Looks like it could possibly be mites/lice. The hard lumps could very well be a build up of parasite feces and eggs. Have you seen any crawling on your birds.
You may have to soften the eggs/feces/hard part with some type of oil like coconut, olive, etc. You would need to treat your birds, as well as, take out all the bedding and clean & treat the coop, then put in fresh bedding, you may need to do this more than once. If you haven't done so, provide them with a clean area to take a dust bath.
Here are a couple of links that have photos and outline treatment for lice/mites: