Without looking at your chickens backsides it is hard to say.
Read up on all the external pest that affect chickens. The following list is not definitive but it is a step in the right direction to get you started.
some common names for mites may be repeated:
Common Chicken Mites
Red Chicken Mites
Gray Chicken Mites
Northern Chicken Mites
Depluming Mites
Black Mites
Scaly Leg Mites
Feather Mites
Then there are the lice associated with chickens. Mites belong to the arachnid family, that is spiders and they all have 8 legs except when the life cycle of the mite in question only has 6 legs. Lice all have 6 legs and they are insects. Lice also (usually) only lay their eggs on the bird and cannot live off its host. Some mites (Common Chicken Mites) may live for months and months without a host to feed on.
Also common chicken mites only spend from 8 to 12 hours a day sucking your chickens life blood while some lice species feed on your chickens' skin and flesh 24/7 and also feed on pooling blood coming from the lice's feeding sights.
Is there any redness associated with the feather loss?
If so sprinkle or dust your birds with Sevin 5% garden dust.
You also need to spray a Pyrethrum insecticide on the roost, and coop frame. An alternative method is to paint the coop frame and roost with used motor oil mixed with Pyrethrum at the recommended rate for a full body dip. Pyrethrum is safe to use on any animal with a back bone or spine. The motor oil carries the Pyrethrum into the cracks and crannies and keeps it there while it preforms a triple whammy on the common chicken mite. First the Pyrethrum kills mites on contact, second the Pyrethrum is a fine mite repellant, third the Pyrethrum impregnated oil suffocates any mites or lice as well as killing un-hatched mite eggs hiding in the coop frame and cracks. If you're still not sure about Pyrethrum remember it is the same pesticide used in creams for use on human infants who have scabies, a human form of the chickens' scaly leg mite, and this body cream is a 10% Pyrethrum formulation at that.