Large-scale change in the sex ratio of a declining eider population

Aleksi Lehikoinen, Thomas Kjær Christensen, Markus ôst, Mikael Kilpi, Pertti Saurola & Aarne Vattulainen

The breeding potential of a monogamous animal population should be maximal during equal operational sex ratio, and empirical evidence suggests that the population-wide sex ratio may be linked to population density. We studied the sex ratio of migrating eiders Somateria mollissima into the Gulf of Finland, the Baltic Sea, in 1979–2005 (9 years), and birds collected by Danish hunters in 1982–2004. The sex ratio during peak migration has reversed from female bias to male bias during two decades, and hunting statistics showed significantly increasing adult male bias. Also the proportion of juvenile males showed a significant increase (Danish hunting statistics 1982–2004), which indicates that either the primary sex ratio of ducklings is exceedingly male biased, or that the mortality of female ducklings has increased. This shift in sex ratio is paralleled by a dramatic decrease of the Baltic eider population since the early 1990s. The proportion of juveniles in the hunting bag, an indicator of breeding success in the Baltic, significantly decreased over the study period. The sex ratio of migrating eiders showed seasonal fluctuations, the pattern of which has changed during the study period. The proportion of late-migrating females experienced a particularly dramatic decrease from the early 1980s, suggesting a declining influx of subadult females. Both the increased male bias and the decreased breeding success are likely to be linked with the population decline. A primary contributor to the shift in sex ratio and the declining trend in breeding success and population size is possibly differential mortality of the sexes during breeding, as the mortality of breeding females has increased sharply in the western Gulf of Finland, mainly due to predation by white-tailed sea eagles (Haliaeetus albicilla) and american minks (Mustela vison), the former of which have recently increased. Differential winter mortality of the sexes unlikely explains our results, as the wintering area of eiders from the Gulf of Finland has remained the same, and the Danish hunting bag reflects the existing sex ratio. Our study highlights the need for future empirical and theoretical work on the relationship between population sex ratio and population density.