Kingfisher Colours ~ A kingfisher imaged near Paris by Denis Joye. The small bird’s iridescent turquoise blue wing and bright orange breast are unmistakable. Its colours are the result of some remarkable optical engineering.

 Kingfisher images ©Denis Joye shown with permission
(a) Kingfisher feather barbs. Only the barbs are sponge filled and coloured.


(b) The sponge. Keratin with air filled holes. The hole spacing has a pseudo order and is responsible for the wavelength selective iridescence.

A 'wave' of kingfisher blue light is shown for reference.


(a) and (b) from Kinoshita S. & Yoshioka S, 'Structural colors in nature:The role of regularity and irregularity in the structure'. Chem Phys Chem 6:1442-1459 (2005).  

Reproduced with permission from Professor Shuichi Kinoshita, Osaka Univ., Japan.
     
Peacock feathers have spaced rods that produce structure based colours.   Morpho butterflies have stacked shelf-like structures in their wing scales.   Sea shells rely on interference from regular spaced structures.  
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Breast feathers of bright orange are paint box stuff. The feather barbs contain small grains of pigment.

Blue wings and tail are a different matter. They have no pigment. Electron microscopy shows the feather barbs to contain a transparent spongy keratin structure surrounded by few micron thick cuticle.

The sponge generates the colour. At first sight (right) it appears random in form and dimension. It is not. There is order of a sort. Mathematic analysis (2D Fourier Transform) reveals a quasi periodicity in the sponge spacing with a characteristic length for the back feathers of 211 nm. Interference between light waves scattered across the sponge strongly enhances blue and green wavelengths.

Interestingly, the tail feathers have a slightly smaller sponge dimension of 187 nm and indeed, these feathers are a deeper shorter wavelength blue. If more proof be needed of the sponge’s role, microscopists filled it with xylene which has a refractive index close to that of the sponge backbone. The blue colour disappeared.