Most ducks of the North Temperate Zone mate in the winter, then the pair flies to their breeding grounds, where the female builds a nest and lays eggs. When she begins to incubate the eggs, the male deserts her, and there is no pair bond until the next fall, when the cycle begins again.
Ruddy Ducks (Oxyura jamaicensis) are quite different, and this might have been predicted, as they are the only northern ducks in which males molt in the spring into a bright breeding plumage, much as many shorebirds and passerines do. This is the rich red color that gives the species its name. The male bill, gray during winter, turns intense sky blue from structural changes.
There is no courtship or mating on the wintering grounds, but as soon as the birds arrive on their breeding grounds in spring, intense mating activity takes place. The males display actively at each other and at nearby females. The display consists of the male bumping its bill on its chest rapidly, creating bubbles in the water around it. This Bubbling Display is usually followed by a rush across the water in Display Flight, looking and sounding like a little motor boat. The loud noise accompanying the latter is made by the feet.
Male Ruddy Ducks are fiercely aggressive to one another and to other waterfowl when in mating mode. Females also can be very aggressive. Grebes and coots are not happy sharing ponds with Ruddy Ducks, and vice versa. They are not territorial, but the male stays very close to the female with which he is mated, even if only temporarily. Many males form a monogamous pair bond with a single female, some form pair bonds with a second and rarely a third female, and some appear to form no pair bonds at all, merely attempting to mate with any receptive female.
Now, about duck breeding. Most birds copulate by the male and female pressing the opening of their cloacas together, during which time sperm is transferred. But some male birds, including ducks, have a copulatory organ, the penis. This organ is not homologous to the mammalian or reptilian penis but is an erectile extension of the cloacal opening, as if a glove were turned inside out.
Ducks mate in the water, and cloacal appression might not be adequate for effective sperm transfer in the aquatic medium, so the evolution of a copulatory organ would have been an appropriate adaptation. The unequal sex ratio in ducks, with intense male competition for females, might also play a part in this adaptation. The penis is corkscrew-shaped, and the female vagina is similarly shaped in the opposite direction, so most mating attempts, especially between unmated birds in which the female does not cooperate, may be unsuccessful.
The Lake Duck (Oxyura vittata) of South America apparently has the longest penis in relation to its size of any vertebrate. It may be up to 40 cm, as long as the body. This species is closely related to the Ruddy Duck, which – as far as the present record books show – can develop a penis only about 25 cm long. A very interesting recent finding is that in a given wetland, only certain males develop the longest organs, and these are dominant to the others and the most successful breeders.
There is plenty more of interest about this odd duck. Ruddy Ducks lay the largest eggs with respect to body size of any duck, the average clutch of 7 eggs weighing about as much as the female herself. Thus the young hatch in a very precocial stage, grow quickly, and are abandoned by the female when about three weeks old, before they can fly. Speaking of flying, Ruddies are the poorest fliers among the ducks, rarely seen performing this activity. They apparently move around between wetlands and migrate entirely at night.
They are very highly adapted divers, with big feet that propel them under water, a compact body, and a long tail that may provide steering. They seem to be specialists on midge larvae, which make up the majority of their diet wherever studied. Because their prey is abundant and often evenly distributed, Ruddies can aggregate in large flocks that can find enough to eat even though there are many of them. They feed by diving to the bottom and slurping through the mud to strain out the larvae. They can feed actively for a while, fill up with midges, and then sleep, so our encouters are often with sleeping flocks.