Feather loss at tail

[Dmm9979]

Two of my hens are showing feather loss near their tail feathers. I have attached pictures of the worst one. Have 9 hens (no rooster) that Ive had for over a year and have never observed aggressive behavior. They free range in the backyard a third of the time, two thirds of the time they are in the coop/run. Also haven't noticed them pick at their back feathers. Any thoughts?

 

[sourland]

Another bird or birds is/are feather plucking them. Probably the bird in the flock with the most perfect feather condition.

 

[Dmm9979]

Thanks that's what I was thinking just haven't been able to catch them in the act. I assume the aggression is occurring in the coop. Trying to let them free range as much as possible since it seems the extra space helps.

We have 2 different breeds - all of the barred rocks have flawless feathers from what I can tell. 2-3 of the reds have noticeable damage to varying degrees. Would one breed gang up on the other?

Any suggestions on how to catch the culprit and curbing the behavior?