Factors Affecting the Pairing Chronologies of Three Species of Mergansers in Southwest British Columbia

Abstract

-Most Northern Hemisphere ducks form pairs on the wintering grounds, and timing of pairing varies considerably among species and populations. Hypotheses to explain variation in pairing chronology utilize sex-specific cost-benefit frameworks. These hypotheses have not been adequately tested, largely because of inadequate sample size of pairing chronologies, especially within the tribe Mergini. Wintering Common (Mergus merganser), Hooded (Lophodytes cucullatus), and Red-breasted mergansers (M. serratus) were observed at several sites in southwest British Columbia to determine pairing chronology. Courtship displays of Hooded Mergansers occurred significantly earlier than those of Red-breasted Mergansers. Hooded Mergansers began to pair in mid-November, Redbreasted Mergansers in early February, and Common Mergansers in late February. We tested the prediction that the largest species of ducks pair earliest. This could occur if females derive energetic benefits as a result of pairing, and females of larger species begin to accumulate endogenous nutrients earlier. It could also occur if pairing exerts an energetic cost on males, such that males of smaller species are more prone to winter mortality as a result of fluctuations in energy demand. The much earlier pairing of the smallest species (Hooded Mergansers) is inconsistent with either scenario. We suggest that between-species differences in pairing chronology may be better explained by the constraints imposed by sexual segregation during winter. Received 29 March 1999, accepted 29June 1999.