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I would say you're right, he was asserting his dominance, we have 3 males here and one duck and the poor grl gets ravaged quite frequently! To the point where it looks like she may drown when it happens in the pond.
They either let them get on with it, keep them separate for ever, probably not practical, or serve the make duck with orang esauce!

I've never introduced new ones as I just have my ones that I got all at the same time and they are all girls....... however they do bonk each other in the pond and it's quite a sight to see, and yes the one underneath does look like it is getting drowned! But afterwards they all seem to have had a very good time.