Morphological and isotope data on three passerines in Taiwan

The niche variation hypothesis (NVH) predicts that populations with wider
niches exhibit greater morphological variation through increased
inter-individual differences in both niche and morphology. In this study,
we examined niche-trait relationships in three passerine species
(Cyanoderma ruficeps, Sinosuthora webbiana, Zosterops
simplex). A total of 289 C. ruficeps from 7 sites,
259 S. webbiana from 8 sites, and 144 Z.
simplex from 6 sites were sampled along an elevation gradient
(0-2,700 m) in Taiwan from 2009 to 2017. We measured bill traits (length,
width and depth of bill) and body-size traits (length of head, tarsus and
wing) of the birds, which were reduced to four principal components (bill
PC1, bill PC2, body-size PC1, body-size PC2). We collected feather tissues
for stable carbon and nitrogen isotope analyses to quantify their isotope
niche. We quantified inter-individual differences in isotope space and
trait space with four diversity metrics (divergence, dispersion, evenness,
uniqueness), and tested whether inter-individual differences in isotope
space and trait space are positively associated. We quantified population
isotope niche width by Bayesian ellipse area, and population morphological
variation by variances of the PCs. The results showed that individual
uniqueness in isotope niche and bill morphology (average closeness of
individuals within the population isotope/trait space) were positively
associated across three species. Furthermore, isotope niche width and bill
PC1 (reflecting the size of bill) variation at population level were also
positively associated across the three species, supporting the NVH. Of the
three species, C. ruficeps and S.
webbiana showed stronger support for the NVH than Z.
simplex, possibly due to the latter having narrower elevational
distribution and a more specialized, plant-based diet. The diversity
metrics represented different aspects of inter-individual differences in
niche/trait space, and for the passerines, individual uniqueness appeared
to play an important role in their niche-trait dynamics.