posted September 20th, 2011; last edited October 26th, 2011 –– David Sibley

The basics of iridescence in hummingbirds

After all the discussion of orange-throated and red-throated hummingbirds, I thought it would be helpful to add a brief and simplified summary of how the brilliant iridescent colors of hummingbirds are produced. These are structural colors, not pigment, which means they are reflected by microscopic structural features of the feather surface.

The gray bands at the bottom represent a cross-section of an air bubble in hummingbird feathers. Incoming light waves are shown in gray. Some light reflects from the upper surface of the air bubble, and some light passes through and reflects off of the inner surface as well. When wavelengths of light (red in this example) match the thickness of the air bubbles. the two reflected waves combine constructively so that light of that color is enhanced. Wavelengths (green in this example) that do not match the thickness of the air bubble are "out of sync" when they reflect off the two surfaces and cancel out.

The diagram here shows how this happens. The surface of the feather is composed of layers of tiny air bubbles. When light strikes the surface of the feather, some light is reflected from the outer surface, and some light travels through the air bubble and reflects off the inner surface. Light (red in this example at right) with wavelengths that match the thickness of the air bubble are “amplified” as the reflected waves from the inner surface match up and combine with the reflected waves from the outer surface. Other wavelengths (such as the shorter green waves shown in this example) are “out of sync” when they combine after reflecting off both surfaces, and they cancel out. This is the fundamental process that creates the very pure and brilliant colors we see on hummingbirds.

This is an idealized example. In reality the structures that produce iridescent colors in hummingbirds are much more complex, with multiple layers of air bubbles. The refractive index of the material the light must pass through, along with many other factors, can alter the color that is produced, but it is the combined reflections from inner and outer surfaces of the air bubbles that creates iridescent colors. The entire system must be incredibly precise and uniform. The difference between red and orange could be a difference of a few nanometers, and one of the most amazing things about this is that there is so little observed variation in hummingbird colors.

When Ruby-throated Hummingbirds develop orange throats, that means a tiny shift to reflecting slightly shorter wavelengths of light. This could be the result of a thinner layer of air in each bubble, or a thinner layer of solid material forming the outer surface, or a slightly lower refractive index of that material, or many other possible variables.

Still lots of questions…

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