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 behaviors. 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 behavior exists among the stifftails (Oxyura). Such diverse reproductive strategies have exerted powerful evolutionary influences on interspecies variations in sexual dimorphism, sexual behavior, anatomy, ecology, and other traits. This volume includes more than 63,000 words, plus some 200 maps, photos, drawings, and sketches, and nearly 650 literature citations.