| dc.description.abstract | There is a great diversity of species on the earth; among them birds have the amazing and multiple colors, especially on their plumage. The colors of feathers play several important roles for birds, such as hardness of feather enhancement, protection from the erosion of feather by bacteria, thermoregulation, concealment, advertisement, and disguise. In addition, color can serve as an indicator of nutrition condition of birds. As a result, color is also a criterion of mate choice and competition to others of the same sex, especially for structural color. Particularly, there are high associations between ultra-violet color and courtship display. Therefore, in this research, we have explained the physical mechanism of the structural blue-violet coloration in birds’ feathers and build the corresponding mathematical models.
Blue feathers of two endemic avian species in Taiwan are chosen in this study, Swinhoe’s pheasant (Lophura swinhoii), and Taiwan blue magpie (Urocissa caerulea). A range of iridescent color appearances are presented by male Swinhoe’s pheasants’ mantle feathers. Two distinct regions of the open pennaceous portion of its feathers display particularly conspicuous angle-dependent reflection. A bright blue band appears in one region at normal incidence that spatially shifts to another at higher illumination angles. The two-dimensional photonic crystal-like nanostructures, melanin rods, inside the barbules of these two regions are similar. However, this study found that the spatial variation in their color appearance results from a continuously changing orientation of barbules with respect to the alignment of their associated barb. A multi-layered rigorous coupled-wave analysis approach was used to model the reflections from the identified intra-barbule structures. Well matched simulated and measured reflectance spectra, at both normal and oblique incidence, support our elucidation of the origin of the bird’s distinctive feather color appearance.
Different to the conspicuous iridescence in Swinhoe’s pheasant’s mantle feather, the iridescence of the blue feathers of the Taiwan blue magpie is not obvious when the viewing angle is less than 40-degree. In addition, the biomaterial, medullary keratin, producing the blue color of Taiwan blue magpie exists inside the barbs rather than barbules. The spongy medullary keratin inside the feather barbs is investigated by two-dimensional Fourier analysis of transmission electron microscopic images of various positions on a barb to explain this unique characteristic. The orientation of the quasi-ordered nanostructure varies depending on its position of the feather barb. The predicted reflectance increases with the distance of the nanostructures from the vertex of the feather barb, and this results agrees closely with measurements.
This research provides two optical models to elucidate two different iridescent colorations from nanostructures in barbs and barbules of bird’s feathers successfully. | en_US |