. I watched for almost half an hour as her stood over the OE. Every time the OE tried to squirm past him, he blocked her. Once the BR was fully settled, he finally let the OE into the coop. I also have seen that whenever I give them some corn or some other treat the OE will keep the rooster between her and the BR when she tries to eat. Often the BR will still chase the OE off the treat, but she is more prone to ignore the OE if the rooster is between them. The OE is still missing a few feathers on her head, but it is a FEW, not completely naked down to her shoulders and no blood. We do have a solution now, but we seriously considered just culling the dominant bird, or possibly the OE, since the BR seems to leave all the others alone. However, the two of them are currently our only layers, so it is tough to part with either one. If I had to choose, I would cull the bully.
