Male ducks don’t look anything like female ducks, at least during the breeding season. And you can easily tell the boy Black-headed Grosbeaks (right) from the girls (below)… in fact, they don’t look like they even belong to the same species! But with lots of birds, from Steller’s Jays to Canada Geese, it seems that only they know who is who.
I’ve often wondered just how they do that. What is it that tells male Red-tailed Hawks who the ladies are? How do California Gulls avoid courting a gull of the same sex? Well, I just read some fascinating articles that solved this mystery, at least when it comes to Black-capped Chickadees.
It turns out that the birds do have sexual dimorphism—the males and females look different. The problem is that we can’t see it with our limited human eyes. The markings are in the ultraviolet.
Most people are aware that bees and other insects see a different set of colors than we do. Flowers that appear plain yellow to us actually have ultraviolet runway lights inviting the bees to land and mix up some pollen. Hummingbirds see more colors at the other end of the spectrum, distinguishing reds that we are blind do. That is why they’re attracted to red flowers, and plants have evolved to advertise in that color.
How do birds see colors we cannot? If you remember from your science classes, our eyes have three different kinds of cones—color receptors—each sensitive to yellowish, greenish, or bluish light. Birds have four. So where we see a rainbow of seven colors, they can see more. Birds’ eyes also have more cones than ours, and more connections from the eye to the brain, giving them superior eyesight.
Now it turns out that Black-capped Chickadees, at least, proclaim their sex in the same way. The males are brighter white (into the UV) and deeper black (also into the UV) than the females. In fact, female birds prefer males with the brightest white patches and the sharpest contrast between the white and black patches. This serves not only to maintain the differences, but to strengthen the distinction in each generation.
Chickadees aren’t the only birds to have sexual markings that we can’t see. “In general, practically all bird families present some UV reflectance in their plumage.” In addition to distinguishing males from females, birds’ ability to see into the ultraviolet helps them find prey, establish territories, and proclaim their virility.
Next time we see a pair of apparently identical birds courting, we don’t have to wonder how they can tell which sex is which. Now if only we could see in the same wavelengths, we’d have to rewrite the field guides!
 “The role of ultraviolet wavelength in sexual selection” Revista Brasileira de Ornitologia 14 (3) 191-19, Setembro de 2006