Pestene Insect (Mite and Lice) Powder
Don’t buy this unless you see or feel mites or lice on the chooks.
If you do see them on the birds, dust the birds, coop and favourite hangouts with this stuff and problem solved. Some say you should dust periodically as a preventative but I think that is just a waste of money. Solve the problem when it occurs, not before.
I have only had mites 4 times in 20 years, so it is not like they are often a problem. Powder can also be used as a flea powder for dogs and cats.