BILL AND BODY-SIZE IN THE PEREGRINE FALCON, NORTH VERSUS SOUTH - IS SIZE ADAPTIVE

Visually, the bill size on southern hemisphere peregrine falcons (Falc o peregrinus), especially from Australasia and the Philippines, appear s appreciably larger for their body size than in other peregrine popul ations. Accordingly, we measured the bill 'size' or 'volume' (length, width, depth) as a function of body mass on a sample of peregrines. We used both wing and tarsal length as indicators of body mass. We compa red bill volume between two northern hemisphere groups, a medium-sized tundra breeding group and the largest of all peregrines (F.p. pealei) from the Aleutian Islands of North America, and also among three sout hern hemisphere groups, South American, Australian and Melanesian/Phil ippine. Finally, we compared northern hemisphere and southern hemisphe re birds. Southern hemisphere peregrines have a more massive bill rela tive to body mass than those of the northern group, and can be disting uished from the northern hemisphere birds using ratios of bill volume to wing or tarsus length. Absolute bill volume generally shows a stabi lizing selection and, except for tundra birds which are smaller, all b ills are statistically the same by sex while body size or mass changes geographically showing directional selection. It is not clear why bod y mass is consistently smaller in the southern hemisphere than in nort h temperate regions nor why the convergence in bill volume to body mas s among southern groups should be so evident for such isolated geograp hical locations, especially since there is apparently no gene flow. Fu rther, habitats and thus foods that might alter bill structure differ among locations.