This has happened with me lately. We had 2 old hens and a rooster and took on 3 new rhode island reds and an australorpe~ all 5 months old. The rooster instantly loved the black austrlorpe, but the rir's he picks on and one rir that I think is maybe a month younger than the rest, he is relentless with. He will dive at her from across the pen running at her pecking and flogging... chasing her all around. We made a pen within the pen so that they have to see each other all the time, but the rooster can't get to her no matter how hard he may try. We put a mate in with her so she's not alone even if they are all right there almost together. I couldn't stand to watch them all pick on the youngest pullet. No way. Why in the world the old chickens and roo took to the black asustralorpe is such a mystery to me!