Birds use UV vision to catch prey, mates

Comparing our vision with that of birds, well, at least we can claim opposable thumbs.

Birds see a vastly different world than we do. They see more detail. They see more colors, by a factor of 10. They see ultraviolet (UV) wavelengths, and that is key.

UV rays can damage human eyes, so we filter them out. Throughout nature, we are an exception for UV vision, not the rule.

A few weeks ago I wrote of harriers finding prey in marsh grass by looking for UV reflection from the mammals' urine. But there is even more to it than that. UV vision is an essential all-purpose tool for birds.

Let's start with black-capped chickadees, the ones at your feeder. They look alike to you and me. To chickadees, however, not at all. They see colors other than the black/white/gray plumage we see. Light in the UV part of the spectrum makes that possible.

Birds, like us, want to find the best mate possible. UV color helps the female make a choice. Brighter male color means a healthier, stronger bird. He's likely to have a prime breeding territory. He'll be a good provider. He'll be an aggressive defender.

(It's the human world equivalent of a guy who has a good job, drives a nice car and has a cool apartment.)

This healthy, fit male chickadee will give the female a good chance to pass her genes to future generations. That's what she wants. That's the game nature plays — survival of the fittest.

The male bird has the same genetic concern. Monogamous in that he tends but one nest, his noteworthy fitness is likely to be noticed by other mated female chickadees in the neighborhood. He is likely to try to breed with them, and they are likely to be receptive. It's putting eggs in more than one basket, as the old saying advises.

Brightly marked male bluebirds are sending a signal not only to females but also to male competitors: I am bright, therefore I am fit, therefore you should not even try.

An aside on color: We have three kinds of retinal cones that facilitate color perception: red, blue and green. Each lets us see as many as a thousand colors. Those colors combine to illuminate our world.

Birds have those three cone sets plus a fourth, for the UV wavelength. Many bird species see UV light. So how else does it help them?

We have that harrier, looking for reflective mouse urine. If it ate fruit, it could look for reflective berries or reflective skin on fleshy fruits. The UV colors become visible when the fruit is ripe. This is an invitation to be eaten so the fruit's seeds can be distributed by eventually passing through the bird. Nature loves cooperation.

Some insects reflect UV light. Other raptors use UV as does the harrier. Some bird eggs are UV-marked. In a nest, this might allow the bird to distinguish between its own eggs and those of a parasite bird, like cowbirds or cuckoos. In a breeding colony, UV markings could help a bird identify its own eggs from the thousands that might be visible.

Think of how complicated it is for us to do such things. But, again, we have opposable thumbs and birds do not.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at www.startribune.com/wingnut.