Geographic variation in morphology of Northern Cardinals: possible application of Bergmann's Rule?

Geographic variation in morphological characteristics of a species can be influenced by environmental conditions. Bergmann's rule states that endotherms inhabiting colder climates at higher latitudes are predicted to have larger body sizes. However, application of Bergmann's rule to songbirds has yielded mixed results. Our study examines whether geographic variation in morphology conforms to Bergmann's rule in the Northern Cardinal (Cardinalis cardinalis), a resident songbird with a broad distribution extending from Central America to Canada. Measures of body size (mass, wing chord, tarsus length) and feathers that could possibly serve as ornaments (tail, crest length) were compared in populations of cardinals near the northern (Ohio, New York; USA) and southern (Mississippi, Florida; USA) extremes of the species' North American distribution. Cardinal populations in Florida and Mississippi had significantly smaller body size than populations in Ohio and New York. Southern birds weighed significantly less than northern birds and had significantly shorter tarsus and wing chord lengths. Cardinal populations from higher and lower latitudes were clearly distinguished by a primary discriminant function comprising a linear combination of mass, wing chord length, and tarsus length. Northern and southern populations were not as clearly distinguished by tail or crest feather length. Our findings suggest that Bergmann's rule could apply to the Northern Cardinal. Given the broad distribution of this species, it is plausible that larger body sizes could be necessary to conserve heat, avoid starvation, or some combination of both factors in populations of cardinals at higher latitudes and that these adaptations are less necessary closer to the equator.