Bald bottoms...more than one chicken

My biggest Australorp started losing feathers on her bottom last month. Yesterday, I noticed that my other Australorp, 3 of my BR, 1 of my GLW & 1 of my BO all have feather loss on their bottom/tummy under their vent. I have witnessed all the birds pecking each other's feathers before...there isn't one culprit. Don't think it's molting since it's just their bottoms, although I am finding a few have new feathers trying to pop out on their bare bottoms. They are a bit red/irritated where the feather loss is. It also looks a bit dry/rough.

I'm just at a loss as to what is going on. This is my first experience raising chickens.

They seem to all be in good health & are still laying. Poop looks normal. No signs of egg bound or vent gleet, I have noticed a few BO laying on the laid eggs if I don't get them fast enough during the day. But they go about their business after I grab them & don't seem "broody". We don't have a rooster. Do not see lice/mites on any of them. Went out last night to check for mites with a flashlight while they were roosting...nothing.

Location: Suburb of Portland, Oregon

Age: 8mo old (all of them)

Rooster: None

Breeds: (all hens)
Gold Laced Wyandotte (2)
Silver Laced Wyandotte (1)
Australorp (2)
Barred Rocks (3)
Buff Orpingtons (3)

Feed: They get Scratch & Peck Naturally Free Layer feed (7.5 cups), 1/2 cup of UltraEgg Omega supplement, herbs (1tsp each: dill, basil, oregano, parsley, garlic powder) each day

Water: I put Bragg's apple cider vinegar in their waterer

Treats: occasionally they get scrambled eggs, oatmeal, tomatoes, dried mealworms, black oil sunflower seeds, bananas, kale, cucumber

Coop/run: Their coop is 6x6 with a 20x6 fully covered run w/1/4" hardware cloth fence & 6x6 area under their coop. I'm using the deep litter method with straw inside their coop. In their run, I'm using straw, but need to figure something else out with all the rain in Oregon.

Roosts: My roosts are on the perimeter of my coop & make an "L" shape. They are all the same level. I'm not sure if they are being pecked at during the night or if they are doing it to themselves? (Wish I had electricity out there so I could have a camera.)


Any ideas would be greatly appreciated!

Here's one of my BR from yesterday. My Australorp is worse than this, but I didn't get a picture.

 

I had them in lockdown for a bit because of the Avian Flu (we are a few miles from a wetland preserve), but decided to let them out some. We do supervised free range because of predators (hawks, etc). They did not like being stuck in their coop/ run. I was thinking maybe boredom. I'll try the Blu-Kote & a few treats they have to work for.

Thanks for your suggestions!