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Pair-forming and copulatory displays of the four species of eiders are described and illustrated, based on observations of captive and wild specimens. Behavioral differences indicate that two genera (Polysticta and Somateria) of eiders should be recognized, and that Lamponetta should be considered a synonym of Somateria. Racial differences in the displays of Somateria mollissima mollissima and S. m. v. nigra are explained on the basis of sympatry between the latter and S. fischeri, producing selection for display divergence that has not affected S. m. mollissima. Similarities in the behavior patterns of the eiders and the scoters suggest that the eiders are directly related to the scoters and other sea ducks, probably through such intermediate forms as Camptorhynthus, Clangula, and Histrionicus, for which the behavior patterns are largely or entirely unknown.