DOI: 10.5061/dryad.bj17r
title: Data from: Trophic niche width increases with bill size variation in a generalist passerine: a test of the niche variation hypothesis

description: 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.
description: Morphological and
 isotope data of the vinous-throated parrotbills in
 TaiwanThis file contains data
 on 170 individual adult Paradoxornis webbianus captured between 2009 and
 2012 from eight sites in Taiwan. The columns include, from left to right:
 sampling data (short date format: yyyy/mm/dd), species scientific name,
 latitude of the site, longitude of the site, site full name in English,
 site short name used in the associated manuscript, tag showing the band
 number of the bird (a number without a beginning letter "K"
 indicates the bird died during handling), sex of the bird, stable carbon
 isotope values of the bird's feather tissue, stable nitrogen isotope
 values of the bird's feather tissue, altitude of the site, the
 bird's bill length in mm, the bird's bill width in mm, the
 bird's bill depth in mm, the bird's wing length in mm, and the
 name of the technician who performed the morphological measurements. Sex
 of the bird was determined by molecular sexing. Stable carbon and nitrogen
 isotope values were meausured on a ThermoFinnigan Delta Plus Advantage
 Isotope Ratio Mass Spectrometer (Bremen, Germany).Parrotbill_NVH_Hsu2013.xlsxIsotope data of plant foliar samples in TaiwanThis file contains stable carbon and nitrogen isotope data of foliar samples from 69 plants (50 unique taxa at species or genus level) collected at six sites in Taiwan. These are used to calculate baseline isotope values for the consumer species, Paradoxornis webbianus, in the associated publication. The columns are: plant species scientific name, stable carbon isotope value of the foliar tissue, stable nitrogen isotope value of the foliar tissue, elevation, latitude and longitude of the site, full name of the site and short name of the site.Plants_NVH_Shaner2013.xlsx