Did you know that not all male birds lose their reproductive organs during evolution? Some of which that still possess penises are ducks, especially mallards. The organs are usually 1.25 cm in length with some reaching 40 com in length.

The odd thing about these organs is that they are rather conflated with twists, pouches and dead ends. Quite interestingly, female ducks have also evolved "maze-like" vaginal tracks.

To understand the rationale behind this, you must be aware first how the social setup of ducks work. Male mallards usually rape females in a violent act which results at times with drowning and injuries. This occurs especially when female ducks arrive in a new and unknown breeding ground.

The females, on the other hand, fight back by preventing successful fertilization. Yale University reproductive biologist Patricia Brennan in Connecticut explained,

Females are trying to remain in control of which male sires her offspring. Because they have already established a pair bond when they arrive to the breeding grounds, they may be trying to avoid having their eggs fertilised by a male of unknown quality, that tries to pursue forced copulations.

It sounds kind of counter-intuitive, but if a female can resist insemination by a forced copulation, the end product is good quality offspring. This is because males that can successfully circumvent the female defence mechanisms are more likely to sire offspring, and pass that advantage on to their own young.