On the Possibility that Intercontinental Landbird Migrants Copulate En Route

birds undertake long-distance migrations between temperate breeding grounds and tropical wintering areas in the Caribbean Basin, Central America, and South America (Keast and Morton 1980). Although they spend over one-half of their annual cycle away from their breeding grounds, either en route or "wintering" in Neotropical areas, it is generally accepted that songbird migrants form pair bonds and copulate only after they arrive on their breeding grounds following spring migration (e.g. Nolan 1978). The occurrence of en-route copulations and perhaps even pair-bond formation prior to arrival on the breeding grounds has not been seriously considered by most ornithologists. If found, such behavior would have important implications for studying avian mating systems, testing assumptions and predictions of current sexual-selection models, and understanding the relationships among different phases of a migrant's annual cycle. This paper examines the possibility that copulations occur before migrants reach their North American breeding grounds. First, we address some of the obvious arguments against the premise that en-route copulations occur. We then describe some requirements, including anatomical and physiological, for copulations to have reproductive consequence. Finally, we discuss expectations and predictions if enroute copulations were to have reproductive consequences for male or female migrants. At the outset, it is important to distinguish between forming and maintaining a pair bond initiated during migration and en-route copulations. This paper does not directly address the former, but rather the general occurrence of en-route sexual behavior. We ask how likely is it that such behavior will have reproductive consequences, while assuming that a pair bond is formed with a mate shortly after individuals arrive on the breeding grounds (e.g. Nolan 1978). Arguments against copulating en route.--The possibility that migrants copulate before they arrive on their breeding grounds has seemed unlikely because males commonly precede females during spring migration (e.g. Ramos 1988, Yunick 1988, Chandler and Mulvihill 1990), which decreases the probability that a male would encounter a female en route. Whereas