My first recourse is to bathe them with pet flea shampoo. If they are that infested they will probably calm down quickly and enjoy the bath. Then you need fresh bedding in the coop, diotemaceous earth in their favorite dust baths, and a thorough coop cleaning.
Dust baths are important for your birds. It is their natural pest control method. Contrary to everything I expectand am told, feather lice aren't terrribly contagious. Birdd lice eat only feather sheath protein - can't live on mammals. Plus in my coop, only my rooster gets them very often. I think he just gets distracted from personal grooming sometimes ;-) Busy fella
if you want to get rid of them for good use ivermectin pour on for cattle..... get an eye dropper, put 3 or 4 drops on the skin, between wings on back...do this again in 3-4 weeks...... this will kill mites, lice, and worm them.... the only downside is you don't want to eat the eggs for a few weeks...... treat your birds, then clean the coop real good and dust every crook n cranny with whatever dust you decide to use.. some use 7 dust, others de..... this is very effective and simple..... you can buy a bottle from your local feed store or from amazon.com.... using just dust can be an on going battle that never ends..... good luck!
DE will not get RID of lice or mites. Dust ALL your birds with Sevin dust by putting it in a plastic bag then putting your chicken in up to it's neck. Massage the dust all through the feathers down to the skin. Do this for each bird...then segregate it from the living quarters. Clean out ALL bedding from the coop, and either dust or use liquid Sevin mixed according to directions and THOROUGHLY dust/spray it being sure to get in all the cracks and crevices. Dust their known dustbathing spots too.
Repeat the process in 10 days to get any new hatches.
Best thing ever was some spray I got from the vet (like frontline) for chickens. You spray once under each wing, and once on their bottom....and wa la! NO MORE LICE!! Quickest ever!!! It's called FRONTIER SPRAY (Fipronil spray 0.25%). Actually looking at the bottle now, it is FRONTLINE SPRAY, but for birds. I spray mine every 3 months, and never, ever, have any issues.
YOU WON'T HAVE TO WORRY ABOUT SEVEN DUST, OR ANY OTHER NON-SENSE. THIS IS, BY FAR, THE QUICKEST WORKING, AND EASIEST TO PERFORM.
Where do you see it at? I got mine from a aviary vet. But, I am sure you can probably buy online a whole lot cheaper. I would think it would be ok. Like I said, I spray my BANTAM chickens once under each wing, and once on backside. I stand about 6" away. Maybe with the stronger solution, I would spray less frequently, or stand at a further distance. When I took my BTW Japanese to vet, he had HORRIBLE lice, but he was very ill, too. Lice seem to harp on the weaker birds. The vet sprayed him in the office, and when we got home, (about an hour later), there were no lice to be seen. Good luck!!