Sexual displays of the Musk Duck and Blue-billed Duck are described and illustrated. The displays of male Musk Ducks comprise a series of three forms exhibiting increasing ritualization, complexity, and time-interval constancy. All of them have conspicuous auditory characteristics as well as variously conspicuous visual features. Displays in the species appear to have evolved under the influence of intense sexual selection resulting from what is probably a more completely promiscuous mating system than occurs in any other species of Anatidae. These selective pressures have also probably promoted the evolution of such features as large size and extreme sexual dimorphism that distinguish the genus Biziura from the typical stiff-tails (Oxyura). Displays in the Blue-billed Duck likewise embody a combination of auditory and visual characteristics, and include a large number of variably ritualized postures, several of which are clearly derived from comfort movements or intention movements. Certain similarities between the major displays of the Blue-billed Duck and the North American Ruddy Duck are thought to be the result of convergence, and a close relationship between these species is not indicated.
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