Environmental Effects on Body Size of Canada Geese

Abstract

Canada Geese (Branta canadensis interior) breeding on Akimiski Island, North- west 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 envi- ronment 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 interme- diate 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 avail- ability explains the smaller size of Canada Geese on Akimiski Island. Received 24 October 1996, accepted 29 May 1997.