About 2-3 months ago, I picked up a couple of 5 month old hens to add to my flock from a friend of mine. My flock was 3 RIR's (hens), one of the chickens is very similar looking to a RIR, not sure exactly what it is. The other hens picked on her a little at first, but then things settled down. The other chicken I added is an Americauna, my 3 RIR's picked on her from the start and it hasn't stopped. She spends most of the days perched on the top roost outside, even in the pouring rain sometimes. When she goes down to the ground, they jump all over her. I think eventually they might kill her. Either she hasn't figured out where to lay her eggs, or when it is time, she just cant get in the coup out of fear, because her eggs will be on the ground outside.
Here is my plan. For one thing I don't really want to invest in adding a coup just for her. But, I thought of building a nesting box and mounting it on the roost where she spends most of her time. She could lay in there, get out of the rain, and maybe I could put a small feeder in there. None of the other chickens go up to that high roost, at least not often.