There's got to be clear advantage, or at least no strong disadvantage across the morphological choices they are constrained to, in the forms. Isabella Rossolini aside, there's no dance here which is pure beauty, its form-follows-function all the way.
Ducks are savage. York University where I studied, in the 70s had a huge shallow lake and it had a regrettable sexual population imbalance which caused fatalities. Randy drakes drowned the decreasing number of ducks. I think they had to do culling to fix it. You think nature would right itself, but in human time for small local populations sometimes it doesn't do a good job.