I call this the butt AKA bum …and have experience with this. Worried me tremendously until I found my answer.
According to the
Chicken Health for Dummies book the most probable cause of vent area feather loss is pecking by flock mates.
If all of yours have this, then there's more than one doing it. But look for who still has a fluffy bottom and bet they are the feather pecker. If the feather follicles are intact, they'll get new fluff when they molt...
MY STORY:
Last year, 5 of my 6 hens got bare bottoms, started where yours has and most also lost feathers around the vents. This occurred not long after they all started laying eggs. None had lice, no mites, but I treated them anyway. I searched for answers and advice everywhere. There are a lot of folks who see the same thing, and many have different opinions. But all evidence kept pointing to a feather pecker.
When my hens started molting last month, I was so excited to see new feather growth on behinds. And that's when I saw the only one who never lost her big fluffy butt pecking the other's new quills and feathers. She is not aggressive, just does it. Mystery for me was solved.
Most will tell you feather pecking is due to stress or over crowding, but I doubled my coop/run space this year, they have a luxury of space. They are very happy, spoiled hens. The don't fight unless one is broody, and get along well. Guess my Big Buff just likes to pull feathers.
Hope this helps.