Environmental Effects on Body Size of Canada Geese

Canada Geese (Branta canadensis interior) breeding on Akimiski Island, Northwest Territories, and those breeding on the Ontario mainland southwest of James Bay, share a common wintering range and are considered to be part of the same population, but differ significantly in size (e.g. skull length of mainland birds averages ca. 8% greater than that of Akimiski birds). We collected eggs in each area and raised the goslings in a common environment to determine environmental effects on differences in body-size traits observed in wild birds. We found no differences in asymptotic size or growth periods for skull, culmen, and tarsus length among birds from different origins (P > 0.05) but significant differences between sexes (P < 0.05). Estimated asymptotes for skull length of captives were intermediate to those of wild insular and mainland adults but closer to those of the larger mainland birds. Captive goslings from Akimiski Island had structural measurements that averaged 8 to 17% larger than those of wild goslings of the same age on the island. These results suggest a significant environmental effect on observed differences in body size between insular and mainland populations in southern James Bay. We suggest that lower per capita food availability explains the smaller size of Canada Geese on Akimiski Island.