Effects of Sunlight Exposure on Carotenoid-Based and Structural Coloration of the Blue-tailed Bee-eater

Abstract

After a bird's molt is complete, the coloration of its plumage may be altered by external factors such as soiling. We tested how exposure to sunlight affects plumage coloration derived from carotenoid pigments and feather nanostructure. We studied these changes in the Blue-tailed Bee-eater (Merops philippinus) because that species combines yellow chin feathers (colored by carotenoid pigments), green back feathers (colored by a combination of carotenoid pigments and feather nanostructure), and turquoise-blue rump feathers (colored by feather nanostructure). We measured reflectance of feather samples and then exposed them to sunlight in boxes that allowed penetration of both UV and visible wavelengths of light. After exposure to sunlight, reflectance spectrometry revealed that all three regions became less colorful. According to models of avian sight, chromatic aspects of color change were too small to be perceived in the majority of individuals. In contrast, the reduction in brightness after exposure to sunlight was likely visible to birds in most cases. Moreover, our results suggest that types of feather coloration differ in sensitivity to sunlight-induced change in color that is visible to birds. Structural coloration appears to be the most resistant to sunlight, carotenoid-based coloration appears to be the most sensitive, and colors produced by a combination of carotenoid pigments and feather microstructure are intermediate in sensitivity. Overall, our study demonstrates that sunlight modifies plumage coloration between successive molts but the strength of this effect on the Blue-tailed Bee-eater is relatively small and depends on the mechanism of color production.