Laura Erickson's For the Birds

One of the fascinating areas of ornithology has to do with how birds recognize each other in individually. Males are dramatically different from females in some species, such as tanagers, hummingbirds, many finches, ducks, and Red-winged Blackbirds. There are more subtle but noticeable differences between males and females in species such as woodpeckers, kingfishers, robins, and bluebirds. But for some species, including geese and swans, cranes, eagles and most hawks, chickadees, jays, crows, cuckoos, and many flycatchers, male and female appear identical to our human eyes. When we see a pair in a family unit of geese, we can distinguish the male from the female because he tends to be the warier one, with his head up watching our approach, but when migrating geese are down feeding in a small lake, they are just about impossible for us to sex. How do they tell each other apart? Behavior probably plays at least some role for many species. ln songbirds, males are the ones that sing, and females tend to be less dominant interactions, but there are important exceptions. Black-capped Chickadee females, for example, do sing occasionally, and they often set up dominance hierarchies with other females.

But now some researchers at the Brookfield Zoo in Chicago have come up with an interesting theory, which experiments are verifying. Ornithologists have long known that birds can see in the ultraviolet wavelengths. Kestrels take advantage of their UV vision to detect rodent urine, so they can hunt in the best areas. Hummingbirds detect when certain flowers are at their peak for nectar production by seeing the flowers in full color, including the ultraviolet wave lengths. And it turns out that male and female birds apparently look very different from each other under ultraviolet light.

The first researcher to notice a difference between males and females in a monomorphic species—that is, one in which the males are identical to the females—was a British researcher studying Blue Tits, relatives of our Black-capped Chickadee. He noticed that under special instruments that make UV light visible to humans, the head patterns of males and females were dramatically different.

The Brookfield Zoo was asked to do some captive breeding experiments with three declining Hawaiian species, the Amakihi, Apapane, and Iiwi. But they simply could not get the birds to breed in captivity, though they tried all kinds of environmental changes to induce reproduction, including day length, diet, temperature, and humidity. Out of more than 100 birds, they had only one or two successful pairs.

Then Patty McGill read about the Blue Tit study, and enlisted the zoo’s research manager to design a study that won a $50,000 federal research grant. They placed groups of three cages tightly joined together and outfitted with black lights. A female bird was placed in the center cage, and male birds of the same species were placed in the two side cages.

These birds were originally sexed with blood samples, and marked with bands so the researchers were certain of their sex. They separated one cage from the center cage with a pane of UV-filtering glass. They separated the other cage from the center cage with glass that allows UV light to pass through. And then they monitored the behavior of the female, and quickly discovered that the females show a pronounced preference for the males they can see through the pane that allows UV light through.

Based on their preliminary results , they’ve been setting up UV lights in other enclosures, and turning them on for two weeks at a time, then off for two weeks at a time. And zookeepers have noticed a definite difference.

The world is more colorful to birds, perhaps actually lovelier than what our poor, limited human eyes can see. They may not be visionaries, but they do see farther, and better, than we do without even standing on the shoulders of giants.