Morphological and plumage colour variation in the Réunion grey white-eye (Aves: Zosterops borbonicus): assessing the role of selection

The Réunion grey white-eye (Zosterops borbonicus), a small passerine endemic to the island of Réunion (Mascarene archipelago), constitutes an extraordinary case of phenotypic variation within a bird species, with conspicuous plumage colour differentiation at a microgeographical scale. To understand whether natural selection could explain such variability, we compared patterns of variation in morphological and plumage colour traits within and among populations. To quantify morphological variation, we used measurements obtained by Frank Gill in the 1960s from 239 individuals collected in 60 localities distributed over the entire island of Réunion. To quantify colour variation, we measured the reflectance spectra of plumage patches of 50 males from a subset of Gill's specimens belonging to the five recognized plumage colour variants and used a visual model to project these colours in an avian- appropriate, tetrachromatic, colour space.We found that variants occupy different regions of the avian colour space and that between-variant differences for most plumage patches could be discriminated by the birds. Differences in morphology were also detected, but these were, in general, smaller than colour differences. Overall, we found that variation in both plumage colour and morphology among variants is greater than would be expected if genetic drift alone was responsible for phenotypic divergence. As the plumage colour variants correspond to four geographical forms, our results suggest that phenotypic evolution in the Réunion grey white-eye is at least partly explained by divergent selection in different habitats or regions.