The stiff-tailed ducks are part of the Oxyurinae subfamily of ducks. All have, as their name implies, long stiff tail feathers, which are erected when the bird is at rest. All have relatively large swollen bills.
These are freshwater diving ducks. Their legs are set far back, making them awkward on land, so they rarely leave the water.
Their unusual displays involve drumming noises from inflatable throat-sacs, head throwing, and erecting short crests.
Plumage sequences are complicated, and aging difficult.
A fossil species from the Late Pliocene or Early Pleistocene of Jalisco (Mexico) was described as Oxyura zapatanima. It resembled a small Ruddy Duck or, even more, Argentine Blue-bill. A larger Middle Pleistocene fossil form from the southwestern USA was described as Oxyura bessomi; it was probably quite close to the Ruddy Duck.