For the Birds – Color Vision and Beyond

Among the mammals, humans and the great apes have the best color vision. In the retinas of our eyes, cells called cones are sensitive to different wavelengths of light. Humans and apes have cones with three different light-sensitive pigments called photopsins. One type of photopsin is most sensitive to yellowish-green light, another is most sensitive to blue-green light and yet one more is most sensitive to bluish-violet light. However, all three pigments are sensitive to all colors of the rainbow to varying degrees. The combination of the response of the three pigments to a colored object allows our brains to determine color. For instance, red is perceived when the yellow-green photopsins are stimulated much more than the blue-green photopsins in our cones.

Some humans, including yours truly, only have two types of photopsins in our cones. Such people are colorblind. Color blindness is a misnomer in that most colorblind people can discern some colors; color blindness in which the world is seen in shades of gray is very rare. A colorblind (or better, color deficient) person usually has difficulties in distinguishing red from green and distinguishing pastel colors.

Most mammals have vision similar to a colorblind human because they only have two types of color pigments in their cones. But even humans with normal color vision see the colors of the world much less vividly than birds. Birds have four different photopsins in their eyes. Birds are able to distinguish shades of color in ways that humans really can’t experience. The colors of the world must be really vibrant to a bird.

The cones of birds are more numerous than those of humans. The cone density of human in the center of the retina is about 200,000 cones per square millimeter. For a bird the density ranges from 400,000 to a million cones per square millimeter.

In humans, the cone density falls off rapidly away from the center portion of the retina. In birds, the density of the cones is relatively high in the outer portions of the retina.

Albatrosses and other seabirds have dense ribbons of cones around the retina. These strips are thought to play a role in allowing these oceanic birds to see the horizon to maintain proper orientation.

In recent years, ornithologists have discovered that birds can see into the ultraviolet (UV, for short) part of the spectrum. Human eyes are not capable of seeing these short wavelengths. Birds have an extra set of cones that have peak sensitivity in near-UV wavelengths. How different a bird’s vision must be from our own. For instance, we perceive the color of a Scarlet Ibis as a rich red color. However, the plumage of a Scarlet Ibis also reflects UV radiation invisible to us. Birds however detect both the red color and the UV reflectance, causing the birds to see a Scarlet Ibis as a purple bird!

Recent research had identified a number of ways that birds use ultraviolet reflectance. European Starlings preferentially feed nestlings with UV-reflecting skin. The Redwing, a Eurasian relative of the American Robin, prefers to feed on Viburnum berries that reflect UV compared to berries that do not reflect UV. European Kestrels use their UV-sensitive eyes to find voles, their favored prey. Voles use their urine to mark their trails across meadows. These urine markings reflect UV radiation, giving the kestrels a huge advantage in finding prey.

UV reflectance seems to play an important role in mate selection in birds. Blue and violet feathers are particularly rich in UV reflectance. Male Eastern Bluebirds with bright UV patches tend to make better fathers; males with UV-rich plumage pair earlier, feed young more often, and have greater success in fledging young. Human eyes cannot see these differences that bluebirds clearly can. Thanks to technology we can measure UV reflectance even if we can’t see it ourselves.

[Originally published on December 2, 2006]