Individual diet specialisation in sparrows is driven by phenotypic plasticity in traits related to trade-offs in animal performance.

Individual diet specialisation (IS) is frequent in many animal taxa and affects population and community dynamics. The niche variation hypothesis (NVH) predicts that broader population niches should exhibit greater IS than populations with narrower niches, and most studies that examine the ecological factors driving IS focus on intraspecific competition. We show that phenotypic plasticity of traits associated with functional trade-offs is an important, but unrecognised mechanism that promotes and maintains IS. We measured nitrogen isotope (δ15 N) and digestive enzyme plasticity in four populations of sparrows (Zonotrichia capensis) to explore the relationship between IS and digestive plasticity. Our results show that phenotypic plasticity associated with functional trade-offs is related in a nonlinear fashion with the degree of IS and positively with population niche width. These findings are opposite to the NVH and suggest that among individual differences in diet can be maintained via acclimatisation and not necessarily require a genetic component.

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 (δ15 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.