It might just be lice. In fact if the hens are otherwise healthy, it's more likely lice than red mite, which severely debilitate the birds by sucking blood.
Catch one of the hens, turn it on it's back and part the feathers around the vent and on to the tummy. If you look carefully, you will be able to see the little blighters running about. They are clear cream colour, oval and fast. You may also see clusters of grey / white eggs on the base of feathers. The main place for lice are round the vent and under the wings. They don't harm the hen per se (unlike red mite) but are irritating and can put them off laying / eating etc.
If there are lice, and there probably will be, dust with louse powder, available from pet and farm stores. You'll have to do it again in a week or so, as the eggs take 7-10 days to hatch. If any of the feathers have a high egg burden and you're brave enough, you can pluck them out (hens pluck their own feathers so it can't hurt that much!) and dispose of them preferably by burning. Don't throw them on the grounds where they can hatch and infect another bird.
Don't worry if you get lice on you - they are host specific and will jump off in due course ;-)