Just in the last couple weeks my duck has became real aggressive toward my hens. He has been in with them since they were babies. But now its like he is trying to breed the hens. He gets on top of the hens and won't let them up and two of my black hens are now missing feathers on their back. They are bald on their backs now. He only seems to pick on the two black ones. I separated them last night but have got to find a better way to separate. Does anyone know what is going on with my duck?