Origin of two-color iridescence in rock dove's feather

Iridescence is observed in various kinds of animals that utilize optical interference phenomenon of microstructures to produce their brilliant colors. It appears according to the interference condition that relates the wavelength of the reflected light with the angle of view or incidence. However, the iridescence of the neck feather of rock dove looks very peculiar; the color change is limited only in two colors, green and purple, and the change occurs very suddenly by only slightly shifting the viewing angle. We show that this two-color iridescence originates from the surprisingly simple physical mechanism-thin-layer interference. The peculiarity lies in the fact that the higher-order interference condition is satisfied. This causes the sophisticated correspondence in the spectral line shape between the reflectance and the visual color sensitivities of human eye, and results in the two-color nature of the iridescence. It is also suggested that the rock dove's vision perceives this two-color, iridescence as a tool for visual signaling among rock doves.