..ducks are one of the few birds that still have a penis. It's a very weird structure. It has an explosive erection, its erection mechanisms are lymphatic instead of vascular, it's stored outside-in inside the cloaca and comes flying out, and they can get very lengthy—up to 40 centimeters, which is over a foot long on a duck that is itself not even a foot long. It's an extraordinary piece of biology. What's going on in these ducks?
In lots of ducks there's forced copulation. It's the equivalent of rape in ducks. In species where there is a lot of forced copulation, females have evolved or co-evolved complex vaginal morphologies that frustrate the intromission or frustrate entry of the penis during forced copulation. For example, the penis of ducks is counter-clockwise coiled and often has ridges or even teeth-like structures on the outside. In this case, in these species, the female has evolved a vagina that has dead end cul-de-sacs, so that if the penis goes down the wrong direction it’ll get bottled up and doesn’t perceive to be closer to the oviduct, or further up the oviduct and closer to ova. And then above the cul-de-sacs, the duck vagina has clockwise coils, so there's literally anti-screw devices that prevent intromission during forced copulation.