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In recent years an increased interest in the use of behavioral characteristics in evolutionary studies has developed, and this is particularly true in the case of waterfowl. The classical studies of Heinroth (1911), who was one of the first to apply knowledge of waterfowl behavior to systematics, have been elaborated on by Lorenz (1941; 1951-1953) in his important contribution toward the understanding of relationships in the Anatinae. These, and other, studies have stressed the qualitative behavioral differences occurring among different species as providing possible isolating mechanisms through their presumed function of conveying species-specific recognition signals. To the present, no extensive quantitative studies of the behavior of very closely related forms of waterfowl have been undertaken, although Dr. D. F. McKinney's still uncompleted studies on the races of the Common Eider (Somateria mollissima) will provide an important contribution in this field. By studying the reproductive behavior of such closely related forms the evolution of isolating mechanisms can be fruitfully studied in their early stages and thus provide an insight into the general process of speciation. As part of a more general study (Johnsgard, 1959) concerning the evolutionary relationships between the Mallard (Anas platyrhynchos), the Black Duck (Anas rubripes), and other closely related forms, behavorial characteristics were utilized as taxonomic characters. The results of this general study, which will be published later, indicate that the Black Duck is much more closely related to the Mallard than is generally supposed and that the two forms should probably be considered to be only subspecifically distinct. The purpose of the present paper is to summarize the quantitative aspects of the behavioral studies and to discuss their probable significance in terms of (1) the evolution of behavioral isolating mechanisms; (2) the relative importance of display and plumage in species-recognition signals of these birds; and (3) the concepts of response specificity and response thresholds, or "drive."