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 sou th west of James Bay, share a common wintering range and are considere d 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 gos lings in a common environment to determine environmental effects on di fferences in body-size traits observed in wild birds. We found no diff erences in asymptotic size or growth periods for skull, culmen, and ta rsus length among birds from different origins (P > 0.05) but signific ant differences between sexes (P < 0.05). Estimated asymptotes for sku ll length of captives were intermediate to those of wild insular and m ainland adults but closer to those of the larger mainland birds. Capti ve goslings from Akimiski Island had structural measurements that aver aged 8 to 17% larger than those of wild goslings of the same age on th e island. These results suggest a significant environmental effect on observed differences in body size between insular and mainland populat ions in southern James Bay. We suggest that lower per capita food avai lability explains the smaller size of Canada Geese on Akimiski Island.