Those do look like mites. Mites reproduce every 5-7 days. Treatment with permethrin spray works, as well as permethrin garden dust, or products with spinosad, such as Elector PSP. Treat every 7 days at least twice, then remove all bedding from nests and the coop, and treating the coop, roosts, and nests with permethrin spray before replacing with clean bedding. Straw has been known to harbor mites, and some may accidentally bring in mites that way. Mites can cause severe anemia and kill chickens, so is much more serious than lice.