Feather Iridescence: Now You See It, Now You Don't

They say that a picture is worth a thousand words. Sometimes I'm not sure a thousand will do. When it comes to the magic of refracted light, it's probably best to sit back and enjoy the view without getting too wrapped up in the mental gymnastics of trying to understand how we see what we see. It is important, however, that birders have a basic understanding of how the colors we think we are seeing are affected by a number of factors.

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Perhaps no other group of birds captures our imaginations like hummingbirds. The iridescent flash of a hummingbird's gorget (bright throat feathers) can be almost blinding  at just the right angle. Then, with a quick turn of the head, it's gone! These two images of the same male Ruby-throated Hummingbird were taken moments apart. In the bottom image, incoming light is being absorbed and not reflected back to the camera lens and our eyes. When this happens, feathers generally look black. (Photo by Mark Szantyr)

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In general, I'm a person of modest curiosity, the kind who needs to know what time it is, not how the watch was built. Yet, on occasion some topic catches my attention and I can't resist drilling down into the minutiae. My initial interest in the variability of feather iridescence got started when I noticed that a Eurasian Green-winged Teal photographed on Adak Island, Alaska appeared to show wing patches that were different colors (see "Can We Trust the Colors We See? published in this journal in March 2010).

One of the first things that I learned as I started researching the color variation in the wings of the teal was that blue coloration in birds is not the result of pigment. Instead, it is produced when light is refracted by the uneven surface of the feather. In effect, the structural characteristics of the feather bend (refract) the light like a prism, resulting in a spectrum of color that is not produced by pigment. Since feather structure plays a role in this process, feather condition (wear) will affect how much or how little color we perceive (see photos below).

I have to admit that since learning about how blue appearance occurs in birds, my appreciation for Western Scrub Jays has increased dramatically. This is a species that I see many times a day, yet, until recently, I have rarely stopped to take a closer look. A few days after posting the teal discussion, I found a roadkill Western Scrub-Jay. I took advantage of the opportunity to capture some close-up images of one of its tail feathers, which normally appear to be a rich azure blue on a live bird. 

The pictures were taken from a variety of angles with light reflecting off the surface of the feather back to my eye. Then I took some pictures holding the feather up so the light took a direct path through the feather to my eye. As I learned in my earlier research, when reflected, the incoming light waves are altered by the prismatic effect of the feather, producing the blue coloration one expects to see on a Western Scrub-Jay. Conversely, when the light passes directly from its source (the sun in this case) through the feather to my eye, the feather appears to be a dull medium gray.  The images below show a single scrub-jay tail feather (rectrix) from a variety of angles. As you can see, light angle has much to do with the intensity of color that we perceive.

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This image of a Western Scrub-Jay tail feather was taken with the light source directly behind the feather. At this angle, little if any refraction occurs, thus the feather shows only the color that is produced by pigments. Under such conditions, a scrub-jay tail feather looks gray. It is important to note that the outer half of wider outer web of this feather is getting quite worn. Notice how it shows less structural pattern and is paler gray. (Photo by Dave Irons)


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In this picture, the incoming light source is from above, thus it hits the feather and reflects back to the camera lens (and my eyes). In this low-light situation, the blue is darker and fairly rich because it is not being washed out by glare. (Photo by Dave Irons)

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This image, taken in much brighter sunlight, shows the effect of glare. Notice how the blue on the inner web (the narrow section below the feather shaft) looks much paler than it does on the photo above. Also, note how feather wear affects the amount of refraction. The outer third of the outer web (the wide section above the feather shaft) is quite worn, reducing its prismatic qualities. Thus it looks flat gray rather than blue like the unworn inner two-thirds of the outer web. (Photo by Dave Irons)

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Compare the amount of structural pattern showing in the less worn inner section of the outer web (above) to the heavily worn outer section of the outer web on the same feather (below). Feather condition plays a significant role in the amount of color we see in birds, particularly when it comes to colors like blues and greens, which are produced by refracted light and not pigment.

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The lesson to be learned from this discussion is that colors we see are far from absolute. In fact, they are quite plastic. The combined effects of light angle, light intensity, feather wear, and the vagaries of our individual eyesight should be taken into consideration as we assess the colors we think we are seeing. Whether a bird appears blue-green or greenish blue, greenish yellow, or yellow-green is a matter of perception.

1

I always enjoy the posts at BirdFellow. They are always so polished and enlightening, worthy of any birding magazine out there.

2

And I tguhhot it was because the shovels were dark blue and dark green. And they shown through the snow…Interesting, because the snow was the same color in Norwich.

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