About a week ago my rooster singled out one of my hens. He keeps her in the coop not letting her out to eat or drink. When she comes out he chases her back in and attacks her. She was a little pecked up around her wings so we separated her from the flock. He was aggressive to all the hens but particularly to her. These chickens have been together for a year and a half with no issues. My friend suggested I reintroduce her to the flock and separate him for a few days. A little time out so to speak. There are only four hens and the rooster so I need to restore some peace to the group especially with winter coming. Any suggestions?