Assortative Mating and Reciprocal Difference in the Blue–Snow Goose Complex

Abstract

THE lesser snow goose Chen hyperborea and the blue goose Chen caerulescens are maintained as good species by the standard taxonomic works¹ though the morphological difference between them is confined to colour of plumage. The geographical distribution of the two is different. The snow goose ranges through Siberia, western Canada, the Hudson Bay area and Baffin Island; west of Banks Island the populations are unmixed. To the east, however, most populations contain some blue geese and in the Bowman Bay area of Baffin Island there exist several populations in which more than 90 per cent of the birds are blue. Where the populations are mixed, interbreeding between the two occurs, and the progeny produced are viable, fertile² and phenotypically distinct³. This has led some authorities⁴ to consider the two forms to be polymorphs of one biological species. This communication is concerned with observations made by one of us (F. G. C.) on breeding behaviour in the mixed Boas River population, and for convenience the terms blue and snow are used in a purely descriptive sense. Table 1 shows the observed and expected (random mating) distribution for three types of mating : (1) both snow birds ; (2) both blue birds ; (3) one snow and one blue bird in a random sample of 520 breeding pairs from the Boas River population in 1952.