Effects of winter weather conditions on abundance fluctuations in a nest box breeding population of the Common Merganser Mergus merganser in the southern coast of Finland

Earlier studies based on censuses of adult birds in spring suggest that the Common Merganser Mergus merganser population in Finland has been rather stable during the last few decades. However, in the beginning of the 1990s, a population increase was followed by a decline. Contrary to the trends detected in breeding populations, wintering populations of the species have increased in Finland and elsewhere in Europe. Along with warming winters, wintering areas have shifted gradually towards northeast. In the present study, a coastal population of the Common Merganser breeding in nest boxes was monitored between 2004 and 2022 in southern Finland. The role of winter weather conditions in population fluctuations and changes was examined. The weather conditions during two preceding winters reflected in the population fluctuations of mergansers. The number of nests in nest boxes increased following increasing winter temperatures in the northeastern end of the wintering area (near the breeding area). The breeding population decreased after warm winters in the southwestern end of the wintering area (far from the breeding area). The present results suggest that established populations decrease after warm winters, because young birds that have not yet bred have shifted to overwinter and breed further eastwards than in general.