Sexual selection can operate throughout the annual cycle and likely shapes the winter plumage and courtship displays of many northern waterfowl that choose mates during winter. Less conspicuous effects of sexual selection are likely and in this study I asked whether winter distribution patterns and grouping behavior of Harlequin Ducks (Histrionicus histrionicus) are partially shaped by sexual selection. Harlequin Ducks are typically dispersed in small groups and observed grouping behavior supported the hypothesis that unpaired adult and immature birds will show sexually-selected changes in their spacing to facilitate courtship and mate sampling. Unpaired birds occurred in larger groups than paired birds during October-February, and group-related differences in the sex ratio and in the proportion of females that were unpaired indicated that unpaired birds were aggregating specifically for courtship. Behavior similar to lekking was observed at one site. Males gathered at this site at daybreak, unpaired females visited the site each apparently to attract a group of courting males, and these courting groups left the site without feeding. When herring spawn was available in March, unpaired birds were more likely to move to exploit it and gained both direct nutritional benefits as well as indirect benefits related to changes in time budgets and spacing behavior that facilitated courtship and mate sampling. Overall, results suggest that sexually-selected behaviors that affect the process of mate choice and the timing of pairing are important to consider if we are trying to explain winter spacing patterns of waterfowl.
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Vol. 29 • No. 4