Eider duck in flight

A male eider duck (Somateria mollissima) in flight at the Isle of May, Firth of Forth. Eider ducks are the only species of marine duck found in Britain, and are interesting because their stomachs are alkaline, not acidic like other animals. This is because they feed on shellfish which have calcium carbonate (chalk) shells that would react with acid to produce gas, causing a bout of indigestion!