Local variation in sex ratios in three species of wintering ducks: the need for large-scale sampling

This study investigates the hypothesis that site characteristics and flock size influence the sex ratios of wintering ducks by examining three species: the Mallard (Anas platyrhynchos), the Goldeneye (Bucephala clangula), and the Long-tailed Duck (Clangula hyemalis) in the Gulf of Gdańsk, southern Baltic. Birds were counted in mid-January along approximately 160 km of shoreline in the western part of the Gulf of Gdańsk and the estuarine sections of the Vistula River on the southern Baltic coast. Only flocks in which the sex of all individuals was identified were included in the analysis. The sex ratios of the three study species varied across the five habitat types within the study area but did not vary with species-specific flock size. In Mallards the lowest proportion of males was recorded along the open seacoast and municipal beach, suggesting that these areas are primarily occupied by paired birds. The higher proportion of males at other sites is likely due to the shift of unpaired individuals to these locations, where the overall number of this species is the highest. In the Goldeneye the highest percentage of females was observed in the most sheltered areas, while the lowest percentage, alongside low individual numbers, occurred along the open seacoast. These patterns suggest that Goldeneye density and sex ratio are influenced by female tolerance to adverse weather conditions, as this species shows the greatest size difference between males and females among the species studied. Long-tailed Ducks exhibited a more uniform sex ratio across the study area, likely due to their frequent movements to access areas with abundant food resources. This study highlights spatial variations in duck sex ratios within a single water body and underscores the need for large scale sampling to obtain representative sex ratio estimates.