Ditto the advice to treat with ivermectin (or eprinomectin - sounds like it would work too). May need to retreat several times (doesn't kill eggs, severe infestation). 0.2mg/kg seem to be a standard dosage for many species. Topical treatment can be higher. As I wrote on another thread, an older version of Exotic Animal Formulary by James Carpenter DVM claims "...dosages exceeding 0.2mg/kg are probably unnecessary in birds." A few species actually have a higher dose listed. He doesn't mention chickens specifically but does state "Most species" and goes on to list many types of bird.
I almost lost my chicken - did not realize she had mites until she went into full molt, I picked her up, and I ended up covered with mites. They do eventually die (my jar of vent feathers, I could look at them with a usb microscope - it took about a month for them all to kick off). Once I realized she had mites, I treated her immediately with ivermectin. I know it made a big difference.
I almost lost my chicken - did not realize she had mites until she went into full molt, I picked her up, and I ended up covered with mites. They do eventually die (my jar of vent feathers, I could look at them with a usb microscope - it took about a month for them all to kick off). Once I realized she had mites, I treated her immediately with ivermectin. I know it made a big difference.