Basically:

1. Ducks have Corkscrew Penises and Labyrinthine Vaginas.
2. Ducks reproduce through violent acts of hardcore gang-rape.

That is all.

Most birds don't have penises (one of several water conservation adaptations), so ducks picked up the slack by having some of the most nightmarish ones.

The lake duck of South America has one that can grow up to 17 inches, longer than the actual bird. They use it as a lasso/grappling hook in courtship.