Prey choice and ingestion of microplastics by common shelducks and common eiders in the Wadden Sea World Heritage Site

Top predators such as coastal birds are essential components of marine food webs, and understanding their trophic interactions forms an essential basis of food web models. At the same time, the proportion of plastic debris in marine food webs has constantly increased while the degree of plastic ingestion by marine birds is still poorly known. In this study, the diets and microplastic uptakes in two numerous benthivorous bird species in the Wadden Sea were examined microscopically, i.e. the common eider ( Sommateria mollissima ) indicative for the subtidal and the common shelduck ( Tadorna tadorna ) indicative for the intertidal area. Eiders ( n = 42 carcasses; n = 120 faecal samples) mainly ingested common cockles ( Cerastoderma edule ). Blue mussels ( Mytilus edulis ) ranged second and—same as the invasive razor clam ( Ensis leei )—occurred more frequently than reflected by historic data. Proportions of gastropods, bristle worms and crustaceans were low. Shelducks ( n = 20 carcasses; n = 98 faecal samples) fed mainly on small molluscs, especially C. edule , mud snails Peringia ulvae , and amphipods. Plastic debris was found in 92.9% of the stomachs and 74.2% of the faeces from eiders as well as in 95% of the stomachs and 85.7% of the faeces from shelducks. Filaments in shelduck prey remains were significantly larger, whereas there was no species-specific difference in abundance of filaments. Most plastic consisted of brightly coloured filaments ˂ 5 mm. These findings indicate regular uptake and excretion of plastic debris by coastal benthivorous seabirds, both in the subtidal and intertidal realm. The origin and mode of uptake of microplastics are discussed.