Avian facial plumage, bill coloration, and feather microstructure may serve one or more adaptive functions. Several researchers have proposed that dark eyestripes, bills, and facial masks aid in reducing glare, however, there have been relatively few tests of this hypothesis. Dark facial markings have been shown to have an adaptive glare-reduction function in recent field studies of a few species, but this hypothesis has never been tested in a broad multispecies analysis. It is likely that feather microstructure influences feather brightness and has an effect on the efficacy of glare reduction properties of feathers. I examined the link between dark facial markings and glare reduction under natural lighting conditions in several bird species, using a spectrometer probe placed in the eye-position of museum specimens. As a measure of glare, I quantified the reduction in irradiance in full, natural sunlight, for specimens varying in bill and head plumage coloration and pattern. Each specimen was tested with the head held at various angles to mimic natural foraging positions. I also quantified the brightness of bills and plumage surrounding the eye of these same specimens using reflectance spectroscopy. Correlations between irradiance measurements and the bill and plumage brightness were analyzed. Facial feather microstructure, proximal and distal barbule density, and pith:cortex ratio were examined using scanning electron microscopy. I then correlated these characteristics to plumage brightness of both light and dark patches. A significant relationship with average head darkness and reduction in irradiance values was found when the eye faced directly into the sun, and when it was rotated horizontally 45° away from the sun. Dark patches in the anterior and posterior dorsal quadrants are most important in this reduction in irradiance. Of feather microstructural features, the pith:cortex ratio affected plumage brightness of the entire head, with a larger pith:cortex ratio being associated with darker plumage. Proximal and distal barbule density also play a role in feather brightness. Increased proximal barbule density was correlated with darker plumage, while in an opposing trend increased distal barbule density was correlated with lighter plumage. Future research could expand on the link between these and related features to plumage coloration, with an emphasis on glare reduction or their functions in the feathers of diurnal species
