Spatial Patterns in the Body Sizes of Bird Species in the New World

Spatial patterns in animal body sizes have primarily been studied with the aim of evaluating Bergmann's rule, which states that body size is inversely related to ambient temperature, and hence increases with latitude across closely related species. However, geographic range sizes and species richness also show spatial variation, and there is evidence that both are correlated with body size. It is surprising, therefore, that there has been no attempt to examine covariation in body size, species richness and geographic range size patterns simultaneously in a single assemblage. Here, we do so, using the assemblage of birds in the New World. The mean body mass of New World birds is lowest in equatorial areas, and increases with latitude either side of the equator, as expected from Bergmann's rule. However, this pattern closely mirrors that in species richness, and there is a strong inverse correlation between the number of species in an area and their mean body mass. This correlation is stronger than that of either variable with latitude, and hints at a common underlying cause. In contrast, the relationship between the mean body size and the mean geographic range size of species in an area is very weak. Nevertheless, the spatial patterns mean that the interspecific relationship between range size and body size varies across latitudes, which has important consequences for conservation assessments based on species' range sizes. If there is an ecological reason why larger body size is favoured at high latitudes, where energy availability is lower, this could in part explain the coincidence of spatial patterns in body size and species richness. Spatial patterns in range size also may in part be mediated through a weak correlation with body size. Thus, spatial patterns in all three variables may be inter-related.