Testing the niche variation hypothesis in a community of passerine birds.

The niche variation hypothesis (NVH) predicts that populations with broader niches should exhibit greater between-individual diet variation or individual specialization (IS) relative to populations with narrower niches. Most studies that quantify population niche widths and associated levels of IS typically focus on a single or few species, but studies examining NVH in a phylogenetically informed comparative analysis among species are lacking. Here we use nitrogen isotope (δ N) analysis to measure population niche widths and IS in a single bird community composed of 12 passerine species representing different foraging guilds. We found support for the NVH at the interspecific level; species with broader population niche widths were comprised of more individual specialists. Moreover, our results suggest that this relationship is influenced by foraging guild; specifically, omnivores have higher degrees of IS for a given population niche width than insectivores. Finally, the levels of IS among passerine species, in contrast to population niche width, were associated with their relatedness, suggesting that the potential phylogenetic effect on the prevalence of IS is higher than previously recognized.