Rees EC, Bowler JM, Butler L. Bewick’s and Whooper Swans Cygnus columbianus bewickii and C. cygnus: the 1990-91 season. Wildfowl [Internet]. 1991 Jan. 1 [cited 2025 Dec. 17];:169-75. Available from: https://tidsskrift.dk/Wildfowl/article/view/154066