Hi-
I had this problem..... the short answer is: duct tape! I am not kidding.
I tried Blue-Kote, vicks vaporub, etc but the others still pecked her.
(The purple is from the Blue Kote)
I did a lot of online research and concluded that the root cause was a protein/amino acid deficiency (they were no longer eating worms/bugs since its winter) so I began adding proteins. Black Oil sunflower seeds alone didn't help. They're now getting daily protein in the form of:
organic organ meats
organic table scraps of meat & fat
plain yogurt
sunflower seeds
canned tuna
fish scraps
and -I think this helps a lot- Avia Charge 2000 in the water.
The pecking has stopped, except that the one victim hen could not regrow her feathers because they would get pecked out (this was at the start of adding the protein supplements).
I kept her inside in a dog crate for a while for her skin to heal (also gave her the Avia Charge) but when I put her out they went after the new growth. Tried a "saddle" but it didn't cover enough.
Here's what worked: DUCT TAPE. No kidding. Just tape it on, overlapping onto the good feathers. it won't stick to the raw skin, which is kind of dusty I guess, just the feathers. Eventually it falls off, I've had to reapply once in the last few weeks. It also adds a layer of cold-protections, especially as the new feathers begin to regrow underneath, like a down comforter. My poor hen is doing well now. The tape will probably fall off in another week or so and her feathers are looking god underneath. I was really at my wit's end before the duct tape and it REALLY WORKED for me.
Good luck.
Stacey