Bowler, Jonathan M, Linda Butler, and Eileen C Rees. “Bewick’s and Whooper Swans Cygnus Columbianus Bewickii and C. Cygnus: the 1992-93 Season”. Wildfowl 44 (November 1, 1993): 191–199. Accessed April 15, 2026. https://tidsskrift.dk/Wildfowl/article/view/154692.