Cortex Thickness Is Key for the Colors of Iridescent Starling Feather Barbules With a Single, Organized Melanosome Layer

Abstract

The iridescent plumage of many birds is structurally colored due to an orderly arrangement of melanosomes in their feather barbules. Here, we investigated the blue- to purple-colored feathers of the European starling (Sturnus vulgaris) and the blue and green feathers of the Cape starling (Lamprotornis nitens). In both cases, the barbules contain essentially a single layer of melanosomes, but in S. vulgaris they are solid and rod-shaped, and in L. nitens they are hollow and rod- as well as platelet-shaped. We analyzed the coloration of the feathers by applying imaging scatterometry, bifurcated-probe- and micro-spectrophotometry. The reflectance spectra of the feathers of the European starling showed multiple peaks and a distinct, single peak for the Cape starling feathers. Assuming that the barbules of the two starling species contain a simple multilayer, consisting locally only of a cortex plus a single layer of melanosomes, we interpret the experimental data by applying effective-medium-multilayer modeling. The optical modeling provides quantitative insight into the function of the keratin cortex thickness, being the principal factor to determine the peak wavelength of the reflectance bands; the melanosome layer only plays a minor role. The air cavity in the hollow melanosomes of the Cape starling creates a strongly enhanced refractive index contrast, thus very effectively causing a high reflectance.

How keratin cortex thickness affects iridescent feather colours

“…Some studies have suggested that the keratin cortex surrounding multilayered melanosomes affects the colours of bird feathers [16,37,38,45]. Of these, most have considered keratin cortices of different thicknesses with theoretical models [36][37][38]46], statistical analyses [33,45] and by observing barbule samples where the keratin cortex had been accidentally removed [36,37]. Specifically, based on a quantitative optical modelling of multi-layered melanosomes in mallard barbules-a very similar system to the pheasant feathers examined in this study-Stavenga et al [38] have convincingly explained the effect of changing barbule cortex thickness on the iridescent colours produced by multi-layered melanosomes on the hue.…”

“…Most previous research has studied the effects of changing the thickness of the keratin cortex on the glossiness of black barbules [44], iridescence from poorly ordered melanosomes [36] and iridescence from single-layered melanosomes [37]. Some studies have suggested that the keratin cortex surrounding multi-layered melanosomes affects the colours of bird feathers [16,37,38,45].…”

“…In the relatively recent work, Freyer et al . [37] have suggested cortex thickness is a key for the plumage colour from single-layered melanosomes by applying effective-medium multi-layer modelling, and the melanosomes play only a minor role. Our direct experimental evidence on altering the thickness of the keratin cortex around multi-layered melanosomes clearly indicates that the keratin cortex could govern the feather colour.…”

“…Whereas many studies assumed that the refractive index of a melanosome is 2.0 [39,40,48], some studies reported that the refractive index varied from 1.7 to 1.8 depending on the wavelength [41,49]. Those studies suggested that the overly high refractive index of melanosomes could have a strong influence on the optical properties of multi-layered biophotonic structures [16,37]. In this study, the optical simulations were conducted using most recently reported empirical data for the refractive index of the melanosomes.…”

Measuring Photonics in Photosynthesis: Combined Micro-Fourier Image Spectroscopy and Pulse Amplitude Modulated Chlorophyll Fluorimetry at the Micrometre-Scale

“…Research has focused on the fundamentals underlying those natural photonic systems, as well as on their implications in nature and how to utilize them as inspiration in biomimicry. There is a vast amount of work on understanding the photonic properties of nano-scale structures found in a variety of animals, spanning from insects [ 1 , 2 , 3 , 4 ] to shellfish [ 5 , 6 ], birds [ 7 , 8 , 9 ] and beyond [ 10 , 11 ]. Animals employ photonic nanostructures for varied behaviours, such as displaying colour in mating or as aposematism, in pattern production and vision [ 12 ].…”