Many birds species exhibit a resource based mating system where males defend breeding territories. This mating system is thought to have lead to male-biased philopatry as male birds are better able to defend familiar territories. In contrast, female waterfowl are more likely than males to return to their breeding grounds. Waterfowl are the exception because males cannot economically defend breeding territories and a mate-defense system has evolved. However, waterfowl pair in the winter. If male waterfowl can defend winter territories a resource defense mating system may evolve and, consequently, male-biased philopatry to the wintering grounds. Since August 1994 I have studied a small population of wintering Harlequin Ducks (Histrionicus histrionicus) in coastal southwestern British Columbia. Locally, a total of 122 individuals have been caught during the wing molt and marked with individually identifiable tarsal leg bands. Weekly surveys were conducted to assess population size, sex ratio, age structure and as many bands as possible were read. The local population size and sex ratio fluctuated considerably over the two winter seasons. An influx of young males and unpaired males occurred in April of each year, before departure to the breeding grounds. Males moved around the study site more than females. Unpaired males moved around the study site more than paired males. Return rates from one season to the next were high (males: 72%, females: 66%) and did not differ between the sexes. Pairing occurred early in the winter (November) and pair re- formation occurred in 4 pairs where both members returned to the study area. Harlequin Ducks appear to have a mate-defense mating system and male- biased winter philopatry would not be expected.