The mites described in the post above are called red mites. They are red in the morning because they have moved to the chickens at night and fed on their blood. Look for them in the cracks and crevises around your roost.
The other kind is the Northern Fowl Mite which lives on the bird and clusters in the soft feathers just beneath the vent. If allowed to infest the bird for a long time there will be a very large, firm clump containing mites of all ages.
Dusting is one way to quickly get rid of the mites that are hatched. A dust containing "Sevin" or carbaryl is the most effective and will kill the bugs overnight a long as you get it right into the clump and down to the skin. The treatment must be repeated in 10 days. In that time any eggs in the clump would have hatched but the mites would not have had time to mature and reproduce, so by dusting again on the 10th day the cycle will be broken. The dust will also kill lice so should be applied to the underside of the wing feathers and neck or hackle feathers. Again re-apply in 10 days. A dusting in the nest boxes, under the straw will prevent the nests from becoming infested.
As Sunni Ten advised, a clean out of the coop should be done when an infestation is found.