Try getting some mite dust and fully dusting all chickens. Then try petroleum jelly and cover the entire leg. Try to keep it covered in it to suffocate the mites. When the mites get into the leg, they are really hard to get rid of. If they cripple the bird, about the only way to get rid of them is perscription. Yet, the best way I have found with the legs involve suffocating them. Permetherin (sp) or another type of insectacide should be used to treat the entire area. If there are mites at all, they need to be eradicated through spot and environment treatment. I have also found that alot of the IGRs (insect growth regulators) work well on mites (area treatment), but they need to be used way away from food or water. DE, or diatamaceous earth (food grade) may be spread all throughout the area to eradicate the mites, just make sure that there is contact (with the birds and the area) for five days (in order to dehydrate the insect).