Winter distribution of Greater Scaup Aythya marila in relation to available food resources

The European wintering population of Greater Scaup has declined significantly over the last 20 years and some of the main threats appear to be at the wintering grounds. This contribution aims at describing the local distribution of Scaup in the Dutch Wadden Sea, one of the most important wintering sites for Scaup in relation to available food resources. Data on the distribution of Scaup was obtained from annual aerial counts in January during 1999–2009. This data was related to benthos data of regular shellfish surveys in the sublittoral part of the western Dutch Wadden Sea. Boosted regression tree models were applied to quantify the relative importance of different potential prey items to explain the presence of Scaup. Scaup preferred sites with high abundances of small sized Mya arenaria. A negative influence on Scaup preference for sites with high abundances M. arenaria was found when there was a high abundance of large Cerastoderma edule at the sites, which increased Scaup searching time for M. arenaria.

► We modelled Scaup distribution in relation to food resources.
► We used boosted regression trees to examine changes in Scaup presence probability.
► Increasing abundances of small sized prey will increase Scaup presence probability.
► Mya arenaria is the main prey item affecting Scaup presence probability.
► Co-occurrence of suitable and non-suitable prey decrease Scaup presence probability.