Lice and other mites on chickens can be a common thing unfortunately. The best way to treat them is with preventative measures like sanitary coop conditions and providing your chickens with a a place to dust bathe.
In a study performed by the University of California Riverside, chickens that rolled in a dust bath of sand and diatomaceous earth showed a huge reduction in the amount of external parasites after just a week. If you have chickens that aren’t laying as well, it might possibly be caused by mites. If your chickens have to fight of an infestation, they have less energy for egg production.
The best way to treat them is what I call the Shake & Bake method.
Use an old pillow case and place it inside a large plastic bag.
Place diatomaceous earth, food grade, about a cup. You can mix in Sevin if you wish to use it.
Make sure to wear a breathing mask and gloves.
Here's a good video explaining it: