Isn't there a topical spray for this - something like "pick no more"? I think it' something like Bitter Apple but for birds. I don't have much experience with this, but I did have an issue when the flock was bored. I had to keep them locked up last April die to a persistent fox. Once they could free range again, the issue disappeared.I've had a few feather eaters before, and had to cull. Nothing I tried worked, and on one occasion another bird was starting to learn that behavior! I hope you can come up with a solution better than I could.