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Citation. McKinney, F.; Derrickson, S. R.; Mineau, P. 1983. Forced copulation in waterfowl. Behavior 86:250-294. [1422 CC]
Summary. Most species of birds form pair-bonds and, until recently, it was assumed that the male always fertilizes eggs laid by his mate. Although extrapair copulations had been recorded in a number of monogamous species, most observers dismissed them as abnormal or exceptional events of no consequence in reproduction. In his discussion of how natural selection may be expected to act on the sexes, TRIVERS (1972) suggested that such extrapair copulations could be functional. He argued that, because sperm are less costly to produce than eggs, males may be expected to maximize their chances of leaving offspring by inseminating (and abandoning) as many females as possible. In monogamous species in which males provide parental care, TRIVERS predicted that "a mixed strategy will be the optimal male course--to help a single female raise young, while not passing up opportunities to mate with other females whom he will not aid." In spite of the enormous literature on breeding behaviour in birds, evidence for the existence of such mixed male strategies is scarce. Extrapair copulations have been reported as regular occurrences in several species belonging to distantly related families (e.g. albatrosses, herons, gulls, swallows, corvids) (GLADSTONE, 1979) and in some cases they are known to be made by paired males during periods when they could lead to fertilization of eggs (McKINNEY et al., in press). But most of these studies were carried out before the differences between male and female reproductive interests were fully appreciated, and observers probably failed to record or report data that could have been relevant to the question of paternity. New studies are needed, even on the best known species, and it will be some time before generalizations can be made about the incidence of mixed male strategies in monogamous birds. Forced extrapair copulations (often called "rapes") have been reported in the literature on waterfowl (family Anatidae) for at least 70 years. They are especially well known in certain dabbling ducks (tribe Anatini), species that form strong pair bonds and are generally considered to be monogamous. In this paper we review what is known about forced copulation in waterfowl and examine the possibility that some species have mixed male strategies of the kind envisaged by TRIVERS.