Two of our five nearly year-old hens are missing feathers too. I'm wondering if we should try treating them for lice/mites just in case, even though we don't see obvious signs of infestation...
One has bare patches where her wings meet her back and lost all her tail feathers; she's one of the bossy ones, more likely to bully than be bullied. The second ,has a completely bald belly and is starting to lose some around her neck, and she's been spending a lot of time burrowing in the dirt.
We don't see obvious signs of mites or lice, but we're not very experienced — the specks we do see look like dirt to our eyes. And we don't see much bullying.
Egg production has dropped slightly, from 4 a day to 3. They seem happy otherwise, but this is frustrating!