The effect of the temperature on local differences in the sex ratio of Mallards Anas platyrhynchos wintering in an urban habitat

Abstract

In Mallard, as in other ducks of the northern hemisphere, males outnumber females in wintering flocks and the temperature seems to be one of the most important factors shaping the sex ratio at a given site. In this study, we checked the influence of winter harshness on the sex ratio of Mallards overwintering in urban waterbodies with an emphasis on differences at the local scale within a single town. Our study shows that in harsh winter seasons, there was a lower share of males among wintering Mallards. It seems that females, despite their smaller body size and higher specific metabolic rate, found favourable conditions in the urban area during cold weather. Towns are characterized by a milder winter climate than rural areas, moreover birds profit from the supplementary feeding by humans. In severe weather conditions, females remained in urban waterbodies together with their paired mate, whereas unpaired males possibly left the town in search of sites with less competition, as they occupy a lower position in the flock hierarchy than paired individuals. We also found such differences on a local scale, with the proportion of males decreasing with decreasing temperature. Obtained results suggest that the sex- and status-dependent response to even a slight temperature change during the winter period is probably the most important factor shaping the variation in the proportion of male and female Mallards in flocks, at least in areas with a high abundance of anthropogenic food, which is likely to weaken competition between the sexes for resources.