Can We Trust the Colors We See?

Over recent weeks frequent visitors to this site will have noticed a lack of new posts to the our online journal. During this period our efforts have been focused on gathering and organizing photos into galleries that will eventually appear online. Steve Mlodinow, who has been a regular contributor to this journal, is among those who are supplying us with photos for our future online galleries. While sorting and loading some of his images, I found a picture of an interesting female Eurasian Green-winged Teal (see below) in a collection of photos that he took during a May 2009 visit to Adak Island in the Aleutian Island chain off the mainland of southwest Alaska.

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Note the apparent difference in color between the blue inner most secondaries of the left wing and the green inner secondaries on the right wing. This female Eurasian Green-winged Teal was photographed at Adak Island, Alaska in May 2009 (photo by Steve Mlodinow).

Though a bit out of focus, the image above serves to remind us that the colors we think we are seeing can be both subjective and/or misleading. At first glance, the mis-matched colors in the wings of this female teal are a bit confusing. However, there is a rather simple explanation for why one wing appears to have a green speculum while the speculum on the opposite wing looks blue.

First, it is important to understand how the colors we see in a bird’s plumage are produced. Plumage coloration is produced one of two ways. Like human skin, most bird feathers contain pigments that give each feather color. These pigments result in the blacks, browns, reds, oranges, and yellows that we see in the plumages of many bird species (www.ducks.org/DU_Magazine 2007). Though changes in light angle and intensity will cause some variation in the lightness or darkness of the color our brain perceives, the actual feather color is to some degree constant.

It should be noted that feather wear and sun bleaching often cause pigmented feathers to fade as they age. Most of us have seen gulls with bleached-out flight feathers in the summer months. At that time of year, the flight feathers of a gull are nearly a full year old. Most birds, including gulls, replace their flight feathers just once a year during a prebasic molt that usually occurs during the summer and fall months. During this molt, retained flight feathers will generally show worn and tattered edges and appear much paler than newly grown feathers in the same feather tract. In some cases, the old feathers will be so worn that they appear to be little more than quills.

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This first-cycle Glaucous Gull, photographed at San Francisco, California on 7 May 2009, offers a classic example of a gull with plumage (particularly flight feathers) that is both very worn and sun-bleached. Its flight feathers would have been acquired during late summer or early fall of 2008, meaning that they were nearly a year old at the time this image was captured and due to be replaced during its next prebasic molt. (photo by Dave Irons)

In addition to colors that result from pigmentation, we also perceive colors that are created by light refracting off of the minute structural components of a feather (www.birds.cornell.edu/allaboutbirds). These structural characteristics cause light to refract (bend or change direction) in ways that, depending on the angle of incoming (incident) light and our viewing angle, may cause us to see several colors reflected from a single feather. Colors resulting from this type of refraction are often iridescent. The gorget color of a hummingbird and the speculum (wing patch) color of many dabbling ducks result from light refraction and not pigmentation. Depending on the reflection of incident light, such feathers may appear highly colorful or black. When the feathers look black, little or no light is being reflected off the feather surface back to our eyes. Instead the light is being absorbed (www.birds.cornell.edu/allaboutbirds). When we are seeing an array of highly iridescent colors, lots of light is being reflected off of the feather surface back to our eyes.

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More often than not, when one sees a Double-crested Cormorant, its plumage looks mostly black or very dark brown. However, when lighting conditions and the viewing angle are just right, as was the case when this cormorant was photographed at Estero San Jose, Baja California Sur, Mexico on 8 January 2009, various green, bronze, and even purplish hues become apparent. (photo by Dave Irons)

After all this explanation, we are still left to wonder, “why are the specula of the teal in the picture above two different colors?” It would seem highly unlikely that there would be a difference in pigmentation in the two wings. More importantly, it has been determined that blue coloration in feathers results from light being scattered by small barbs in the feather rather than pigmentation (http://www.birds.cornell.edu/allaboutbirds). It is also pretty safe to assume that the structural characteristics of the individual wing feathers should be the same in both wings. What is different in this case is that the wings are not being held at the same exact angle in relation to the light source . Also, our viewing angle of the two wings is slightly different as well. These two factors cause light to refract through the feather and reflect back to eyes in ways that cause us to see color differences between the two wing patches.

As you look at birds and try to quantify plumage colors, it is important to recognize that lighting conditions and viewing angle play a significant role in producing the colors your brain tells you that you are seeing. Further, if colors appear iridescent, be assured that what you are seeing is the product of refraction and not actual pigment in the feathers.

Literature Cited:

http://www.birds.cornell.edu/allaboutbirds/studying/feathers/color/document_view

http://www.ducks.org/DU_Magazine/DUMagazineNovemberDecember2007

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A nice primer on bird colors, put forth in straightforward Irons fashion by none other than Irons himself. I thought light was overrated as a modifying factor in bird ID for years, until I moved to Humboldt Bay, where it’s persistently foggy during morning birding hours much of the year. // I can add that the Mallard-like double white border to the speculum is a feature of Eur. Green-winged Teal, visible in the shot of the Adak bird. This mark proved useful in identifying a puzzling duck of this form that was record-late for the species in early May at Humboldt Bay NWR about three or four years ago.

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Again, BirdFellow provides a concise and interesting short spot on a topic of interest to birders of all levels. In addition to the information contained herein, be aware that lack of coloration is also a sometimes confusing issue both in terminology and in identification. Two melanine are responsible for almost all bird coloration. Pheomelanin handles all of the pale browns to almost whitish tan, while eumelanin is responsible for blacks, grays, and dark browns. A lack of either (or an illness or condition that does not allow the transfer the melanins to feathers) results in leucism, or partial leucism, in birds resulting in white or dilute colored feathers. This condition often creates dilemmas of identification for birders.
Conversely a condition where inordinate amounts of melanins are present is known as Melanisitic or Melanism and darkens all affected feathers, or groups of feathers. Where there should be pale (or I might say ‘normal’) feathers they are now black or dark brown. A fine article on these color phenomena can be found at http://www.vogelringschier.nl/DB28%282%2979-89_2006.pdf and is authored by Hein von Grouw, who has been the curator of the Netherlands National Museum of Natural History for many years.
It is also worth mentioning that for many birds the bright reds, oranges, and yellows are often the result of carotenoids in the diet. A good example is the flamingo. The pink or red coloration is a direct result of the deposition of carotenoids from their diet which is transferred to the feather during development. Without the carotenoids the resulting bird ends up white. Nice job guys.

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“Depending on the reflection of incident light, such feathers may appear highly colorful or black. When the feathers look black, little or no light is being reflected off the feather surface back to our eyes. Instead the light is being absorbed (www.birds.cornell.edu/allaboutbirds). When we are seeing an array of highly iridescent colors, lots of light is being reflected off of the feather surface back to our eyes.”

I ran into this in some Mallard Drakes I photographed over the weekend. They tended to stay in the shady areas, but strayed into sunlit areas on occasion, when their dazzling green head color flared for the camera. http://www.youtube.com/watch?v=0vjRIjPaRsg

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