1. The energetic costs of reproduction in birds strongly depend on the climate experienced during incubation. Climate change and increasing chance of extreme weather events may severely affect these costs, especially for species incubating in extreme environments.
2. In this three-year study, I used an experimental approach to investigate the effects of microclimate and nest-shelter on the incubation effort of female common eiders (Somateria mollissima) in a wild Arctic population. I added artificial shelters to a random selection of nesting females, and compared the body mass loss during incubation between females with and without shelter.
3. Non-sheltered females experienced a higher mass loss during incubation than females with artificial shelter. This manipulation allowed me to identify wind as the key climatic variable leading to an energetically challenging incubation and was positively correlated with mass loss for non-sheltered females. Increasing ambient temperatures tended to decrease mass loss, but this effect was negligible in absence of wind. Humidity had no effect on mass loss.
4. This study is one of few that clearly demonstrate a direct effect of climate on avian lifehistory. By showing that increasing wind speed counteracts the energetic benefits of a rising ambient temperature, this study emphasizes that climatic variables other than temperature may also affect wild populations and need to be taken into account when predicting the effects of climate change.