Sex ratio among wintering Pochards Aythya ferina at Erstavik, Sörmland

Abstract

Data on the sex ratio of Pochard Aythya ferina have been collected during eleven consecutive winters (December—February) at Erstaviken (59° 16'N, 18° 15'E) in the vicinity of Stockholm, Sweden. The average fraction of males was 0.89±0.054. No difference in sex ratio was found between different winters or between mild and cold winters, i.e. if the bay froze or not during the winter. Neither was there any difference between different months. Flock size did not affect sex ratio either. The high proportion of males at Erstavik fits with a general pattern of declining sex ratios from north to south in Europe. However, presence or absence of seasonal variation in sex ratio and its dependence on flock size seem to vary from site to site in Europe.