Huh... I've never seen that before. Your roost doesn't look like it is super conducive to feather pecking in that area. Have you ever seen your head hen mount her?
Maybe the condition of her feathers is in large part due to standard wear and tear... Are you getting eggs from her still?
Some parasites are microscopic and can live inside the feather shaft and will never be seen by the naked eye (though depluming mites are not that common, just sharing information). Some live in stumps or wood cracks like under the roost and only come out to feed on the birds, so you may not see those during a standard inspection either.
Sorry for being intrusive... just looking for clues. Can you post a pic of her lovely vent area?

To me feathers with their shaft exposed is a SYMPTOM of parasites or maybe over preening... which is another symptom by itself... but I don't know what constitutes "over" preening... I am also here to learn, thank you for sharing.
