Chickens will pick at each other's feathers and vents when they are over crowded, bored, or not getting enough protein in their diet. They can actually cannibalize each other if this isn't dealt with, and can become a habit. Try giving them some extra protein or some flock raiser feed to increase protein to 20%. Make sure they have plenty of room, and get outside to roam every day. Limit lighting in the coop to 12-14 hours a day. Apply some Nustock cream to the pecked areas to heal and hide wounds.