Measuring and modeling the inconspicuous iridescence of Formosan blue magpie's feather (Urocissacaerulea).

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.