Bowler, J, L Butler, R Hesketh, C Liggett, and E C Rees. “Bewick’s and Whooper Swans Cygnus Columbianus Bewickii and C. Cygnus: The 1994-95 Season”. Wildfowl (January 1, 1995): 176–186. Accessed December 17, 2025. https://tidsskrift.dk/Wildfowl/article/view/154654.