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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.
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.
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.
In a recent post to Oregon's statewide birding listserv, veteran observer Wayne Hoffman shared that on two different days during the second week of April 2010, he had seen alternate-plumaged Common Loons (Gavia immer) at Newport, Lincoln County, Oregon that showed green iridescence on their lower neck collars. This observer went on to note that he had not ever seen a Common Loon showing bright green on the lower neck collar. His subsequent search of field guides and other references found very little mention of green iridescence in connection with this species.
Wayne's post reminded me of a similarly plumaged loon that I photographed at the same location in April 2009. The alternate-plumaged bird that I observed (see images below) showed the same green sheen he was describing on both its head and on its lower neck collar, with the lower collar being particularly colorful. I can remember being a bit taken aback by how green it looked at the time and how I was completely unaware that a Common Loon might show such coloration. Then it occurred to me that I'd never before been so close to an alternate-plumaged loon in crisp sunlight.
A quick check of the most popular North American field guides revealed that nearly all of them illustrate alternate-plumaged Common Loons with a black head and black lower neck collar that are separated by an incomplete upper collar that is mostly white with narrow, dark vertical lines. The accompanying text in these volumes generally describe the head as black, which is how we perceive it in most light conditions.
Of the guides that I own, the 4th edition of National Geographic's Field Guide to the Birds of North America seems to best depict this green-headed appearance, but offers no description of the summer plumage in the text. The next best illustration that I found was in my 3rd edition of Peterson's Western Guide (1990). I don't own the most recent editions in the Peterson series, but I was a little surprised to see that the Common Loon in the 5th edition of Peterson's Eastern Guide (2002) showed a more purplish-blue gloss to the head and none of the green tones that are apparent in the earlier Western Guide. Maybe this was just a matter of printing, or perhaps the result of an editorial decision to show this species as we most often see it. In the Sibley Guides, which to my eye offer the best overall set of illustrations, alternate-plumaged Common Loons are shown to have black heads, which is pointed out with arrows and labeling. Perhaps, like others, Sibley made the editorial choice to show his Common Loons with black heads because that is how they appear to us most of the time.
One point that is important to remember, iridescent colors are not produced by feather pigmentation. Instead, the glossy blue, green, and purple hues that we perceive are the result of light refracting off of the intricate structural characteristics of the feather. That being the case, whether we actually see these colors and how intense they appear is highly dependent on lighting conditions. Generally speaking, the brighter the sunlight, the brighter the apparent coloration.
At the very least, we hope this discussion inspires you to take a closer look the next time you see an alternate-plumaged Common Loon and to better understand how lighting affects the colors our brains are telling us we are seeing.