I'm guessing you have mites, and not lice. Do a google search so you can see the difference. If it's mites, they live in the environment. It will not do you any good to treat the birds, if you don't treat their coop at the same time. That entails stripping all of the bedding out of the coop, washing any surfaces where they might hide out (especially cracks and corners). I'd then use poultry dust to treat both coop and birds. You will need to repeat the entire procedure 7 - 10 days later to get any mites that hatch after the first treatment. It's a gruesome job. You will also need to destroy the old bedding as it will harbor mites. If it's lice, it's a bit easier b/c they live mainly on the bird. I've never dealt with lice, you'll need to do a thread search on that.
Unfortunately, there is no easy, quick fix. It entails all out war if you have that much of an infestation.