3 chickens are missing feathers just under vent

[bluebird2009]

I have 4 chickens. All healthy. I just noticed today that 3 of them have bald rear ends ( near vent on underside of chicken). I checked all their vents they look normal. They are eating normally. They Seem to be drinking a lot of water. They are all laying eggs. Even in the cold weather I am getting 4 eggs every other day with 1 or 2 eggs on days in between.
I looked at each of them today and I do not see any mites/lice or eggs anywhere on them.
Are they pecking themselves? Are they pecking each other? The one lowest in the pecking order does have more feathers missing then the other 2. None of them have raw soars but two of them have some scabs that are healing.
Are they bored? They are all about 9 months old. They have a big run but they do not seem to use it unless I put feed out there then they go back into coop. They like to run around backyard but I have not been letting them because they poop everywhere. Not sure what to do.

 

[nurse_turtle]

What size coop/run do you have? Sounds like feather picking on one another.
Maybe consider closing the coop when you let them out into the run each day.

 

[carlislechicks]

Did anything come of this discussion? I just found one chicken with a bare bum under the vent area, and am not sure what I should do to help her heal. Will the feathers grow back? What should I do to help her?