Saturday, 9 December 2006

Why blue birds are blue

Remarkable bird, the Norwegian Blue … Beautiful plumage!

What's even more remarkable than a Scandinavian parrot is that the blue colour of birds isn't produced by blue pigments but by the interaction of light with microscopic structures in the feathers. And not just blues. White, some greens and all iridescent hues are also structural colours.

These colours are generated when light passes through a material of one refractive index into another. Things happen at the interface between the materials. Some light is scattered back to the observer whereas the rest continues on its path—more or less. If there are several layers, the process is repeated at each junction. The type and number of layers (single- vs multi-layer) and the way in which the light is scattered (coherent vs incoherent) creates a range of colours. Whether a colour is iridescent depends on the part of the feather that generates it: barbs produce non-iridescent colours, barbules iridescent ones.

Although all blues are structural, their production is often governed by other factors. The startling blue of male fairy wrens (Malurus spp), for example, is under hormonal control—the more testosterone a bird has, the earlier he undergoes the moult into his bright new suit.

Some greens are also structural colours, produced in the same way as are blues. The wattles above the eyes of the velvet asity (Philepitta castanea, a fruit-eating bird from Madagascar, contain tightly-packed arrays of collagen that scatter light. (Unlike blues, not all greens are structural—the plumage of African turacos is coloured by turacoverdin.)

Iridescent colours change with the angle of light. They may either be a single hue, which seems to shimmer and shine when the light strikes it, or vary through all or part of the spectrum. In some animals, iridescence is a result of fine incised lines on a surface but in birds (and many insects), they result from the same multi-layer refraction that gives rise to blue and green. The plumage of a male satin bowerbird (Ptilonorhymchus violaceus) is iridescent in a range of hues from deep blue to ultraviolet. That of the brush bronzewing (Phaps elegans) covers most of the visible spectrum.

White results when pockets of air inside the feathers scatter all wavelengths of light. The brightness of the white depends on the number and distribution of those pockets. Compare an egret with a barnyard chook to see the difference. Or this stoic silver gull, still a just-washed white despite the surroundings.


Duncan said...

Fascinating stuff Snail, my education continues!

Snail said...

Nature really is wonderful!

Corinne said...

How amazing nature is and how clever are some people to have worked out what is happening to produce the gorgeous colours in plumage.

Snail said...

It's taken 400 years for physists and biologists to work out some of the biological mechanisms. David Nelson has a more in depth post on colour in blue butterflies.

mark said...

As a graphic designer I have long pondered the question of plumage colour that you have explained so well. Many thanks to you and Denis Wilson for making me a little and wiser.

Mark Greeenhill
(BiBY - "tarkineus")

Snail said...

What makes it even more fascinating is that certain blues reflect UV light, which is visible to some species of birds but not others! Amazing stuff.