Nilsson, Leif, Ove Andersson, Rolf Gustafsson, and Mikael Svensson. 1998. “Increase and Changes in Distribution of Breeding Whooper Swans Cygnus Cygnus in Northern Sweden from 1972-75 to 1997”. Wildfowl, January, 6-17. https://tidsskrift.dk/Wildfowl/article/view/154602.