White clumps at the base of feathers is probably lice eggs. You should be able to also spot the fast moving tiny tan bugs when parting the feathers. Permethrin 5-10% spray or the garden dust is safe for treatment of lice. Treat once, then change out bedding and treat the coop, nests, roosts, and in 10 days retreat the chickens to get the hatched eggs. Check them again after another 10 days to make aure there are no eggs or lice seen.