Introduction

The word color actually has a broad definition. Although color can be thought of as a scientific concept of energy, reflective material, visible light, and colorimetry, only the definition of color associated with the optical phenomenon was focused on in this study. In this sense, color can be divided into pigmentary color and structural color. Pigmentary color is the color seen by humans in nature, and melanin and carotenoids are represented largely in pigments (Stephen et al., 2011). These colors are the most common in living organisms and ecosystems. The pigments include red, black, brown, gray and yellow (McGraw et al., 2004; 2005). On the other hand, structural color refers to the color caused by microstructure and light diffraction and interference. Many known examples of structural color are rainbows, colorful feathers of birds including peacocks, and the colors of Morpho butterflies and abalone shells. Birds have colorful feathers, and the iridescence of feathers attracts researchers from many different scientific disciplines - physics, material science, biology, and more. In nature, the brilliant feathers of colorful birds are classified into structural colors (Kinoshita, 2008; Prum, 2006). Structural colors of feathers include metallic blue, red, green, and purple, and the colors give off a brilliant sheen because of their high reflectance (Kinoshita et al., 2008; Lopez-Garcia et al., 2018). Generally, structural color is produced by interactions of light and microstructures in organisms (Prum, 2006).

The structural color can be divided into an iridescent color and a non-iridescent color (Kinoshita & Yoshioka, 2005). The iridescent color is that the color that appears different depending on the angle at which it is viewed, and non-iridescent color refers to the appearance of a constant color regardless of viewing-dependent geometry (Fu et al., 2016). For example, peacocks, and mallards exhibit iridescent color (Medina et al., 2015), and cotingas and manakins exhibit non-iridescent color (Kinoshita, 2008). The iridescent color of feathers produced in the barbules is related to the shape and the reflective layers of air, keratin, and melanin granules (Doucet et al., 2006). The non-iridescent colors of feathers are generally produced with light scattering by air vacuoles within the keratin of the barbs (Kinoshita, 2008; Noh et al., 2010).

In particular, non-iridescent blue feathers have attracted the attention of researchers, because the color blue is rarely produced by the light and the pigments and, in many cases, is produced as the structural colors. . Some studies have shown the optical analysis of blue feathers of cotingas, bluebirds, jays, and blue penguins (D'Alba et al., 2011; Noh et al., 2010; Parnell et al., 2015; Shawkey et al., 2006). In this study, we studied a bird with blue feathers who inhabit in Korea. The purpose of this study was to determin the blue coloring mechanism through the microstructure and feather reflectance of blue feathers and to show the possibility of an application technique of biomimicry in the future.

Materials|Methods

Results|Discussion

The structures of the dorsal, ventral and barbs of blue feather vanes were observed with a stereomicroscope (Fig. 2). The feather vanes showed different colors on the dorsal and ventral sides, and the bluish shining of the barbs was observed when they were enlarged. Common Kingfishers have blue color only on the dorsal side feathers from the back, tail and wing (Fig. 2).

In the cross-section of blue feather barbs, barbs were composed of a keratin cortex and sponge matrix, which are medullary to the keratin rod and air space (Fig. 3). The vacuoles were located in the center of the barbs, and melanin granules were irregularly distributed in the sponge matrix. Interestingly, the wing feather barbs occupied approximately half of the keratin honeycomb structures in their barbs (red circle in the Fig. 3C). The diameters of the four categories (CKR, CAS, IKR, and IAS) of feather microstructures are given in Table 1. The feather barbs of the three parts showed significant differences in the four categories (P<0.05).

The reflectance spectra of the back and tail feathers of common kingfisher showed a flattened pattern between approximately 350 to 550 nm without peaks (Fig. 4A). On the other hand, the wing fathers showed a low but certain peak, which was approximately 8% on the reflectance spectrum at 350 nm (Fig. 4).

Several studies on feather structural color have reported that non-iridescent blue feathers are produced by coherent scattering with a β-keratin spongy layer and light (Kinoshita, 2008; Kinoshita et al., 2008; Noh et al., 2010; Prum, 2006; Shawkey et al., 2006; Stavenga et al., 2011).

The barbs of common kingfishers also have a β-keratin sponge layer, air vacuoles and melanin granules. Honeycomb structures are observed with the keratin sponge layer in the secondary feather barbs.

The reflectivity of common kingfisher’s back and tail feathers was high, and the wing feathers had lower reflectivity. Given that, although there are significant differences in CKR, CAS, IKR, and IAS, the relationship between reflectivity and feather microstructure is not seen as a simple measurement, so a more precise experiment is needed. However, it is assumed that the reflectivity of wing feather barbs is low due to the keratin honeycomb structures. The back and tail feathers are composed of the keratin sponge layers in the barb, but because of the keratin sponge layers, wing feathers occupied approximately 49.3% and 31.7%, respectively (unpublished data).

The keratin rods in the sponge layers were arranged irregularly according to several studies, they are quasi-ordered arrays with short-term periodicity (Noh et al., 2010; Prum, 2006; Prum et al., 2009; Stavenga et al., 2011).

The keratin rod in the sponge layer is formed at a specific size to produce the color and then halt, and the rod must be very small to produde the blue color (Table 1; Prum et al., 2009). In the Peacock feathers (although it is not non-iridescent color), the melanin granules in blue feathers were smaller and more densely packed than those in other colors, and the keratin rod in the feather of the eastern blue bird also had a nanoscale diameter suitable for blue (Shawkey et al., 2006; Yoshioka & Kinoshita, 2002). Thus, the keratin rods in this study are suitable for producing blue colors as quasi-ordered arrays with keratin rod nanoscales.

In barbs, the melanin granules are rod-type and irregularly distributed in the keratin sponge layers and around air vacuoles. The role of melanin granules is light absorption in a non-iridescent blue color, which also plays a role in the feather barb of common kingfishers.

We presumed that the color mechanism of the three parts feathers of common kingfishers was caused by incoherent scattering. In particular, we need to carry out further studies to determine more precise feather color mechanisms, feather microstructures and whether the flat spectrum of the back and tail feathers can be explained by the mixture distribution model.