They are mites. You can get rid of them, but it's going to take some work.
You need to pick a product and method of dealing with them. The easiest is dusting them with Sevin dust, which can be picked up at any garden center. There are several other potential ways of dealing with them, but this is quickest and easiest, so let's focus on this one. You can dust them one of 2 ways-
1. Make a powder puff out of a small section of panty hose. You can fill the section with Sevin and use it to puff the bird all over focusing mostly around the vent and under the wings.
2. Put a quantity of dust into the bottom of a pillowcase. Put the chicken into the pillowcase up to their neck. Cinch the pillowcase closed and then shake the bird and case around to get the bird coated in dust. Keep the dust out of the face.
Once the birds are dusted then you must treat the coop. All the bedding needs to be removed. The house needs to be sprayed with an insecticide. I like to use Adam's Flea and Tick Spray (although I have used straight bleach and activated Oxine, as well with good results). Once the coop is dry then put a light layer of bedding in and dust that lightly with Sevin, as well. You are only putting a light layer in because you will need to treat the coop again.
The birds should be OK for now, but you are going to need to re-treat everything and everyone in 7-10 days to catch the mite eggs that hatch in the meantime.
You need to care for this problem ASAP. Mites can be deadly to birds in large numbers. I nearly lost my flock a few years back to mites.
Good luck.
