Feather Colors: What Birds See.

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Author: Marcus, Adam

The latest study throws water on the long-held belief that for most songbirds -- nearly 70 percent of the world's 5,000 passerines -- the sexes look alike. During graduate studies at the University of Minnesota's Bell Museum of Natural History, Muir Eaton was examining feathers with a spectrometer -- a machine that analyzes the way light is absorbed and reflected by a given material.

"I noticed that the spectrometer revealed a lot of variation in color that I could not see," he recalls. "In light of the fact that there were a couple of examples of 'hidden' sexual dichromatism in the literature, I thought it would be good to test our assumptions of species as sexually monochromatic on a broad scale."

The idea proved a worthy one -- more so than Eaton imagined. He analyzed 139 species of passerines, from pipits to waxwings, that were considered sexual monochromes. Over 90 percent turned out to be dichromatic. Reading the data conservatively, Eaton estimates that 73-82 percent of all songbirds are dichromatic, upending the accepted rate for plumage similarity.

Eaton, now a postdoctoral researcher at the University of Kansas Natural History Museum, reported his findings in the August 2, 2005, issue of the Proceedings of the National Academy of Sciences (available at www.pnas.org).

For his study, Eaton used skins from the Field Museum of Natural History in Chicago and the American Museum of Natural History in New York. His sample included at least two species from every passerine sub-family throughout the world.

Richard O. Prum, head curator of vertebrate zoology at Yale's Peabody Museum of Natural History, says Eaton's findings, while intriguing, have yet to be firmly linked to bird behavior. Eaton is eager to fill the gap with additional evidence.

"Now, when you look at these birds, you can think to yourself, maybe they're behaving in a certain way because they know who the males and females are in a flock," Eaton says. "That's the sort of thing that's missing." He speculates in his paper that plumage signals we can't see "might be a pervasive feature of avian coloration."

Indirect evidence suggests that the differences Eaton saw on his spectroscope do influence behavior. A 2004 paper in the journal Animal Behavior showed that Yellow-breasted Chats, long considered sexual monochromes, could recognize the sex of stuffed specimens placed in their territories without any other identifying clues, such as songs or displays.

Herman Mays, a postdoctoral researcher at Auburn University, helped conduct the study. The differences between the sexes are subtle, he says. "But there probably is still some sex selection operating to make males brighter than females."

Eaton studied chat specimens and looked at the way they scattered UV light. "The breast coloration of the sexes was pretty different if you analyzed the spectral readings," he notes. In calculating what he calls "avian color space," the full range of colors that birds see, Eaton found significant differences between chats' face, head, and chest plumage. (For examples of other birds' differentiating feathers, see the chart at right.)

More evidence for UV-related mate-selection behavior comes from a 2004 paper on two nearly identical species: the Black-chinned and Blue-winged Mountain-Tanagers, found in the northern Andes Mountains of South America. The birds are considered "sibling" species and are difficult to differentiate in the field. (See "Birds in Ultraviolet," page 53.)

Both have black faces, yellow crowns and underparts, blue wing edges, and black and blue tails. The key field mark is back color. Black-chinned's is olive, and Blue-winged's is black. Some Blue-winged subspecies, however, also have olive-colored backs, yet the two species do not interbreed. Somehow, the birds know how to tell each other apart.

What tells the birds who's who, according to zoologist Robert Bleiweiss of the University of Wisconsin Madison, are patches of UV-reflecting plumage on the Black-chinned's back. The mountain-tanagers appear to rely at least in part on the UV reflections to select mates. He published his study in the Proceedings of the National Academy of Sciences.

Ultimately, Bleiweiss concluded, calling species "siblings" because they look similar to the human eye is taxonomical hubris.

"Ignorance of humans to UV-based plumage differences," he wrote, "highlights that sibling species really are no different from other biological species except that their distinctions are more obvious to the organisms involved than to humans."

Eaton believes his findings throw into question a prevailing assumption about the evolution of sexual plumage patterns in birds. "If my results are correct and most species are sexually dichromatic, that suggests that maybe it's the ancestral characteristic for most groups of birds, and that maybe the monochromes are evolving from dichromes," he says.

Prum marvels at the recent advances in the understanding of avian visual acuity. The new knowledge does not reflect favorably on human eyesight. Consider the difference between us and birds: While you see a beach full of sunbathers, the Herring Gull overhead can tell who's slathered in UV-blocking sunscreen. "Those kinds of details," Prum says, "can really make you realize that birds in some neurological ways have superior abilities to sense the world than we do."

The two South American tanager species shown above have similar plumage patterns, making them difficult to identify in the field. But ultraviolet (UV) feathers and superior vision enable the birds to tell who's who.

University of Wisconsin scientist Robert Bleiweiss made still video images of museum specimens of the Black-chinned and Blue-winged Mountain-Tanagers in human-visible light (left) and in UV (right). Our eyes see yellow crowns and underparts, black heads, dark backs, and black and blue tails. The birds, though, can see strongly reflected UV in the Black-chinned's back and in the blue wing trim of both species (arrows), suggesting that they use UV color to differentiate one another.

A Kansas researcher discovered that the sexes of the vast majority of songbird species have color differences that we don't see. North American and Hawaiian species are listed below with the feather patches that, to avian eyes, separate males from females. The few birds in which the sexes truly do look alike include American Dipper, Townsend's Solitaire, Warbling Vireo, and Harris's Sparrow.

Tropical Kingbird crest, belly

Northern Shrike belly

Yellow-throated Vireo throat, rump

Green Jay malar, back

Mexican Jay chest, head

Barn Swallow throat, belly, head

Verdin head

Pygmy Nuthatch head

Brown Creeper rump

Cactus Wren chest, belly

Canyon Wren head

Hermit Thrush back

Wrentit chest

Northern Mockingbird chest

Bohemian Waxwing throat, tail tip, chest

Cedar Waxwing tail tip, belly

Painted Redstart throat

Yellow-breasted Chat face, chest, head

Western Meadowlark throat, belly

Altamira Oriole rump, head, throat

'I'iwi back, chest

'Apapane head

PHOTO (COLOR): Black-chinned Mountain-Tanager

PHOTO (COLOR): Blue-winged Mountain-Tanager

PHOTO (COLOR): The Green Jay reflects ultraviolet light in its malar and back feathers.

PHOTO (COLOR): Like most songbirds, male and female Cedar Waxwings appear identical to humans. New research shows how the birds tell the difference.

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By Adam Marcus

Adam Marcus is a science writer. He lives in New York City and spends spring weekends looking for warblers in Manhattan's Inwood Hill Park.