Bill and body size in the peregrine falcon, north versus south: is size adaptive?

Visually, the bill size on southern hemisphere peregrine falcons (Falco peregrinus), especially from Australasia and the Philippines, appears appreciably larger for their body size than in other peregrine populations. 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 compared 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 southern hemisphere groups, South American, Australian and Melanesian/Philippine. Finally, we compared northern hemisphere and southern hemisphere birds. Southern hemisphere peregrines have a more massive bill relative to body mass than those of the northern group, and can be distinguished from the northern hemisphere birds using ratios of bill volume to wing or tarsus length. Absolute bill volume generally shows a stabilizing selection and, except for tundra birds which are smaller, all bills are statistically the same by sex while body size or mass changes geographically showing directional selection. It is not clear why body mass is consistently smaller in the southern hemisphere than in north temperate regions nor why the convergence in bill volume to body mass among southern groups should be so evident for such isolated geographical locations, especially since there is apparently no gene flow. Further, habitats and thus foods that might alter bill structure differ among locations.