Breeding sex ratios in two declining diving duck species: between-year variation and changes over six decades.

Sex ratio variation has important consequences for population dynamics and viability. Adult sex ratio is male biased in many duck species, and data from wintering areas suggest male bias has been increasing in some European duck species. However, the reasons for changes in sex ratios remain poorly understood, and information on sex ratio variation among breeding ducks is limited. We studied long-term changes in breeding sex ratios and breeding numbers in Common Pochard Aythya ferina and Tufted Duck Aythya fuligula in southern Finland, the former species being currently assessed as critically endangered and the latter as endangered in Finland. In addition, we tested the hypothesis that between-year variation in breeding sex ratios is affected by the severity of weather conditions in the wintering areas. The proportion of females among breeding Tufted Ducks decreased from 42.9% in 1951–1970 to 36.9% in 1996–2015, while no statistically significant change was observed in Common Pochard (41.8% and 39.5%, respectively). Both species showed a decline in breeding numbers over the same period. Severity of the preceding winter did not affect the proportion of females in the breeding area. [ABSTRACT FROM AUTHOR]