Broad Support for Bergmann’s Rule in Birds

Abstract

The observation that species, and individuals within species, tend to be larger in the colder parts of their range was long considered to be one of the few rules in ecology (Bergmann’s Rule). However, recent efforts to test the generality of this rule at large scales have failed to find support for Bergmann’s Rule. In birds, one challenge that has precluded a largescale test of Bergmann’s Rule has been a lack of data to estimate size variation between and within species across a sufficient gradient of temperature. We use 3,016 skeletal specimens from 734 passerine species across the Americas to test whether variation in body size is associated with temperature. We find a negative relationship between temperature and both inter- and intra-specific differences in size. Our results are evidence of widespread conformation to Bergmann’s Rule in birds.