Origin and biomimicry of weak iridescence in black-billed magpie feathers

The feathers of the black-billed magpie exhibit distinct iridescence, which is much weaker than that of other bird feathers well known to display structural colors. Close examination of the cross-sectional microstructures of the magpie feather barbules revealed that the vertically layered structure of melanin tubes along the barbule surface (cortex) is responsible for the weak iridescence, while their lateral arrangement is of little importance. Artificial barbules that complied with the microstructures observed in the magpie barbules were realized by rolling a flexible polymer grating film. The comparison between experimental data obtained from the artificial barbules and theoretical results simulated for model structures strongly supported our hypothesis on the origin of the weak iridescence. By controlling the single and most significant structural parameter, the optical thickness of an individual cortex layer, we successfully demonstrated that the color hues of the resultant artificial barbules can vary across the entire visible range.