DIRKSEN, Sjoerd; BEEKMAN, Jan H. Population size, breeding success and distribution of Bewick’s Swans Cygnus columbianus bewickii wintering in Europe in 1986-87. Wildfowl, [S. l.], p. 120–124, 2013. Disponível em: https://tidsskrift.dk/Wildfowl/article/view/155000. Acesso em: 17 dec. 2025.