Meeting Abstract
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 feather microstructure influences feather brightness and has an effect on the efficacy of glare reduction properties of feathers. We tested the hypothesized 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. Reduction in irradiance in full, natural sunlight, used as a measure of glare, was quantified 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. We 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, barbule density, pith and cortex size were examined using scanning electron microscopy. These characteristics were then compared to plumage brightness of both light and dark patches. We show a significant relationship with average head darkness and reduction in irradiance values when the head was held at 45 degrees from horizontal. Dark patches in the anterior and posterior dorsal quadrants are most important in this reduction in irradiance. Preliminary findings on feather microstructure will be discussed.
