During the breeding season, which can last anywhere from April to September for domestic ducks, you are going to most likely see some aggression. The ratio of drakes to hens is very important. We've managed with a 2:5 ratio, but we currently have a 1:6. We keep only silver Appleyard ducks.
Even with one drake and six hens, it takes observation and management during breeding season. Our drake will usually pick one or two hens that are his favorites, and they will be fine together. He will also usually pick one or two that he really picks on (e.g., nips, runs them off, chases them down and stands on them (not mating), etc.). During mating season, we will usually keep the flock in two groups during the day (separated in two pastures by a wire fence so they can interact), but we will constantly mix those groups up just making sure the drake and the hen(s) he's picking on are separate.
At night, our drake has his "special fort" (also called "the hole" if he is being a turd). His special fort is in the run but separated from the rest of the flock by a wire fence. Sometimes his best girl spends the night with him and sometimes he's alone, but the girls will usually sleep beside him just on the opposite side of the fence. We do let them all free range together at least twice a day, but we keep an eye on them.
I'm not sure if that helps, but that's the way we deal with it. Even though it is kind of a pain, we think it's worth it to have a drake with the flock. Our drake (Angus) is also often a really good boy, and he looks out for the flock. Also, if you want ducklings... ;-)
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