By now, you should know for sure if the rouen (or was it a mallard?) is a male or female. He/She should have its colors and curled tail feathers. Also, if it quacks, it's a female. If it has a whispery, quiet quack or grunt, it's a male.

IF it is a male, then this is a display of dominance. It's not unusual. My hens do this to each other. My drakes (when I had more than one) did as well.