Symptoms of lice are damaged feathers, not loss of feathers. Also, decreased laying, but if they're only 8 months old, that wouldn't be an issue. Lay them upside down on your lap and look closely at the skin around the vent and surrounding areas. Also, look at the base of the feathers around the vent, to see if there are any very small, white, eggs, glued together and to the shaft of the feathers.
Lice will usually infest the very young, the very old, and the immunocompromised. One may not show any signs, where another will, so you need to check several or all of them.
You can put diatomaceous earth (DE) in their dust bathing places and/or in their nests, and some people use Sevin dust the same way.
Anyone else??