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This bizarre group of diving ducks differs from the rest of the Anatidae in so many respects that by any standard it demands special attention. Of the eight species that are presently recognized, most are placed in the genus Oxyura, which name refers to the stiffened, elongated tail feathers typical of the group. In these species the tail feathers extend well beyond the rather short tail coverts and are usually narrow-vaned, so that the individual rectrices tend to separate when spread. The feet are unusually large, and the legs are placed farther to the rear of the body than in any other waterfowl tribe, increasing propulsion efficiency during diving but rendering the birds nearly helpless on land. This grebe-like adaptation is paralleled by the evolution of numerous short, glossy body feathers, presumably increasing the effectiveness of waterproofing. In the typical stiff-tails the bill is rather short, broad, and distinctly flattened toward the tip, and virtually all the foraging is done under water. At least in the North American species of stiff-tails, most of the food taken is of vegetable origin. Nests of the typical stiff-tails are built above water, of reed mats or similar vegetation, and often a ramp leads from the nest cup to the water, providing easy access. The birds are quite heavy-bodied and have relatively short wings, so that flight is attained with some difficulty in most species. The masked duck is something of an exception to this point, since its combination of small body size and fairly long wings allows it to land and take off with surprising agility from water of moderate depth.
Only two species of stiff-tails have ever been reported from North America, and it is most unlikely that any others will ever occur here by natural means. The ruddy duck is much the more widespread and abundant of these, while the little-known masked duck barely reaches the Mexico-United States border as a breeding species. Indeed, the masked duck is the species most recently added to the list of known breeding North American waterfowl, since it was not until 1967 that firm evidence of its breeding in Texas was established.