That is a favorite area for birds to pluck each others feathers, the tail/vent area. Make sure you have enough room in your coop. At least 5 square feet per bird. 10 Square feet in the run per bird. Boredom can cause feather picking as well as nutritional deficiencies. So give them things to do...hanging veggies or cabbages, seed pecking blocks, ladders, mirrors, swings, what ever it takes to keep them entertained. Make sure they are eating a good quality food and keep the treats down to a dull roar so they are getting enough protein.
If any of your birds have bald spots that the others can see, spray the areas of the skin with blu-kote. This will hide the skin color and stop them from going after any bare skin.
If you have an aggressive bird or birds, you may need to separate them out or use pinless peepers on them. I have a hen that I have had to use pinless peepers on in the past due to her aggressive nature.