Feather Iridescence

A tiny blue tit, Cyanistes caeruleus, blazes with iridescent colour as it flies close to the sun.
An image by Rene Winter who has honed the art of capturing bird iridescence.

All images ©Rene Winter, shown with permission

Atmospheric
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Bird feathers iridesce in several ways. A feather has several levels of order. Its main stem has regularly spaced branches called barbs projecting outwards. In turn, each barb has projecting from it regularly spaced sub-branches called barbules. Look inside the barbules with an electron microscope and they might even have ordered internal structures.

Rene Winter’s images were taken close to the sun. Take extreme care or do not do it!  Sight is far too valuable to play with. We are probably seeing the effects of light scattered through relatively small angles by the arrays of barbs or barbules. The arrays act as diffraction gratings taking in white sunlight and directing shafts of colour into specific directions. Each grating element scatters light into outgoing waves. Adjacent waves overlap and interfere. In directions where the sets of wave crests for a particular colour are in phase there is coloured light. Directions where the waves are out of phase are dark. The regular circles of dots on a CD or DVD act the same way.

Scatterers do not have to be regularly spaced – witness those of a corona around the moon from randomly spaced droplets. Feather barbs and barbules can similarly individually scatter light to produce colour. However, the effect is likely to be weaker than the coherent scatter from regular barbule arrays.

Peacocks, humming birds, some doves have bright colours by astoundingly sophisticated mechanisms. Ordered structures within barbules act as three dimensional photonic crystals to produce them – structural colour.

The intense but non iridescent blues of kingfishers, parakeets and blue jays were previously thought to come from yet another light scattering mechanism – Tyndall or Raleigh scattering. Very small randomly spaced particles scatter blue light much more strongly than longer wavelengths – our blue skies are so made. Material inside barbules was thought to act in this way. It is now known that this explanation is over simple and that the inner structures are not quite random – although they might well have originally evolved from random ones.

Iridescent colours adorn a backlit European skylark.
A backlit Bohemian Waxwing shows ordered iridescence from its wing and tail feathers.