Spatial and environmental correlates of intraspecific morphological variation in three species of passerine birds from the Purus–Madeira interfluvium, Central Amazonia

Biogeographic studies in Amazonia typically describe biodiversity across interfluvia, rarely within them, where geographic variability in morphological traits might be observed. We tested for intraspecific phenotypic variation in three bird species within the Purus–Madeira interfluvium (Central Amazon) and whether phenotypes were correlated with environmental heterogeneity or geographic distance among sites. We compared coloration indexes derived from reflectance spectra and morphometrics of up to five adult individuals of each sex among 11 sites within the interfluvium and contrasted them with proxies for geographic distance and environmental variation (tree basal area and bird community). Environmental heterogeneity was minimally spatially autocorrelated, and there were no obvious geographical barriers to dispersal in the study region. The null hypothesis was that we would see either no phenotypic variation or random variation that was not explained by the tested variables. Half of the cases analyzed showed intraspecific morphological variation. Coloration varied more frequently than morphometrics, and color was better explained by environmental heterogeneity, particularly in males, whereas brightness also varied with geographic distance. Geographic distance explained the only case of variation in morphometrics. Our results indicate that coloration, particularly plumage brightness, is more labile than morphometric traits and that plumage color might be under stronger effects of local adaptation than brightness, which also seems to be under effects of neutral drift and gene flow among populations. Higher frequencies of association between male coloration and the environment suggest a role of non-arbitrary mechanisms of sexual selection on the expression of male phenotypes, whereas arbitrary intersexual selection might explain the randomly distributed variation that is not explained by environmental heterogeneity or geographic distance. We revealed intraspecific phenotypic variation in a spatial extent usually not considered in biogeographic studies in the Amazon and demonstrate that both local adaptation and neutral drift are important to explain intraspecific trait diversification at this geographical scale. © 2018, Springer International Publishing AG, part of Springer Nature