My flock consists of 5 females and one drake. The drake is only 8 months old, along with one of the ducks. The other ducks are 3 years old. The drake is a fawn and white runner, so pretty small. He keeps trying to mate with the head duck, who is an older pekin. She tolerates his attempts at mating for a little bit, but the size difference seems difficult for him and he can't get the job done, and she gets tired of his attempts. In the meantime, he ignores smaller ducks who are in the pond with him and seem to be showing signs of availability to mate. Then if he doesn't get the deed done he is a big bully to her. He fights with her face to face and chases her in a way that is intimidating to her. Today she's limping, and I'm concerned that it's because he was chasing her and she hurt herself. Why do drakes choose one female and behave in such a relentless way? He has mated with the other ducks, but he keeps bullying the main pekin, and I'm having to now separate him to keep her safe.