ABSTRACT
The iridescence of the blue feathers of the Formosan blue magpie (Urocissacaerulea) is not conspicuous when the viewing angle is less than 40°. 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 result agrees closely with measurements.
Subject(s)
Birds/anatomy & histology , Colorimetry/methods , Feathers/chemistry , Feathers/ultrastructure , Models, Biological , Refractometry/methods , Animals , Computer Simulation , Models, Chemical , Skin PigmentationABSTRACT
A range of iridescent colour appearances are presented by male Swinhoe's pheasants' (Lophura swinhoii) 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 inside the barbules of these two regions are similar. However, this study found that the spatial variation in their colour 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 colour appearance.
