Seasonal weather effects on the common eider, a subarctic capital breeder, in Iceland over 55 years

Changes in bird populations and their phenology (i.e. timing of nesting and migration) are increasingly linked to global climatic changes, particularly at temperate and Arctic latitudes. These pat- terns arise from local- and regional-scale effects of weather on demography but long-term time-series data necessary to explore these relationships are rarely available. Colonies of the common eider Soma- teria mollissima are often monitored annually for nest-down harvesting. We use long-term data from 2 nesting colonies in northwest Iceland (Bildsey, Brei7afjor7ur: 29 yr and Laekur, Dyrafjor7ur: 55 yr) to examine the effects of weather conditions in each season on breeding numbers, arrival dates and clutch sizes. Numbers of nests in Bildsey increased following warm, wet winters and first nests were produced later following windy and wet winters at Laekur. In spring, windy conditions tended to be followed by earlier female arrivals at Bildsey. Warm, wet springs were positively correlated with larger clutch sizes at Laekur, and clutch sizes at Laekur decreased following especially wet and warm autumns. The over- all population trends vary among sites and are likely to be influenced by both climatic and management conditions. These findings indicate that the effects of global climatic changes will depend on the details of changes in local weather conditions within each season. The strong effects on nesting dates and clutch sizes suggest that accumulation of body reserves, which sustain females during incubation and into brood rearing, is a key mechanism that could determine how altered distribution and frequencies of depressions within each season will affect future numbers of this species.