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The 12 species described in this volume are not closely related, but they provide an instructive example of adaptive evolutionary radiation within the much larger waterfowl lineage as to their divergent morphologies, life histories, and social behaviours. The whistling-ducks (Dendrocygna), with three known North American species, are notable for their permanent pair-bonds, extended biparental family care, and strong social cohesion. In contrast, males of the five typical pochards (Aythya) maintain monogamous pair-bonds only long enough to assure that the female's eggs are fertilized. The extreme of this behaviour exists among the stifftails (Oxyura). Such diverse reproductive strategies have exerted powerful evolutionary influences on interspecies variations in sexual dimorphism, sexual behaviour, anatomy, ecology, and other traits.