Lice is fairly common and easier to fight than mites. Use the sevin dust since it will kill the lice on contact. Then in exactly 10 days, dust them again to get any lice eggs hatching. 10 days later, look at the hens, and you may need to dust a third time. You will also need to empty out your coop and nests, and remove the litter. Get some Permectrin II at Tractor Supply and spray roosts, walls, nests, and floor before adding clean pine shavings or bedding.
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
http://anrcatalog.ucdavis.edu/pdf/8162.pdf
http://edis.ifas.ufl.edu/ig140
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
http://anrcatalog.ucdavis.edu/pdf/8162.pdf
http://edis.ifas.ufl.edu/ig140