Song Sparrows: As Temperature Decreases Body Size Increases, Always?

Schimmel/Conrades Science Center 151

Bergmann’s Rule predicts that individuals within a species have larger body sizes in colder habitats, either closer to the poles or at higher elevations. There are 24 subspecies of song sparrows (Melospiza melodia) in North America with three endemic to the eastern half of the continent and the remaining subspecies are located in the west. Agreement with Bergmann’s Rule is well documented in the western song sparrow subspecies, but the eastern subspecies have not been tested. I sampled one metric of body size in three western subspecies: M. m. fallax (south), M. m. morphna (middle), and M. m. kenaiensis (north) and three eastern subspecies: M. m. melodia in Georgia (south), New England (middle), and northeastern Canada (north) by measuring the tarsometatarsus of 123 sparrows with digital calipers. In the west, M. m. kenaiensis (24.8 mm) was the largest, M. m. morphna (22.6) intermediate, and M. m. fallax (21.8), the most southern, was smallest, which matched the predictions of Bergmann’s Rule. The differences among subspecies in the east were less extreme, but still consistent with Bergmann’s Rule: northeastern Canada (22.2), New England (21.8), and Georgia (21.4). However, when compared statistically, only the western subspecies exhibited any significant differences among subspecies as M. m. kenaiensis was significantly larger than all other subspecies, none of which differed significantly from each other. Thus, I found some support for Bergmann’s Rule in western subspecies, but only if the northernmost subspecies is included in the analysis. The eastern subspecies exhibit a latitudinal cline in tarsus length that follows the pattern predicted by Bergmann’s Rule, but the differences between subspecies and latitudes are not statistically significant.