I never lost a bird to lice.We put a small amount of the dust under their bedding, and under the bedding in the nest boxes. Treat everyone! Some birds will have a heavy load, and some nearly none, but all need to be managed. retreat at least once, seven to ten days later, and maybe three times.
Lice or mites will kill birds!!! Happened here, long ago, before I knew what was happening.
Reinfestations happen often, from the wild songbirds who visit.