I have some that do this. Mine do it to themselves. At one stage I had four of them doing it I think prompted by a brief red mite infestation in the summer. Only two still do it but they are basically half naked. I have given up with the anti peck spray but I do spray them purple if they get red and sore. Mine have loads of space (about 25 square metres) and lots of chicken type entertainment (perches, things to get up on, options on feeders etc.).
Hopefully more space will help yours but sometimes I think it becomes a habit. I'm considering putting mine in jumpers for a few weeks so they can't reach their backs!
Yes it is probably boredom and over crowding I'm afraid. You can buy anti peck spray etc but its not great at stopping. You need to get them more space. I have 3 chickens and they have a run that is a good 15' x 10' and they are MUCH happier than in the run that came with their coop which was about 4' x 3' and NOWHERE near big enough.
Hi we have 5 chickens housed in a coop/run suitable for 4-6 chickens. I've noticed a couple of our girls are plucking and eating the feathers of the others, two have large bald patches on their rear ends, though one seems to be getting it worse than the other. Is it more likely to be due to overcrowding or boredom, and is there any way we can discourage it?