Skewed sex ratio in a northern wintering population of mallards

Abstract

The object of this study was to document the chronology of female disappearance in an isolated population of mallards (Anas platyrhynchos) relative to changes in the winter environment. A sex ratio biased towards males was formed at the onset of winter during both years of study, but during one environmentally harsh winter the proportion of females in the population, as well as the size of the population, continued to decline. Differential emigration at the onset of winter is probably the major factor creating biased sex ratios in most northern wintering duck populations, a bias that apparently can be exacerbated by adverse environmental conditions.