From colour to heat: linking visible and near-infrared reflectance in birds

Feather reflectance in visible (VIS) and near-infrared (NIR) wavelengths influences avian heat balance by modulating the absorption of solar energy, yet the evolutionary and ecological patterns of NIR reflectance remain poorly understood. We developed a cross-taxonomic predictive framework linking NIR to VIS reflectance using a full-spectrum dataset of 531 bird species from 23 orders and 106 families, integrating 439 newly measured spectra with published data. Across species, VIS and NIR reflectance were strongly correlated (R² = 0.667), but regression slopes differed significantly among orders, indicating lineage-specific spectral scaling. When controlling for visible coloration, NIR reflectance still varied markedly across orders, revealing taxon-specific thermal properties beyond colour. Morphological and ecological traits further predicted these patterns, with hand-wing-index, migratory status, habitat types, habitat temperature and precipitation all exerting significant effects on NIR reflectance. Residual analysis revealed that Gruiformes, Pelecaniformes and Charadriiformes (i.e. waders, gulls, terns and allies) showed low and variable NIR reflectance, possibly reflecting structural adaptations to heterogeneous thermal environments. Collectively, our results provide the first large-scale, phylogenetically informed framework for predicting NIR reflectance from VIS data, advancing mechanistic understanding of avian thermal adaptations.