Red butts

[chickbetts]

We have 7 Buff Orpington chickens. Just a year old now. We have 3 of them with really red butts and missing all the feather. I have included a photo. I have tried to look with a flashlight around their vents and don't see any signs of mites or lice. What could be happening? They seem healthy otherwise and acting normal. Appreciate your help.

 

[Roo5]

Looks like their picking each other’s feathers out aka bullying.