Sparkling feather reflections of a bird-of-paradise explained by finite-difference time-domain modeling

Abstract

Birds-of-paradise are nature's prime examples of the evolution of color by sexual selection. Their brilliant, structurally colored feathers play a principal role in mating displays. The structural coloration of both the occipital and breast feathers of the bird-of-paradise Lawes' parotia is produced by melanin rodlets arranged in layers, together acting as interference reflectors. Light reflection by the silvery colored occipital feathers is unidirectional as in a classical multilayer, but the reflection by the richly colored breast feathers is threedirectional and extraordinarily complex. Here we show that the reflection properties of both feather types can be quantitatively explained by finite-difference time-domain modeling using realistic feather anatomies and experimentally determined refractive index dispersion values of keratin and melanin. The results elucidate the interplay between avian coloration and vision and indicate tuning of the mating displays to the spectral properties of the avian visual system. biophotonics | body colors | courtship | signaling | reflectance B irds-of-paradise are best known for their magnificent coloration. Living isolated on Papua New Guinea and its satellite islands (1), the absence of predators has allowed these birds to become extremely specialized for female sexual selection (2). Male birds-of-paradise have evolved extravagant ornamental traits, with intricate sounds and ritualized sets of dance steps and movements accompanied by simultaneous elaborate feather movements, all combined in beautiful displays to win the favor of females (1-4). Among the 39 species of birds-of-paradise almost all colors of the rainbow can be found, and often the males advertise themselves with brilliant, vivid colors framed within a jet-black background. The females on the other hand have dull brownish plumage which has remained in its ancestral color state (1, 2).Whereas the biological purpose of the colorful displays is relatively well understood (1, 2), the coloration mechanisms of the birds' displays and the connection to the visual system of the animals are poorly explored. Feather coloration can be generally categorized in two forms: pigmentary and structural. Randomly arranged, inhomogeneous media containing pigments are colored, because the pigments absorb the diffusely scattered light in a restricted wavelength range. For instance, carotenoids cause the colorful yellow or red feathers of many songbirds (5), and the ubiquitous, broad-band absorbing pigment melanin causes feathers to be black (6). Structural colors occur in feather barbs due to quasiordered spongy structures, and in feather barbules due to melanosomes--nanosized, melanin-containing granules--regularly arranged in layers within a keratin matrix, resulting in directional reflections because of constructive interference (7-11). Differences in the morphology of the structural colored feathers, i.e., in the dimensions of the spongy structured barbs or the melanosome multilayers in the barbules, can modify the color of the ...

High refractive index of melanin in shiny occipital feathers of a bird of paradise

“…In fact, finite-difference time-domain modeling 6 yielded reflectance spectra matching those of the multilayer modeling (compare Figure 2 of Ref. 6 and Supplementary Fig. S3).…”

“…The averaged reflectance spectra resulting from the set of 36 parallel lanes nevertheless showed a close correspondence with the experimental spectra concerning the peak wavelength in the infrared and a similar hypsochromic spectral shift with increasing angle of light incidence, indicating the validity of the heuristic approach. In fact, finite-difference time-domain modeling 6 yielded reflectance spectra matching those of the multilayer modeling (compare Figure 2 of Ref. 6 and Supplementary Fig.…”

“…6 In order to understand the barbule optics in further detail, we wanted to know the material properties of the rodlets and its surroundings, and therefore we performed microspectrophotometry on single barbule cells. The reflectance spectra of small areas of different barbule cells (Figure 3a, inset right) were broad-band and peaked in the blue-violet, and the far-red wavelength range featured striking oscillations with amplitude increasing with wavelength ( Figure 3a).…”

“…Compared to the breast feathers they are less colorful, but the barbules of the occipital feathers exhibit a mirror-like, directional reflection due to nanostructured melanin. 6 The uniquely colorful breast feathers allows the breast color to switch sharply between yellow, blue and black as the bird moves, during the ballerina dance, which is performed as part of the courtship display. [7][8][9] The shiny occipital feathers have a similar function.…”

“…Recent behavioral observations on the closely related Wahnes's Parotia demonstrate that the occipital feather reflections are sharply directed to the observing females during part of the courtship performance, presumably to impress a potential mate, viewing from an elevated position on a tree branch. [6][7][8][9][10] To unravel the optical basis of the shiny occipital reflectors, we investigated the barbule anatomy. This revealed very regularly arranged melanosomes, i.e., small melanin rodlets, arranged in layers.…”