I've already discussed the iridescence of my little fish and how that vivid colouring originates from interference and phase-shifts within a thin layer of material that is as deep as the light's wavelength. Now, I'm going to move on to the iridescence in a male hummingbird's gorget. In a medieval world, a gorget was armor designed to protect one's throat – a term that has been extended to include the metallic-like iridescent feathers on the hummingbird's throat, the purpose of which is likely to attract lady hummingbirds. It takes a combination of optical effects at different scales acting together to create iridescence in a feather. Iridescence is considered a 'structural' colour because it is produced by optical effects instead of pigments.
Hummingbirds are fascinating. Their wings beat so fast I can hardly see them and these tiny creatures are the only group of birds that can fly backwards, sideways or straight up. With a revved up metabolism, their little hearts beat around 1200 times a minute and they only live about three to five years. Which leads me to the question: what would happen if a hummer drank caffeine? Would it be able to beat its wings fast enough to fly out of our time-space continuum?
As part of Newton's investigations in the early eighteenth century regarding the nature of light, he hypothesized that the iridescent colours found in bird's feathers were due to the presence of thin films. Newton was right about the thin films, however, he didn't discover the actual mechanism. It took another 100 years before interference was put forward as the mechanism behind iridescent colours in birds and many more years until this theory was accepted. Plenty of birds display iridescence in either small ways (like a pigeon's breast) to the scintillating colours of a peacock's display. It's feathers that provide the light-reflecting layers that create iridescence.
Feathers are complex in structure and fill a wide variety of roles. They keep a bird warm, prevent it from getting a sun burn, provide a streamlined shape, and aid in waterproofing. More importantly, tail and wing feathers allow a bird to fly (with the exception being the birds that don't fly, where feathers provide buoyancy, insulation and even formal wear for penguins). Feathers supply colour for camouflage and for the males these colours help with attracting mates. Birds have evolved to use many different techniques to produce the colours in their plumage including pigments, structural colours and a combination of the two. Through pigments they can produce yellows, reds, browns, and blacks. Whites and blues are often formed by selectively scattering incoming light in tiny air pockets within the feather barbs, while greens are produced by combining yellow pigment with air-pocket scattering effects. Iridescence is the most intensely striking and vivid structural colour birds produce.
On a hummingbird, the small-scale external structure of the feather is not flat, but raised in a v-shape as a series of ridges and troughs. This microscopic structure preferentially reflects light towards an observer directly facing the bird. At all the other angles almost no light gets reflected, which makes the feathers appear black.
On an even smaller scale, within the hummingbird's feathers there is a stack of three (usually) thin films. Each film has a thickness equal to approximately half the wavelength of the light it intensifies most (visible light wavelengths range from 380 to 740 nanometers, and a nanometer is one billionth of a meter). The films themselves are constructed from eight to ten layers of an irregular mosaic of thin elliptical discs and little pockets of air. The elliptical disks are about 2.5 microns long by one micron wide – about the size of a yeast cell. Different hummingbird feather colours are produced through a combination of the above structural effects. The elliptical disk thickness tends to decrease as the colours pass from red towards blue while at the same time the size of the air pockets increases. For more complexity, iridescent feathers often also contains pigment which absorbs the background light, allowing the structural colours to be even more vivid.
The iridescence-producing feather structure I've described is a complex phenomenon which is actively being researched. There are even on-going attempts to simulate feather iridescence. If this technology pans out, someday I may be able to wear a coat with reds as vivid as an Anna's Hummingbird's gorget.
thanks to G. Hanke for the photo
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