Peacock Feathers

A courtship fan of vivid greens and blues, subtle browns, blues, transfixing 'eyes'. Iridescence? Not really, this is not the iridescence of clouds or oily puddles or of soap bubbles. Their colours are transient, mostly pastel hued and shift with angle of eye and sun. A peacock's display is electrically bright with colours almost constant from all directions.

Let's look at the peacock at higher and higher magnification - not to steal its magic but to reveal more of it.

Top left: The bright blue eye looks part of a tufted carpet pattern. A feather main stem crosses the eye and barbs project symmetrically outwards from it. Along each barb are rows of tiny hair-like barbules and the colour source is within them.

Eyesight alone fails beyond this magnification. At Mid left electron microscopy comes to aid to show a single barb with its rows of barbules. The barbules twist along their length and have a crescent shaped cross-sections.

Slice across a barbule to find a closely packed outer skin with looser structures inside. Magnify yet more and the colour generators are finally revealed. The outer layers are a highly ordered rectangular array of dark circles embedded in lighter stuff. There are no coloured pigments

The dark circles are slices through melanin rods 110-130nm dia. In comparison the peak wavelength of peacock blue light is 450nm some 3-4X greater. The rods are separated by transparent keratin and there are are air holes threaded between squares formed by the rods. Nano-engineering.

Look at different coloured feather regions and the rectangular lattice structure is everywhere - only its dimensions and number of layers change. Blue and green barbules have 9-12 layers of rods, yellow ~6 and browns ~4. The lattice spacing changes with colour, blue has the tightest lattice of rods spaced 140nm between centres.

Lower left: A single layer of rods would form an ordinary diffraction grating. Light waves scattered by each rod would combine with constructive and destructive interference to flash several orders of sunlight spectra in different directions. Add more rod layers and the waves scattered by each rod have extra opportunity to interfere with those from others. More and more wavelengths destructively interfere leaving only a relatively narrow wavelength band (~100nm wide for 525nm blue feathers) of colour. The scattered colour depends on the lattice spacing.

The whole array of rods, melanin matrix and air holes comprises a photonic crystal.   The periodic array of changing refractive index has a band gap where light is of certain colour is blocked. The blocked light is scattered externally to give us the peacock's colours.

There is much interest on mimicking these natural wonders with potential applications in optical engineering and communications. Less seriously, photonic crystal pigment-free paints would not fade, fabrics might be more vibrant, cosmetics and photonic toothpaste could support a whole new marketing industry.

The peacock's 2D photonic crystal is not the whole story. It alone does not account for all peacock feathers' optical effects. The crescent shape of the barbules likely plays a role and there are bulges along the melanin rods thought to have an optical function.

The peacock has reason for its pride.

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Peacock feather at higher and higher magnification. An electron microscope sequence taken by Shinya Yoshioka & Shuichi Kinoshita.

(A) A single blue feather barb (barbs project directly from the main feather stem, not visible here). Barbules are attached to each side of the barb. The ~0.5mm long barbules generate the colour.

(B) Close-up of a barbule. It twists slightly along its length.

(C) Slice through two barbules. They are crescent shaped ~50 micron (0.05mm) across. They have dense outer layers with looser structures within.

(D) Section through the outer layers of a barbule.This is the source of the colour. There are no pigments. The dark circles are high refractive index melanin rods arranged in a carefully sized rectangular array. The rods are embedded in lower refractive index keratin. Between each square of melanin rods there is an air tube (not visible in this image).

Further reading:

Shinya Yoshioka and Shuichi Kinoshita (2002), Effect of Macroscopic Structure in Iridescent Color of the Peacock Feathers, Forma 17,169-181.

Jian Zi, J, Xindi Yu, X, Yizhou Li, Xinhua Hu, Chun Xu, Xingjun Wang, Xiaohan Liu*, and Rongtang Fu, Coloration strategies in peacock feathers (2003), PNAS. 100:12576-12578

Image credits

Les Cowley

Peacock feather close-up
Dr Erica Bower Science Writer, editor, photographer, UK - Shown with permission

Barbule electron micrographs
Professors Shuichi Kinoshita & Shinya Yoshioka
Osaka University, Japan - Shown with permission