Geographical variation in the bill patterns of Whooper Swans

摘要: Field observations in Britain, Iceland and Japan showed that Whooper Swans Cygnus c. cygnus at the eastern end of their range have more yellow on bills than do those western extreme. While bill patterns Bewick's C. columbianus bewickii Whistling are most similar where ranges closest, Trumpeter buccinator then different. The latter pair may diverged earlier or remained consistently isolated.