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.
Highlights
Many bird species feature a brightly colored plumage due to wavelength-selective absorbing pigments and/or more or less orderly arranged nano-scale structures (Durrer, 1977; Hill and McGraw, 2006; Kinoshita, 2008)
In the course of our studies on peacock feathers, we found that a crucial element of the peacock’s photonic structures is the keratin cortex, a superficial layer of keratin overlying the layer of melanosomes
Transmission electron microscopy (TEM) studies showed that the melanosomes of S. vulgaris are solid rodlets with a length of 1.2–1.7 μm and diameter of 0.250–0.254 μm (Durrer and Villiger, 1970)
Summary
Many bird species feature a brightly colored plumage due to wavelength-selective absorbing pigments and/or more or less orderly arranged nano-scale structures (Durrer, 1977; Hill and McGraw, 2006; Kinoshita, 2008). A frequently occurring pigment in organisms is melanin, which causes, for example, the black feather colors of numerous bird species (McGraw, 2006) and has recently attracted attention in bio-inspired materials (Xiao et al, 2020). Instead of being an effective absorber of light, melanised organelles (the melanosomes) can function as reflectors, because melanin has a relatively high refractive index (∼1.75) with respect to the bulk feather material, keratin (∼1.55; Mason, 1923; Durrer, 1977; Stavenga et al, 2015). Striking examples where multilayers of solid melanin rodlets create bright reflectors include the feathers of birds-of-paradise (Durrer, 1977; Stavenga et al, 2011), common bronzewing pigeons (Xiao et al, 2014), and ducks (Eliason and Shawkey, 2012; Stavenga et al, 2017)
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