Pick up your favourite music CD-ROM. Now take it out into the sun and watch the scintillation and magnificent colour – a full spectrum displayed as a result of reflections and diffraction on the surface of the disk. The result is a prismatic effect. The reason is the tiny grooves on the CD-ROM; only 1,6 micron apart. This dimension is comparable to the wavelength of the incident visible light (the range is about 0,4 to 0,75 microns).
Now put that music onto the player, relax, and read on!
The incredible reality is that iridescence in bird feathers is created in a comparable way. Certain birds (my favourite being our sunbirds), have microstructures within some of their feathers (most popular are throat feathers, but also in many other places), the effect falling under the general description of “animal structural colouration”. The effect is a result of a diffraction grating (as for the CD-ROM example), but may also be affected by the natural feather pigmentation. The actual colour (eg. red or blue etc.) would largely be created through differing feather microscopic structure dimensions – where larger microscopic dimensions would as an example create red compared to say blue or violet.
The really interesting point is that the colour and iridescence of birds has been studied for a very long period, and birds have done their part to contribute to the knowledge of the physics of light!
Ironically it was Robert Hooke who in 1665 first described the iridescence of peacock feathers in some detail, and accurately described the microscopic detail of the tail feathers. However, it was his enemy Isaac Newton who first discovered the prism the next year, and Newtons great friend James Gregory who discovered diffraction grating using a bird feather in 1667. When you add to this list the fact that birds had an influence on the work of Christiaan Huygens, Joseph von Fraunhofer, and then Thomas Young (who proposed the wave theory for light), and finally Augustin-Jean Fresnel, you realise just how important those bird feathers were. They were initially observed through the first-generation of microscopes, but the microstructures in bird feathers could not easily be emulated in any man-made items of that time, and the bird feathers were rather complemented by other iridescent natural phenomena such as butterfly wings.
But why do some birds have iridescent feathers? Sadly, a great deal of misinformation has arisen on this score! The initial theory was that birds would gain a reproductive advantage, and it was actually equated to the advantage of better camouflage. The advantage of camouflage is obvious, but the case for iridescence being a camouflage characteristic is patent nonsense. Among the other “contender” theories, only the potential use for increased sexual signal appears to hold any ground. A theory exists that iridescence in animals (a shimmer?) may provide protection by confusing predators, but in the case of birds this does seem a stretch. (There are other theories that iridescence in UV, (higher frequency than the visible spectrum of potential predators), may be used for communication to evade the predators, but this is probably even more of a stretch!) Objectively, the best possible interpretation is that in reality the “prettier” a bird, the more likely to find a mate!
As a truly inconclusive conclusion, we must note that work is ongoing on many, (and some frankly oblique), new theories on bird iridescence (eg. use in flocking). Interesting as these may be, we are greatly blessed by the beauty of these creatures, and our recommendation is to enjoy them first, and try to figure out reasons later.