| Summary: | We studied patterns of geographic variation in structural size and genetic characteristics of white-cheeked geese inhabiting coastal areas of Hudson Bay, Canada, from northern Manitoba to southern Nunavut to determine the degree of morphological and spatial overlap, if any, between Cackling Geese (Branta hutchinsii) and Canada Geese (B. canadensis) in this region. Most Canada Geese occurred in sub-Arctic habitats south of 59°N latitude, and most Cackling Geese occurred in Arctic habitats north of 60°N, but the two species overlapped in a narrow zone between 59°N and 60°N latitude that coincided with the ecotone between sub-Arctic and Arctic ecozones. Mismatches between morphological and genetic characteristics of some individual females suggested that introgression had occurred in this area, and contrasting patterns in the nuclear and mitochondrial DNA (mtDNA) were consistent with female natal philopatry and male-biased dispersal. Evidence of introgression in the nuclear genome was geographically more widespread than evidence of introgression in the mtDNA genome. We suggest that the persistence of Canada Goose mtDNA in phenotypic Cackling Geese is a result of historical hybridization events that occurred when the Arctic—sub-Arctic ecotone was located farther north during a warmer climatic period. Despite evidence of introgression, most birds that we sampled appeared to belong to one or the other parental species, on the basis of their consistent identification using morphological, mtDNA, and nuclear DNA characteristics. We suggest that the area of overlap represents a tension zone between Canada Geese and Cackling Geese that is maintained by behavioral and ecological factors that limit effective dispersal.
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