Fig 1.
Photographs and microscopic details of the uppermost tail feather of the Oriental magpie (P. serica).
The tail feather of the magpie was observed using an optical camera. The barbs extend from the rachis, and the barbules reach out from the barbs in succession. The surface of the flattened and elongated parts (pennulum) of the distal barbules is iridescent green, transforming into iridescent violet and navy around the tip of the feather. The scale bars in left and right represent 50 μm, 200 μm, respectively.
Fig 2.
Optical and electron microscopic images and the structural model of the barbule.
A Surface of the barbule under an optical microscope (x1000) showing multi-colored domains. The red arrows indicate yellow, violet, and blue from top to bottom. The dashed red line indicates the horizontal band. The yellow point represents the direction of light propagation. B Scanning electron microscopy image of the barbule surface. C Transmission electron microscopy image of the barbule cross-section showing the hexagonal structure of the melanin stack. The yellow arrow indicates the direction of light propagation. D Structural model of the barbule and the type of the structure classified as type 5 from Durrel’s classification. The scale bars in A, B, and C represent 10 μm, 20 μm, and 1 μm, respectively.
Fig 3.
Reflectance spectra measured using the experimental setup and calculated using FDTD simulations.
A Reflectance spectra of the barbule in the green area indicated by the red circle measured using the experimental setup. The wide spectrum has a dominant peak at about 520 nm. B−E Spectra calculated using FDTD simulations. The keratin, melanin, and air are represented by black, gray, and white in the simulated images, respectively. The refractive indices for keratin and air are 1.55 and 1.00 [25]. The refractive index of melanin was taken from Stavenga et al. [25] and fit with a Lorentzian function. The B and C spectra agree well with the experimental spectra, while the D and E spectra differ. The solid blue and orange lines represent TE and TM mode, respectively.
Fig 4.
Reflectance spectra obtained from FDTD simulations.
A Four simulated sections of the barbule, each with a height of around 1 μm, corresponding to the size of the color domain. Enlarged images are presented in S3 Fig. B Spectra for the sections of the barbule calculated from the FDTD simulations. Each section has different peaks. Bar (A): 1 μm.
Fig 5.
Fourier transforms for the sections from a single-barbule TEM image.
The Fourier transforms for the refractive index distribution in the barbule are expressed in spatial frequency. The peak wavelengths predicted from Fourier analysis are listed under each image.
Table 1.
Relationship between the structural parameters and the wavelength of the peak reflectance predicted from Fourier transform analysis.