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.