Mites hide in the crevices of the coop and crawl out in the dark to suck the birds' blood.I have been looking for mites on those that die, and on the hens. I haven't seen any.
Check at night using a head lamp or flash light, you can also use double-sided adhesive tape along the underside of the roosting bars so the mites will get stuck to it on their way to the next meal.