Mites. That's the read.

One hen off. What are you seeing that's different?

Feather loss is a symptom, not a cause. Is she losing them in patches or thinning evenly?

Patches point to something specific — mites, a molt, or another hen pulling them. Can you see any mites at the base of the feathers or on the skin?

Mites. That's the read. Treat the hen and the coop — mites live in the cracks when they're not on the bird. Diatomaceous earth in the dust bath areas and a thorough coop clean will break the cycle.

You've got it. One turn at a time.