It's called feather picking/plucking. You need to try and break them of it as quickly as possible.
Do they free range? If not how many chickens do you have and how large is the run? Is the run bare of vegetation?
I have had luck applying pine tar liberally to the pecked areas. It's nasty stuff to work with, and apparently tastes as nasty as it smells. It helped stop the picking for me.
I went through the same thing with the hens pecking my rooster's tail. I believe the biggest cause of this behavior is boredom. Along with the pine tar, I began free ranging my chickens. The behavior ended. I've got a page about feather picking/plucking in my signature that you may find helpful.
If you can't free range, try adding leaves and mown grass to their run and sprinkling black oil sunflower seeds throughout it - or some other snack higher in protein than scratch. You may also want to increase their protein by feeding scrambled eggs, yogurt, fried hamburger, etc. If you do a search on feather picking or feather plucking you'll come up with a lot of threads where this subject is discussed and ways to keep chickens busy are mentioned.
Best of luck,
Penny