Trophic niche width increases with bill-size variation in a generalist passerine: a test of niche variation hypothesis.

The niche variation hypothesis (NVH) predicts that populations with wider niches are phenotypically more variable than populations with narrower niches, which is frequently used to explain diversifying processes such as ecological release. However, not all empirical evidence supports the NVH. Furthermore, a relationship between population phenotypic variation and niche width can be caused by sexual selection or environmental gradients, which should be carefully considered along with competition in explaining niche variation. In this study, we used eight populations of a generalist passerine species, Paradoxornis webbianus (vinous-throated parrotbill), to test the NVH. We assessed evidence of ecological sexual dimorphism and environmental gradients in bill morphology of P. webbianus. A total of 170 P. webbianus from eight sites ranging 24-2668 m in altitude were included in this study. We used two principal components to quantify bill morphology: one describes bill size and the other describes bill slenderness. We used stable carbon and nitrogen isotope values of bird feathers to quantify trophic positions, and we estimated population trophic niche width using Bayesian standardized ellipse area. Paradoxornis webbianus with larger and more slender bills fed at higher trophic levels and population trophic niche width tended to increase with bill-size variation, supporting the NVH. The males had larger bills and marginally higher nitrogen isotope values than the females, suggesting ecological sexual dimorphism. Despite a positive correlation between bill size and wing length indicating sexual selection for larger male size, only three of the eight populations showed both male-biased bill size and male-biased wing length. Sexual dimorphism explained 13%-64% of bill-size variation across sites, suggesting its role in niche variation could vary greatly among populations. The variation in bill slenderness in P. webbianus increased with elevation. However, neither bill-size variation nor trophic niche width changed with elevation. Therefore, environmental gradients that could be reflected in the elevation are not likely to drive the observed morphological and niche variation. This study provides an empirical case for the NVH and highlights the importance of investigating sexual dimorphism and environmental gradients in studies of niche dynamics.