Geographical variation in the bill patterns of Bewick's Swans

Abstract

Individual Bewick's Swans Cygnus columbianus bewickii differ markedly in the pattern of black and yellow on the bill, and can be categorized into different types according to whether the top of the culmen is predominantly black (darky), intermediate (pennyface), or yellow (yellowneb). The proportion of darkies in Japan was significantly higher and of yellownebs lower than among western Bewick's Swans wintering in England. This is consistent with the likelihood of substantial gene flow across the Bering Straits between eastern Bewick's Swans and the conspecific Whistling Swan C. c. columbianus (the latter having considerably less yellow on the bill). Other differences between eastern and western Bewick's Swans and Whooper Swans C. cygnus are discussed and a possible reason proposed for the differing proportions of bill types among Bewick's Swans at various sites in Japan.