.....
No mites...she may be being picked/pecked on, she is the smaller of our flock.
I believe I only have pullets so there shouldn't be any mating going on.
We feed DuMor Grower/Finisher & Chicken Scratch; as for how much...we just put it in the feeder and let them eat as they please.
There are feathers in the coop, but not as many as before and no poop stuck to feathers, we check them pretty regularly for that.
Do the feathers grow back?
Do you know the Ratio of scratch to grower? You NEED to eliminate that scratch and make it an occasional treat totaling NOT more than 10% of your total ration... Or you WILL have feather picking issues that might lead to cannibalism as well as general health and laying issues. Scratch is a TREAT and not a source of whole nutrients. Feed, though made from corn mostly, has been fortified to meet the needs of the chickens. Yes they LOVE scratch because it is like a candy bar to them. Also, I wonder why people are so crazy about scratch when it's just as pricey as feed? Eating as they please should be fine as long as nutrients aren't diminished by scratch (usually 8% protein). So if you are doing say 50/50 and your grower/finisher is 18% (guessing just for example) Than that would be 18+8 divided by 2= 13% total protein. Ok, well I just looked it up and it's actually 15%... So 15+8=23 divided by 2=
11.5% total protein. I don't know the ratio you are mixing, but in my opinion this IS your problem. Gosh, I'm appalled that they sell grower with that low of value...
Feathers are made from 90% protein, or the amino acids that make them up. A hens' protein level should NEVER be less than 16% for light bodies like leghorn and 18% is preferred for heavier breeds like rocks.
While it's mostly true feathers won't grow back until molt, if they were completely pulled out and not just broken... I believe they will.
Regarding mites... I'm guessing you don't actually know, no rudeness intended... There are so many different types some are microscopic so you won't see even though you look and others don't live on the bird but only come out to feed at night, so again unless checked at the right time, you won't know. And there are 3 types of poultry lice (that I'm familiar with). Head, body, and feather. So they might not be noticeable at all places on the body.
Many use and are happy with Dumor. I also have found plenty that think their birds don't do as well on Dumor as other products, maybe a consideration for you. But I have a feeling eliminating or decreasing scratch will make a huge difference in you flock health.
Another question I didn't realize wasn't yet addressed is space. How much roost, coop, run/free range space do you have and for how many birds? Do you keep artificial light on them at night?
One other possibility that can cause vent balding though not my current suspicion is vent gleet/yeast infection.
And you haven't noticed any severe bullying directed at the leghorn? Does she cower down and run away from the other birds? Usually bullying doesn't stop just because you are watching.