A few years ago, a study explored the strange genitals of waterfowl and found that females typically had complex vaginas if males of the same species had complex phalluses. Waterbirds are among the few bird species that have phalluses at all. Duck species in the Anas, Clangula, and Oxytura genera take this to an extreme, with very long, twisting genitals, sometimes equipped with spines. Phalluses twist counterclockwise, and vaginas twist clockwise. (You can see some examples at the post linked above.) Other duck species have shorter and simpler reproductive organs.
Adding such complexity into the reproductive tract seems odd at first glance, since it would seem to impede fertilization. The research team, led by Patricia Brennan, hypothesized that female ducks evolved complex vaginas to counteract forced copulation, a behavior that is well-documented in waterfowl. (See here and this infamous case.) A twisting vagina could give females more control over which males fertilized their eggs.
The same research team is back with more evidence that female ducks evolved vaginas to deter forced insemination. To test their hypothesis, the researchers studied domesticated muscovy ducks at a farm in California. They placed male ducks in a cage to mate with a female but made the males ejaculate into a glass tube instead. A total of 56 males were videotaped in the process of erection and ejaculation. The male ducks at the farm had been trained to provide semen for commercial breeding and were thus amenable to the testing procedure.
Male ducks are unusual not only in having phalluses but also in erecting them very quickly only after mounting a female. (The authors refer to this as "explosive eversion.") A phallus then curves as it erects to match the shape of the receiving vagina or, in the case of this study, the glass tubes. The researchers tested three different types of tubes. The first type, as a control, was a straight glass tube; the second type had a counterclockwise twist, just like a phallus. Neither of these presented an obstacle to erection. The third type imitated the shapes of vaginas, with either a clockwise twist or a 135° bend. The latter tubes either blocked phalluses at the entrance or caused them to erect in the wrong direction. You can see video of erections into the various tube shapes as a data supplement to the paper.
Males encountering a clockwise or bent tube shape could still ejaculate. However, if they were copulating with an unreceptive female duck, their sperm would be left near the entrance of the reproductive tract where they would be less likely to cause fertilization. Willing females, on the other hand, may increase the chances of fertilization by adopting a receptive posture and contracting and relaxing their cloacal muscles to allow easier erection. In that way, a female duck could still produce offspring with the male of her choice, not just whichever male happened to be the most aggressive.
Brennan, P., Clark, C., & Prum, R. (2009). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.2139
Brennan, P., Prum, R., McCracken, K., Sorenson, M., Wilson, R., & Birkhead, T. (2007). Coevolution of Male and Female Genital Morphology in Waterfowl PLoS ONE, 2 (5) DOI: 10.1371/journal.pone.0000418