Argentine Lake Duck: Very Well-Endowed, Can Even Lasso a Female.
The Argentine lake duck may be small, but don’t take pity on it. See, the drake (male duck) of the lowly fowl has the longest penis of any bird species in the world.
From head to tail, the Argentine lake duck measures about 17 inches. That also happens to be the length of its corkscrew-shaped penis when stretched out. The tip of the penis is soft and brush-like, which the drake uses to brush away sperms deposited by a previous suitor.
University of Alaska Kevin McCracken explains that the ducks are promiscuous, and the long penis may be an evolutionary adaptation for the males to become more attractive to the females. That, and the drake also uses his penis to "lasso" a female who tries to escape from it
Associated Files: PDF2/auk-117-820.pdf