How the observed color of the Swallow Tanager (Tersina viridis) changes with viewing geometry

Abstract

Non-iridescent coloration in birds’ plumage is associated with the quasi-ordered nanostructures of short-range order present within the feather barbs. Even though this type of color does not change with the observation direction, a few bird species which exhibit the so called non-iridescent structurally colored feathers, present significant hue variations as the relative angle between incidence and detection directions is varied. Among these, the Swallow Tanager constitutes an excellent system to investigate this phenomenon as its hue changes remarkably from greens to blues as the angle between the illumination and observation directions increases, and independently of feather orientation. In this paper we elucidate the origin of the striking color change of the Swallow Tanager plumage by computing the reflectance spectra of the feathers’ microstructure. We show that the color observed under different illumination-detection conditions can be predicted quite accurately by applying averaging techniques and by considering the air-spheres microstructure of their feathers’ barbs as the main responsible for the observed color change. This work opens up new possibilities for the design of devices with special color characteristics.