Drivers of breeding numbers in a long-distance migrant, the Garganey (Anas querquedula): effects of climate and hunting pressure

Abstract

A multitude of anthropogenic factors are threatening bird populations but their roles as drivers of population changes are generally poorly understood. Several duck species, for instance, have unfavorable conservation status at the Pan-European level but in most cases we do not know why the species have been declining, nor do we know actual drivers of their population dynamics. We studied population dynamics of the Garganey (Anas querquedula), a quarry species with unfavorable conservation status at the Pan-European level. As a trans-Saharan migrant, Garganey is potentially highly vulnerable to climate change impacts. We used long-term (1989–2012) data of breeding numbers from a study area in central Finland and assessed the relative importance of three climatic variables (representing conditions in wintering areas and during spring migration) and local hunting pressure in explaining the interannual variation in breeding numbers. Population size of Garganey showed a decreasing trend over the study period but also considerable interannual variation. Spring temperature in southern Finland was the most important factor in explaining interannual variation in breeding numbers. Rainfall in the wintering areas was also of importance, whereas the NAO (North Atlantic Oscillation) and local hunting pressure appeared not to be important. Our results suggest that weather conditions during spring migration largely drive interannual variation in Garganey breeding numbers at the NW edge of the species’ range. However, positive effects of warm springs may be counteracted by negative effects of drought in the wintering areas.