A longer explanation of your circumstances can help others on here help you more, but I've made a general outline below:
Just re-stating stuff I've read on this site and others (I've never had this issue) but there are four main causes:
Boredom
Nutritional deficiency
Not enough room.
Roosters.
Boredom comes from being cooped up all winter with nothing to do. You can change this by using treats such as cabbage balls or hiding corn in some fluffy clean bedding.
You should have at least 4 square feet of room per hen in the coop if they're in the coop all the time. The run should be 10 square feet per hen, minimal. There should also be some sight-blocks, (I have an upside down bucket in my coop, which serves triple duty as a launch-pad for the younger and older birds, a sight-line break, and a seat)
You should investigate the protein percentage of your feed. 17% is decent, 18-20% is about ideal, from my small understanding of nutrition. Below 16% is bad. Feathers are a great source of protein and the other chickens might be eating them.
Roosters sometimes rip out feathers when they mount. This is especially true of younger, heavier roosters with very active sex-drives. Some people use hen saddles. They're available on
Amazon and Etsy.
Oddly, a rooster can also be the solution--he tends to stop the girls from pecking each other, and he breaks up fights. A good rooster makes for a much healthier flock dynamic, in my experience.
There are spray-on, smear-on, and a few other types of tar-like coating to cover a chicken's back so that the others can't continue to peck at it. You should be able to find some at
Tractor Supply or Rural King.
