Data from: Trophic niche width increases with bill size variation in a generalist passerine: a test of the niche variation hypothesis

1. 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. 2. 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 24-2,668 m in altitude were included in this study. We used two
principal components to quantify bill morphology, one describes bill size
and the other describe 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. 3. 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. 4. This study provides an empirical case for the NVH and
highlights the importance to investigate sexual dimorphism and
environmental gradients in the studies of niche dynamics.