Thanks for that information. I don't have any clear answers for you. No roosters, all mature. and all the same age. Looks like a decent amount of space. All of my easy answers are gone.
I assume they are all acting healthy? Sometimes the flock will try to drive a sick or injured hen from the flock. That's a survival instinct from before they were domesticated. A sick or injured chicken might attract predators so they run out any sick or injured.
Was she previously the dominant hen? Sometimes when there is a change in the pecking order and the dominant hen is deposed, some others gang up on her to keep her from reclaiming the dominant position.
Was she somehow injured with blood on her head. Just some type of accident. Sometimes the sight of blood can turn some of the others into cannibals.
That's all I can come up with and none of these are clearcut.
So what can you do? Heal her up so there is no blood or wound and try to reintegrate her. Watch carefully to see how it goes.
Can you identify one specific chicken as a ringleader in the attack? Sometimes isolating a troublemaker from the flock for a week or so can change her attitude.
Create a separate flock with their own coop and run. See if a couple of the other hens can live with her peacefully.
Get rid of her. If the rest of the flock gets along great with her gone then she may be the problem. It sounds harsh but I try to solve for the peace of the flock, not to benefit of one specific chicken.
