Just because many pullets will skip the molt their first winter doesn't mean all do. They can molt at any time. But that is not molting. Something else is causing that. With that bare spot on that barred one you do need to figure it out.
How did you check for evidence of mites? Roost mites hide in dark cracks and crevices during the day and attack at night in the dark. You need to check for them at night. Still, treating them as 21 mentioned won't hurt and may help. Treat all chickens, clean out the coop, and treat the coop. Then do it again a week to ten days later.
I don't know what is causing that. The bare spot on the barred one looks like feather picking to me. I had a hen do that to a rooster's neck one time. He just stood there as she plucked away. I locked him up for a couple of days and she quit when I released him. I don't know why she did that, they had plenty of room and good feed. She might have been grooming him and gotten carried away.
That other might be feather picking but looks more like mechanical damage to me. Are they laying or roosting somewhere that wears on their feathers? You don't have a rooster, but as 21 mentioned sometimes things can happen when they sort out the pecking order. Some hens have brittle feathers, they easily break. That's typically because if nutrients, I can't remember which nutrients and is normally about how their bodies handle those nutrients more than the nutrients not being present. That's why it affects some and not others. They are the right age to sort our pecking order as they each individually reach maturity.
Has this just started? If it is feather picking, and it could be, a big cause of all kinds of behavioral problems is lack of room. You are in Connecticut, thanks for providing that information. Has this coincided with them being cooped up in a restricted space with a weather change or something else?
I'd have never thought of rats or mice but who's to argue with someone else's experience. You might try setting some traps where chickens and other critters can't get to them to see what happens.
I don't know how much affect what they eat has to do with feather picking. Some people really believe it does, I'm not sure. I don't know how you are feeding them or how much room they have available. That makes it a little hard to come up with specific suggestions.
If they are relatively confined, can you give them more room or improve the quality of their space by giving them clutter they can hide under, behind, or over the others? You might try giving them a little BOSS. That's Black Oil Sunflower Seeds. BOSS is relatively high in protein so that might address that issue if it is a problem, but more important to me the Oil in BOSS can soften their feathers, make them look better and less easy to break. If you scatter BOSS in the coop or run they will scratch for it, giving them something to do.
I don't like that bare spot, that could be dangerous. The other stuff doesn't bother me that much as I don't think it's that dangerous but yeah, you want to correct it if you can.
Good luck!
