Certainly there are individuals who are exceptions, but in general there's an undeniable correlation between testosterone levels and aggression...and not just in ducks :)

Our two male muskovies scuffle and wrestle constantly but don't seem to damage each other except for broken feathers.

My females fight also. They have to get the pecking order sorted out, after all. There is alot of feather pulling in a normal day. But when they really do fight it is with their wings and necks against the other duck, really a wrestling match. Makes a mess all over the place. Knocks things over get tangled in tools.

I also saw my oldest female take on the rooster last year. He got the worst of it, until he used his spurs against her. She seemed pretty darn angry over something. Probably protecting ducklings. It was a pretty strange to see it.