Spatially- And Directionally-Varying Reflectance Of Milli-Scale Feather Morphology

Abstract

Birds have evolved diverse plumage through sophisticated morphological modifications. The interaction of light with these modifications alters the reflectance from feathers, producing complex and directionally-variable visual signals. I hypothesize that structural modifications of the feather produce anisotropic reflectance, the direction of which is determined by the orientation of the structure of the vane. Variation in reflectance originates from the interplay of light with two classes of feather structure: its surface and subsurface volume. Different structural scales within the two structural classes influence light scattering within the UV-visible spectrum. The overall shape and surface of the feather vane (the macro-scale) and of its component members (the milli-scale) scatter light according to principles of geometric optics. Subsurface nano-scale structure in many feathers generate socalled "structural coloration," which is a purely physical optics phenomenon and can differ drastically from ordinary coloration mechanisms such as pigmentation. Iridescence, from which many feathers derive their vivid, eye-catching changeable color, is one type of structural color that varies as a function of viewing angle. This thesis presents investigations into a previously understudied aspect of avian visual signaling: directional reflectance and its relationship to milli-scale structure. Having observed that the stratified nano-scale morphology of structurallycolored plumage contours the milli-scale cortex of the vane, I determined that measurements of the milli-scale could be substituted for a more complex study of directional reflectance from the nano-scale. I thereby hypothesize that the direction of the reflectance from a vaned feather can be predicted from the orientation of its milli-scale morphology-its barbs and barbules. In collaboration with my colleagues at Cornell University, I developed non-destructive tools and methods to investigate the signaling potential of the feather. I correlate measurements of directional light scattering to the milli-scale morphology of select samples of structurally-colored bird plumage. The results of these analyses lead to a more thorough understanding of the relationships between directional reflectance and the structure of the feather itself. Having found the reflectance to be anisotropic, I demonstrate that the change in the direction of the reflectance over the surface of the vane can in fact be predicted from the orientation of the different branches of the barb. The improved understanding of the variation in directional reflectance over the surface of the feather, a phenotypic component, should allow for better comprehension of avian behavior, evolution of morphological adaptations, and the synthesis of more accurate predictive models.