Although I am the world's second worst birdwatcher*, I can tell my Anas from my eider. I'm not sure which came first—a fondness for ducks or the ability to tell them apart—but one inevitably led to the other. Whenever ducks are around, I have my binoculars glued to one eye and the camera to the other**.
Blue-billed ducks (Oxyura australis) are among my favourites. They're pocket-sized birds with tails that look as though someone's shut them in the car door. They're closely related to the stifftails (ruddy ducks, white-headed ducks and others), all of which are characterised by the sparsely-feathered fan-shaped tail that's either laid flat on the water's surface or held vertically on display. (Or at 45 degrees. Darned birds not posing properly!)
These ducks are also known for another characteristic — a penis of prodigious proportions. Kevin McCracken produced a paper on the drakehood of the Argentinian blue-billed duck (O. vittata). He speculated not only on the reason for its length (at more than 30 cm, it's almost as long as its owner) but also for its weird shape (coiled with spines and bristles). It's probably best that you read the original articles from Auk and Nature (both PDF) because I'm struggling to hold back a torrent of puns. And the occasional unfortunate typo.
* The world's worst birdwatcher is a friend who not only mistook a mooring buoy for a pelican but also confidently identified a little pied cormorant among a group of little black cormorants. What was wrong with that? Well, when the white-fronted bird moved, it became obvious that it was, after all, just another LBC ... and the white was a streak of cormorant shit on the rock.
** Apologies for any mental images of Marty Feldman generated by that moment of hyperbole