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The stiff-tailed ducks constitute a unique section of the Anatidae that is possibly the most isolated of all the tribes with the exception of the Anseranatini. There are eight species which almost certainly belong in the group, plus one more that is only very tentatively included. The tribe is of worldwide occurrence. Seven of the species have long, narrow, and stiffened tail feathers that function as rudders in underwater swimming, at which all species are very adept. These species also have a dense and shiny body plumage much like that of grebes, but lack metallic coloration altogether. The typical species have short, thick necks with loose-fitting skin that can be expanded through the inflation of the esophagus or special air sacs. All species have large feet and their legs are placed well toward the rear, which results in a poor walking ability. All species but the most aberrant one (the white-backed duck) exhibit sexual dimorphism. Vocalizations are extremely variable, and male calls are often produced by extra-tracheal structures. Sexual maturity is probably reached in the first year in all but one of the species (the musk duck), and pairs are probably renewed yearly in most and quite possibly all species. Male displays are generally elaborate and tend to produce sound. Nests are generally built over the water surface and the eggs are generally chalky white and relatively large, the young being very precocial at hatching.