Large-scale change in the sex ratio of a declining eider Somateria mollissima population
Aleksi Lehikoinen, Thomas Kjær Christensen, Markus Öst, Mikael Kilpi, Pertti Saurola & Aarne Vattulainen
Lehikoinen, A., Christensen, T.K., Öst, M., Kilpi, M., Saurola, P. & Vattulainen, A. 2008: Large-scale change in the sex ratio of a declining eider Somateria mollissima population. - Wildl. Biol. 14: 288-301.
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 eiders Somateria mollissima migrating into the Gulf of Finland, the Baltic Sea, in nine years during 1979-2005 (1979-1980, 1982-1983 and 2001-2005), and the sex ratio of birds collected by Danish hunters during 1982-2004. In two decades, the sex ratio during peak migration has reversed from female bias to male bias, and hunting statistics have shown a significantly increasing adult male bias. Also the proportion of juvenile males has shown a significant increase (Danish hunting statistics 1982-2004), which indicates either that 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 in the Baltic eider population which started in the early 1990s. The proportion of juveniles in the hunting bag, an indicator of breeding success in the Baltic, significantly decreased during our study period. The sex ratio of migrating eiders showed seasonal fluctuations, the pattern of which has changed during the study period. Particularly the proportion of late-migrating females has decreased dramatically since 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 has recently increased in numbers. It is unlikely that differential winter mortality of the sexes can explain 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.
Key words: Baltic Sea, biased sex ratio, differential mortality, eider, population decline, Somateria mollissima
Aleksi Lehikoinen & Markus Öst*, Bird Ecology Unit, Department of Biological and Environmental Sciences, P.O. Box 65, FI-00014 University of Helsinki, Finland - e-mail addresses: email@example.com (Aleksi Lehikoinen); firstname.lastname@example.org (Markus Öst)
Thomas Kjær Christensen, National Environmental Research Institute, Department of Wildlife Ecology and Biodiversity, Denmark - e-mail: email@example.com
Mikael Kilpi, ARONIA Environment, Åbo Akademi University & Novia University of Applied Sciences, Raseborgsvägen 9, FI-10600 Ekenäs, Finland. - e-mail: firstname.lastname@example.org
Pertti Saurola, Finnish Museum of Natural History, Finnish Bird Ringing Centre, P.O. Box 17, FI-00014 University of Helsinki, Finland - e-mail: email@example.com
Aarne Vattulainen, HankoBird Observatory, Vapaasataman tulliportti, FI-10900 Hanko, Finland - e-mail: firstname.lastname@example.org
*Present address: ARONIA Coastal Zone Research Team, Åbo Akademi University & Novia University of Applied Sciences, Raseborgsvägen 9, FI-10600 Ekenäs, Finland
Corresponding author: Aleksi Lehikoinen
Received 19 December 2006, accepted 28 August 2007
Associate Editor: Kjell Einar Erikstad