Ivy, you have a couple of options for treating mites or lice: topical dust or spray, topical systemic, oral systemic.
For ease and effectiveness, I prefer to use Eprinex. It is a topical cattle systemic that has NO slaughter or milk withdrawl. It was suggested by our poultry leader and got the nod of approval from an avian vet I use. A couple of drops on the skin under the wing, and your birds will lose their bugs and any worms within a couple of days. If you go this route, get some 1 cc syringes. It is the easiest way to apply a correct amount...1/4 cc per 5 pounds of chicken.
Some breeds are more prone to bugs. Cornish, for one. I can grab every other bird and see not one bug, then pick up a shiney feathered, fat and sassy Cornish, and presto, bugs all over the place.
I am a pet groomer, so not unaccustomed to seeing "hitchhikers" on animals, but sheesh, until the Cornish arrived, I never saw a bug on one of my birds. Double uck! For this reason, they are so on my short list of birds to take to auction when the weather improves. Shame too because they are really good free-rangers.
If you wish to go with a dust (Ortho Sevin) or a spray-- I use the Adams Flea and Tick spray (blue bottle) as a preventative when I coop out at every show-- keep in mind it will take repeated applications to keep them bug free (based upon the pest's egg/hatch cycle).
Oh and yep, as mentioned by someone else, them mites and to some extent lice, enjoy hanging out in the cracks and crevaces of the coops/roosts. I have heard that some people use "foggers" when the weather is good and their birds can be turned loose, others spray, and some douse the boards with a pesticide white wash. If I find bugs, the last thing I want to do is wait on weather. Impractical in the snowy Sierras.
Wish you the best in your debugging, whichever avenue you take.