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 more variable than populations with narrower niches, which is frequently used to explain diversifying processes such as ecological release. However, empirical evidence for NVH remains divided. Furthermore, an apparent relationship between within-population phenotypic variation and population niche width may be largely caused or maintained by sexual selection and should be carefully distinguished from natural selection processes underlying the NVH. In this study we used eight populations of a generalist passerine species, Paradoxornis webbianus (vinous-throated parrotbill), to test the NVH and assess ecological sexual dimorphism. We first confirmed that bill morphology reflects individual diet, and then we tested if populations that were more variable in bill morphology had broader trophic niche width. We assessed ecological sexual dimorphism by comparing sex differences in bill morphology and diets, and evaluated the potential role of sexual selection in the relationship between bill variation and trophic niche width. We used two principal components (PCs) to quantify bill morphology: PC1 describes bill sizes, and PC2 describes bill slenderness. We used stable carbon and nitrogen isotope values of bird feathers to quantify their diets, and estimated population trophic niche width using Bayesian standardized ellipse areas.
Individuals with larger and more slender bills fed at higher trophic levels and population trophic niche width increased with intra-population variation in bill size, supporting the NVH. Males and females did not differ consistently in diets across the sites despite a consistently male-biased bill size, providing little support for ecological sexual dimorphism. Because sexual dimorphism on average explained 36% of intra-population variation in bill size, we suggest that some of the observed niche expansion through increased bill size variation may be related to sexual selection. This study demonstrated that populations more variable in bill sizes had wider trophic niches in a generalist bird species and highlights the importance to investigate sexual dimorphism in the studies of niche variation.