A male bluethroat has some impressive plumage, but a female will often be attracted to ultraviolet colors not visible to the human eye. Rachel M. Richardson/U.S. Geological Survey via AP
The focus of today’s column is some recent research that expands our knowledge of bird vision.
But first, we have to delve into some physics.
Energy is propagated throughout the universe in the form of electromagnetic radiation. This radiation emanates like waves on the ocean with alternating crests and troughs. Wavelength, the distance between two consecutive crests, varies hugely. Radio waves have the longest wavelengths, exceeding a millimeter. Visible light wavelengths are measured in the thousandths of millimeters and X-rays and gamma rays have even shorter wavelengths.
Physicists divide electromagnetic radiation into seven categories based on how a particular wavelength of radiation interacts with water. In decreasing wavelength, we have radio waves, microwaves, ultraviolet radiation, visible light, infrared radiation, X-rays and gamma rays.
Do you remember learning the mnemonic device ROYGBIV in grade school to remember the colors of the rainbow? Red, orange, yellow …
We call those wavelengths of radiation visible light simply because our eyes can distinguish them. We visualize that radiation with light receptors in the cones of our eyes. Humans have three kinds of receptors (blue, green and red), each acutely sensitive to particular wavelengths.
About 10% of human males (including me) and 1% of human females are missing one of these three receptors. The condition is called colorblindness and is misleading because we can distinguish colors. But lacking the green or red receptor, I have a devil of time distinguishing red from green at any distance.
The higher primates are the only mammals with three color receptors. Most other mammals have two.
How about birds? Most birds have either four or five kinds of color receptors, each sensitive to a different wavelength. The ability of birds to distinguish color far surpasses human ability. The density of the cones in the retina of a bird is two to three times the density in the human eye, making avian vision even crisper.
Our notion of visible light does not pertain to birds. One of their receptors can detect ultraviolet radiation. In fact, some birds seem to be more sensitive to ultraviolet radiation than visible light. Birds see colors where our human eyes fail to perceive any color.
Ornithologists can use ultraviolet-light detectors to determine the portions of a bird that reflect ultraviolet light and render them into a color that we can see on a screen.
Most birds reflect ultraviolet radiation from their feathers. Like the blue color of birds, the ultraviolet reflectance is caused not by a pigment but rather by selective reflectance of incoming radiation. The hollow feathers capture most of the wavelengths of incoming light and reflect out only the blue and ultraviolet wavelengths.
Birds see each other in ways we cannot appreciate. Consider two tanagers from Ecuador and Colombia: the black-chinned mountain tanager and the blue-winged mountain tanager. Even experienced ornithologists have difficulty telling them apart in the field. It’s no problem for the birds. The black-chinned has bright ultraviolet reflectance on its back; the blue-winged lacks that color. We would never know given the limitations of human eyes.
Some flowers and fruits have colors or patterns that are visible only to animals that can see in the ultraviolet. Many fruits fluoresce in the ultraviolet wavelengths, but leaves do not, making the ripe fruits obvious for fruit-eaters.
Blue tits use their ultraviolet vision to detect camouflaged caterpillars. Female bluethroats choose mates based on the ultraviolet coloration of males. Such selection is likely in many species.
Deaths from window collisions kills millions of birds each year. People sometimes put tape or decals on their windows to try to reduce bird collisions. Recent work has shown that putting films that reflect ultraviolet light on windows makes the glass obvious to birds but is invisible to humans. One study showed a five-fold reduction in bird collisions when ultraviolet-reflecting film was attached to windows.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu
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