Distribution of wintering Common Eider <i>Somateria mollissima</i> in the Dutch Wadden Sea in relation to available food stocks

The number of Eiders Somateria mollissima wintering in the Dutch Wadden Sea has declined rapidly during the last two decades. Changes in the available food stocks are assumed to be an important cause of this trend. In order to extend the knowledge of the importance of particular food sources to wintering Eiders, data on distribution of Eiders obtained from aerial counts were spatially related to routinely collected monitoring data of shellfish. Based on previous diet studies, we hypothesized that the distribution of Eiders in the Dutch Wadden Sea is related to the presence of Mytilus edulis stocks and that M. edulis from the sublittoral areas, especially the ones from mussel culture plots will have the strongest effect on Eider distribution. Boosted regression tree models were applied to quantify the relative importance of different potential prey items on the distribution of wintering Eiders. Indeed, Eiders were found to prefer sites with high densities of medium- and large-sized M. edulis, especially from mussel culture plots. Other bivalve species seemed to serve as alternative prey, since sites with a relatively high abundance of these species increased in importance, when less M. edulis was available on culture plots. The contribution of cultured M. edulis to the diet of Eiders decreased during the course of the winter seasons, indicating that harvesting of M. edulis from the culture plots might reduce this high-quality food source at the end of the winter, forcing the Eiders to switch to less profitable prey.