Flock losing feathers under legs only

Hi all. I have a flock of 6 (Orpington, Barred Rock, Austrolop and RIR, 2 silkies). The silkies are fine, but the other 4 are losing feathers under their legs only. Quite large bare areas, all the fluff is gone. The skin is not irritated at all. I cannot find mites or lice. I have read article upon article and nothing describes my situation.

They are happy, eating, drinking, not molting. Free range organic. No diet changes. Good protein sources. Not broody, not pecking at each other. Good normal egg production.

ANY ideas!?

Do you mean on their abdomen, or around the vent, or what?

Sometimes when it's the underside, it's from resting somewhere that rubs them off, maybe on a roll of fencewire or some such. Broodies will pluck their "belly" feathers, that is, the area that comes in direct contact with the eggs, but it would be odd iff they were all working on going broody at once. Feather picking is the usual cause of feather loss when it's not molting (which should start around the head and neck, anyway.) Feather picking has 2 common causes: lack of space and too little protein. Since they are free range, that kind of leaves protein. If you feed an organic feed, it probably has no animal protein in it, so I'd suggest trying that -- whether from table scraps, mealworms, canned mackerel, whatever. You could also supplement with some game bird food, which has higher (vegetable) protein. Be aware (if you're not) that birds are lactose intolerant, so milk products are not a good idea, at least not in any quantity. Fish meal is sometimes added to feed and is an excellent source of animal protein.