Drakes are hard on ducks. They're also gang-rapers. In my experience, you need at least two girls per boy so that the drakes don't rough the ducks up too much. I like a healthy ratio of 1:4 if I only have one male and 1:5 if I have more than one male.
Separate the drakes if you possibly can. They have big, cork-screwing penises and hens really are not meant to take that.

Separate the drakes if you possibly can. They have big, cork-screwing penises and hens really are not meant to take that.