Factors affecting the pairing chronologies of three species of Mergansers in southwest British Columbia

Most Northern Hemisphere ducks form pairs on the wintering grounds, and tim ing of pairing varies considerably among species and populations. Hypothese s to explain variation in pairing chronology utilize sex-specific cost-bene fit frameworks. These hypotheses have not been adequately tested, largely b ecause 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 se veral sites in southwest British Columbia to determine pairing chronology. Courtship displays of Wooded Mergansers occurred significantly earlier than those of Red-breasted Mergansers. Hooded Mergansers began to pair in mid-N ovember, Red-breasted 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 r esult of pairing, and females of larger species begin to accumulate endogen ous nutrients earlier. It could also occur if pairing exerts an energetic c ost oil males, such that males of smaller species are more prone to winter mortality as a result of fluctuations in energy demand. The much earlier pa iring of the smallest species (Hooded Mergansers) is inconsistent with eith er scenario. We suggest that between-species differences in pairing chronol ogy may be better explained by the constraints imposed by sexual segregatio n during winter.