But to be honest, there really aren’t many birds that are truly purple. Purple Martins are more of a dark indigo. Purple Finches are actually are a reddish raspberry color. These are pretty colors too, but where is that gorgeous, heart-stopping royal purple?
The only local bird that has that special purple is the Black-chinned Hummingbird. Just below the black feathers on the chin of the bird is a band of exquisite purple. It is actually an iridescent patch of color. This means you can only see it when the light hits it just right. It has the effect of flashing on and off as the bird moves, and the angle changes.
When you think about what makes colors, you rapidly get caught up in a complex world of physics. Red objects reflect the longer “red” wavelengths of light but absorb other colors. In other words, we see the object as having a red color.
Purple is on the other end of the spectrum and has shorter wavelengths than any other visible color. Purple objects reflect these short wavelengths and absorb the other colors. We see purple.
Reflected colors like this result from colored pigments in the objects we are viewing. So, the red color in Northern Cardinals derives from actual colored pigments in their feathers. When white light shines on a male cardinal, it will reflect red to us. The color looks the same from any angle.
But this is not how hummingbird feathers get their color!
Hummingbirds have iridescent feathers, especially on their throats and heads. Iridescence is defined as a “play of colors producing a rainbow effect (as in a soap bubble)…that changes as the angle of view changes.” We can see the same rainbow effect on the bottom side of a CD or in a thin slick of oil on water. Sir Isaac Newton, in his treatise on optics in 1704, suggested that iridescent colors in birds and insects might be due to the presence of thin films on their surfaces.
But what causes iridescence? Some definitions attribute it to “differential refraction” (say what?). Others claim it is due to interference and scattering. Needless to say, you need a pretty good understanding of physics to know what this means.
Not really having a brain for physics, I asked my own personal physicist (my husband) to explain it to me. What he said is a little too complicated for this format, but if you want an in-depth explanation, read the online version of this week’s column at www.coastalbendnaturalist.com.
In any case, it is now well accepted that the color in iridescent feathers is due to the interference phenomenon. Some very small physical structures on the surface of the smallest branches of a feather give rise to the color we see by interfering with the wavelengths of light.
These structures are indeed tiny. Microscope examination of the surface of the feather barb has revealed that it is made up of a mosaic of “platelets” that resembles a tiled floor. Each platelet is so small that it would take 10,000 of them end to end to make an inch. And each tiny platelet has a hollow center filled with air. Depending on the thickness of the walls and the amount of air inside, the light waves are interfered with, and a specific color is produced.
In addition, the “tiled floor” is only one of many layers. The layers work together to concentrate some colors and destroy others. This amazingly complex “periodic nanostructure” is what produces that flash of purple on the throat of a Black-chinned Hummingbird. A slight variation in the nanostructure results in the bright red of a Ruby-throated Hummingbird. Another arrangement of the nanostructure gives the bright iridescent green of a Buff-bellied hummer’s throat.
This whole business of structural color blows me away! It is just amazing to me that such complexity evolved, all to provide a bright flash of purple on a tiny bird.
Which brings us to the question, why did iridescence evolve anyway? Many birds display varying degrees of iridescence. Insects, especially butterflies, have it. Fish and marine creatures flash iridescent colors. There have even been recent discoveries of iridescence in plants!
It seems that in all cases, this kind of coloring produces a kind of communication between these organisms. What messages are they sending? It is a whole new field of scientific study. Doucet and Meadows, in their 2009 paper in the Journal of the Royal Society, state that “despite the vast amount of recent work on animal iridescence, our review reveals … an area ripe for investigation.”
Isn’t this exciting? There is so much to learn about in our world.