Latitudinal trends in body size among over‐wintering leaf warblers (genus Phylloscopus)

Abstract

Geographical trends in body size are commonly interpreted in the framework of Bergmann's rule, which states that larger body sizes are found at higher latitudes. Here we demonstrate a negative association of body size with latitude among over‐wintering warblers of the genus Phylloscopus as well as within a single species (Phylloscopus trochiloides) we were able to study in depth. We examine the role of resources in determining body size distributions. In mid‐winter in India there are more large prey at southern sites (occupied by large‐bodied warblers) than at a northern site (occupied by small‐bodied warblers). Phylloscopus trochiloides is a relatively large species. The timing of its autumn migration is correlated with the withdrawal of the monsoon through India and its appearance on the breeding grounds in spring is correlated with the appearance of relatively large prey. We suggest that prey size and abundance are the main determinants of the spatial distributions of Phylloscopus warblers in winter. Cross‐species associations of body size with both time of arrival on the breeding grounds and migration distance may also largely reflect the spatial and temporal distribution of prey. Resources are likely to be more important in determining both the strength and direction of latitudinal associations with body size than is currently appreciated, even in cases where Bergmann's rule is upheld.