Birds of Blue
Pigment vs. Structured Refraction
The concept of color is a trick of the human eye. Blue, for example, as we identify it in the wings and body of a bird, is not color as we know it. It is the structured reflection of the color blue. In other words the wing feather’s microscopic structure reflects the blue light at a certain angle. Since there is no pigment of that primary color, blue, in birds, our eyes are guided by the light refraction to see it as blue. The physical principal of iridescence come into play here: coherent scattering of the light reinforces some wavelengths and diminishes others. Blue is emphasized and other colors fade out.
Rods, Cones and Ultraviolet Light
According to David Sibley, the celebrated author and artist about birds, he says that these flying creatures are able to see about five times more detail than we can (having an ‘eagle eye’ is a real thing). And he estimates that birds see 16 times more color. Sibley goes on to note that birds have five times more light-sensing cells in their eyes than we do, which allows for five times as many dots per inch — so they can see a lot more detail.
Diving more into bird anatomy, Sibley points out that our eyes convert incoming light into electrical signals in the brain. In birds almost all of those light-sensing cells (80%) are cones that see color.
Going back to high school biology – cones as in rods & cones have different functions in our vision. Rods are specialized photoreceptor cells in the retina that do not detect color; they perceive gray, black and white. 95% of human eye cells are rods. Cones are densely packaged inside the fovea at the center of the retina. The cones contain three types of color perception and are sensitive to red, green and blue wavelengths. Only 5% of human eye cells are cones.
What about the color Green?
Interestingly, there is also no green pigment in birds. So when we see the green on a hummingbird’s back or parrot’s head, it is actually a visual combination of two colors: 1) yellow pigment and 2) the structured blue, which the human eye sees as green (combination of those two primary colors).
Bottom line? Birds have the cones and far superior color vision. Our eyes are tricking us. There are no blue or green pigments in birds and color is in the angle of the beholder.
Who knew?
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[1] Sibley, David Allen, What it’s like to be a bird, Alfred Knopf, New York, 2020, page 57.
