I don't think it's the hawk. I have those also.. but my birds hardly notice their presence since they are sitting there silently waiting to ambush. So the girls notice when they fly into the tree but quickly get distracted and go about their business.
It definitely could be influenced by the dirt... maybe the lighter girls are dirt bathing more? Also, YES... feathers do lighten sometimes with molt and get more so every year... but since all are individuals it may not be exactly the same amount. If you were actively breeding, these would be the things you would be selecting for. The less shiny gals may not be absorbing nutrients exactly the same as they did when they were younger.
The balding is definitely not normal and not molting... It does appear as though it could be picking by others or maybe even just excess cleaning/preening.
Unfortunately, depluming mites are microscopic and not visible to the naked eye. They will never be seen, only their evidence. I'm just mentioning this as a possibility... though they aren't super common... They can be treated using pour on Ivermectin... which is not currently approved for use in poultry and does not have a lot of information on egg withdrawal requirements. Topical treatments that don't become systemic such as Permethrin, my usual preferred treatment for lice or mites won't work well for deplumning or scaly leg mites. Also, some mites like Red... only come out to feed on the birds and don't actually live on them. The best time to get a real picture of parasite activity is after they have gone to roost at night and using a flashlight part feathers right below the vent and look for things running away form the light. Also take note if what you see is typical like pink colored skin or is it angry and red? Pink is good, red means something is off.
What you have, *looks* like flock antics... to me. Maybe get a look at who roost where and kinda pay attention to their behavior to see if you can come up with any suspects.
Ha, my hubby's Korean birth name is also Hyun, but he goes by John since he came to the US when he was 12.
One more possibility... if there was a small brawl or short period that had the feather picking going on... AND it HAS ceased... it's still possible the feathers would not grow back in until the next molt... even if the problem no longer exists.
That isn't "typical" of parasite damage I have seen. I do learn something new all the time and see things I would never have expected.