Welcome to BYC!
This could be a mite problem, but it also could be an aggression problem, where they are pulling each others feathers out. Not enough protein will cause them to pluck each others feathers. Do you see mites on them? You can go out side after dark and with a flashlight, shine the light on your birds. If you have mites, you will see them then. Mites come out at night for the most part.
Make sure your birds have enough room. When chickens are cramped, they pick on each other. So 5 square feet per bird in the coop, 10 square feet per bird in the run. Give them things to do...hanging veggies or cabbages, seed pecking blocks, ladders, mirrors, get them out to free range...just things to keep them from picking on each other. Any bare spots of skin or wounds, you will need to spray them with blu-kote to hide these ares to keep them from being picked at more or get infected.
They need to be on a good diet, keep the treats down to 10% of the diet so they are getting enough protein.
I would also give everybody a good exam from beak to tail to make sure they are healthy.
If you do suspect mites, Sevin Dust or permethrin spray will work well. Make sure to get down to the skin and not just the feathers. You will need to clean out the entire coop. All bedding, nest boxes, etc....spray or powder the roost bars especially beneath the bar where the bugs hide, get the nest boxes really well. Repeat this in one week and each month it never hurts to clean everything out and spray everything down.
Good luck with your birds and welcome to our flock!