Abstract

Our objective is to provide ornithologists with a tool for looking at 3D models of birds through the eyes of other birds. This tool overcomes the limits of the human visual system to allow scientists to quantify and analyze avian color in a completely novel way.

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Acknowledgements

National Science foundation grant 0852844. The specimens used in creating 3D models are housed at the Yale Peabody Museum (YPM). We thank Kristof Zyskowski of the Yale Peabody Museum for assisting with bird selections and…More Rômulo Carleial of the Universidade Federal de Minas Gerais for assisting with measurements. Our pipeline uses Patrick Min’s MESHCONV 3D model converter.
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Abstract

Light interacts with an organism's integument on a variety of spatial scales. For example in an iridescent bird: nano-scale structures produce color; the milli-scale structure of barbs and barbules largely determines the directional pattern of reflected light; and through the macro-scale spatial structure of overlapping, curved feathers, these directional effects create the visual texture. Milli-scale and macro-scale effects determine where on the organism's body, and from what viewpoints and under what illumination, the iridescent colors are seen. Thus, the highly directional flash of brilliant color from the iridescent throat of a hummingbird is inadequately explained by its nano-scale structure alone and questions remain. From a given observation point, which milli-scale elements of the feather are oriented to reflect strongly? Do some species produce broader "windows" for observation of iridescence than others? These and similar questions may be asked about any organisms that have evolved a particular surface appearance for signaling, camouflage, or other reasons.

In order to study the directional patterns of light scattering from feathers, and their relationship to the bird's milli-scale morphology, we developed a protocol for measuring light scattered from biological materials using many high-resolution photographs taken with varying illumination and viewing directions. Since we measure scattered light as a function of direction, we can observe the characteristic features in the directional distribution of light scattered from that particular feather, and because barbs and barbules are resolved in our images, we can clearly attribute the directional features to these different milli-scale structures. Keeping the specimen intact preserves the gross-scale scattering behavior seen in nature. The method described here presents a generalized protocol for analyzing spatially- and directionally-varying light scattering from complex biological materials at multiple structural scales.

Acknowledgements

The authors would like to thank Jaroslav Křivánek, Jon Moon, Edgar Velázquez-Armendáriz, Wenzel Jakob, James Harvey, Susan Suarez, Ellis Loew, and John Hermanson for their intellectual contributions…More. The Cornell Spherical Gantry was built from a design due to Duane Fulk, Marc Levoy, and Szymon Rusinkiewicz. This research was funded by the National Science Foundation (NSF CAREER award CCF-0347303 and NSF grant CCF-0541105).
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Average directional reflectance of rectangular regions of Chrysococcyx cupreus…More

Average directional reflectance of rectangular regions of Chrysococcyx cupreus: (A–C) 3 regions of the medial vane, (a–c) 3 regions of the lateral vane, and (Aa & Bb) 2 regions spanning both vanes. Luminance plotted in direction cosine space.

Special Committee

Dr. Susan Suarez, Dr. Steve Marschner, Dr. Kim Bostwick, Dr. Ellis Loew, and Dr. John Hermanson

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 so-called “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 structurally-colored 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.

RGB directional reflectance of a region between 2 barbs of the medial vane was subdivided…More

RGB directional reflectance of a region between 2 barbs of the medial vane was subdivided into 44 linear divisions. The Average reflectance of 22 subsampled divisions is shown: (1) of line 1 along a length of ramus; (2–21) along sequential lines following step-wise movements; (22) of line 22 along a length of the adjacent ramus.