Chicken vent problem

[B and L]

Hi we have had our chickens for about a year now and everything has been great. We wintered them in northern Alberta and had very little issues. But this spring we noticed that a few (7 out of 30) had a scab on the top corner of their vent and are missing some feathers in the area. We read posts in this forum frequently and find most of our problems solved after some reading. We have done the epson salt soak and baths and seem to get this looking better. I have the a few separated to monitor them more closely but I am not sure what is causing it. My first thought is feather pecking which they haven't done at all to this point. We put some red stop picking paste we picked up at Peavy mart on these girls and are checking them but haven't seen this bloody at all. It is still cold below freezing most nights and I don't think that it is lice or mites. Though we did a pretty thorough check of the birds and coop. Not sure if this is molting, picking or what but they have a dime to nickel size scab on the top corner of their vent and and don't seem bothered by it. They are still laying eggs and as active as any of the other girls.
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