please don't use WD-40, or any motor oil...
the chemicals are absorbed into the skin and body system.
The idea is to smother the mites, and recondition the leg scales.
There are safer products to use.
vaseline, Bag Balm (a good product)..mineral oil..all work well..but you have to keep doing it, as another poster suggested.
don't get the oil on the feathers.
as other suggested, wash the legs before applying.
you will also need to treat the hens for mites..
Sevin, poultry dust (such as Ectiban-D by durvet), or Eprinex/Ivermectin...which is given by drops on the skin at the back of the neck near the "shoulders".. similar to Frontline for dogs.
Ivermectin also gets rid of worms.
http://shilala.homestead.com/ivomec.html
also Adams flea products seem to be popular.
here is a link to information:
Pet/Bird Shops also sell products for caged birds to treat mites.
any of these products can be found at most farm/feed/livestock suppliers and online.
you will also have to clean and treat the coop, roosts, and nestboxes...remove all bedding...replace with new.
mites will go back and forth from coop to birds, from one bird to another.
so you'll have to be vigilant till they are gone.
Scaley Leg Mites...will cripple your birds.
they can also get fungus infections, and bacterial infections that can be very serious.