Hi there
There are many causes for missing feathers and you will have to ascertain if it truly is a feather-picker causing the damage, or if there is an alternative cause such as a lice or mite infestation. Some solid observation of your flock is a good idea, and investigation of your coop when it's dark will help you identify a mite infestation if that is the cause. (To check for mites, take a flashlight into your coop and do a visible inspection, and also wipe a damp, white paper towel on the underside of your roosts. You will see smeared, red blood if there are mites under there.)
If it's mites, you will need to treat the roosts, the coop, the nest boxes and the birds with an appropriate product.
If it's feather picking, your girls may be either lacking in protein or just plain bored. I would recommend increasing protein rations (more meat, meal worms, scrambled eggs) and installing some activities for them - hang a head of cabbage just out of reach so they have to jump for it, put in some tree braches and logs to climb in and hide under, provide some fruit frozen into ice blocks etc.
If worst comes to worst, you could isolate the pecking bird for a few days in the hope of knocking her down a peg or two in the pecking order. Also, keep an eye on the injured birds. If blood is drawn the others will continue to pick at the wound and cause severe damage. In that case, the injured bird will need to be separated.
Finally, do you have a rooster? Is there any chance it could be damage from treading during mating? If so, a Hen Apron ('Saddle') might be of some assistance.
Good luck!
- Krista