Rouen duckling bites younger ducks and chicks

[chickenducker]

We've got a Rouen duck that's about 4 weeks old. A couple times since we've had it, we've gotten some new chicks and ducklings and put them in the same brooder. Each of these times, even when the Rouen was only a week or so old, it bites the younger chicks and ducklings pretty hard. We've had to separate the babies until they get old enough to defend themselves because the Rouen will hurt them. Once combined, the biting only lasts about 2 or 3 days, then they all cuddle up together and act as equals. I'm guessing it's a pecking order thing but does anyone know for sure why they do this? Is it gender, breed, or just a duck thing? Thanks for any help!