Figure 4. Distribution of contrasts between feather patches and background elements (dS). Contrasts were averaged among time slots and between light environment type. (A) Species within the highest quantile of dichromatism (average patch-wise dS contrasts between male and female), illustrating the most dichromatic species Ramphomicron microrhynchum. For the group of species within the lowest quantile of dichromatism, we separated those cryptic and conspicuous based on an arbitrary threshold. (B) The least dichromatic-cryptic (<5.5 dS), illustrating the least dichromatic species Patagona gigas. (C) The least dichromatic-conspicuous (>5.5 dS) illustrating the most dichromatic species within this group Heliothryx auritus. (D) All species combined. Illustrations by Birds of the World | Cornell Lab of Ornithology (illustrated by Hilary Burn, Jan Wilczur).
Abstract
The striking diversity of bird plumage colors has long fascinated researchers and inspired foundational evolutionary theories. Both natural and social selection can shape plumage color variation, enhancing either crypsis or conspicuousness, and thereby influencing fitness. Yet how light environments influence color function remains poorly understood. Here, we show that color variation among plumage patches aligns with their presumed functions—either attracting or diverting receiver attention—but is only weakly associated with local light conditions. Using a comprehensive dataset of side-welling irradiance, background reflectance, and hummingbird plumage spectra, we find that site-specific variation in irradiance predicts changes in crown coloration, but not in other patches. In contrast, sex was a significant predictor of coloration in all plumage patches, including those presumed to be cryptic. We found that conspicuousness across patches decreases when shifting away from the angle of peak reflectance, which suggests that iridescence and behavioral mechanisms allow hummingbirds to decrease conspicuousness at different viewing geometries. These results suggest that hummingbird coloration is not adapted to variation in specific light environments, but rather it has been tuned through time by behavior and interactions between sexes and predators to remain effective despite environmental variation.