Dirksen, Sjoerd, Jan H Beekman, and Teus H Slagboom. 2013. “Bewick’s Swans Cygnus Columbianus Bewickii in the Netherlands: Numbers, Distribution and Food Choice During the Wintering Season”. Wildfowl, April, 228-37. https://tidsskrift.dk/Wildfowl/article/view/154973.