These Male Birds Deploy Deceptive Plumage to Win Mates

Male tanager feathers have microstructures that reflect light in ways that make their bearer look more attractive, even if he’s not the fittest bird around

masked crimson tanager
A male masked crimson tanager displays his brilliant red and black plumage in Peru. Francesco Veronesi via Flickr under CC BY-SA 2.0

Male tanagers are birds that use eye-popping colors to woo mates. They’re supposed to be models of what evolutionary biologists call honest signaling in sexual selection. If a female tanager picks the male with the most brilliant colors, it’s supposed to mean he’s been eating lots of healthy food—an un-fakable sign that he’s got genes made for food foraging and breathtaking plumage to show for it.

Sometimes, however, evolution favors a good dupe, reports Emily Anthes for the New York Times. The new study, published last month in the journal Scientific Reports, finds some male tanagers developed light-manipulating microstructures in their feathers to achieve the most alluring hues.

If the male birds were being “honest” with their sexual advertising, the most colorful suitors would have consumed the greatest quantity of food containing antioxidant pigments called carotenoids. Deep, saturated colors on males are doubly significant because the antioxidants in the carotenoids are also essential for boosting the male’s immune system. So, if a male can consume enough carotenoids to spruce up his outfit then he must be quite fit indeed.

“Furthermore, many times they're not using the same types of carotenoids that they eat,” Allison Shultz, curator of birds at the Natural History Museum of Los Angeles and co-author of the paper, tells Matt Simon of Wired. “They're actually changing them into another type of carotenoid, and that's another kind of expensive cost because they have to have enough energy to do this conversion.”

But the microstructures that researchers discovered on the feathers of male tanagers subvert this evolutionary logic.

“They're making themselves essentially look brighter and more colorful without necessarily putting in these expensive pigments. So they're essentially dishonestly signaling their color to females,” Schultz tells Wired.

Speaking with the Times, Schultz’s co-author Dakota McCoy, a doctoral student at Harvard University, puts it like this: “Many male birds are colorful not just because they’re honestly signaling their quality, but because they’re trying to get chosen. This is basically experimental evidence that whenever there’s a high-stakes test in life, it’s worth your while to cheat a little bit.”

For the study, researchers looked at 20 birds from ten different tanager species or subspecies, with one male and one female representing each species, housed in the ornithology collection of the Harvard Museum of Comparative Zoology.

The broadest finding was quite expected: males had darker blacks and more saturated colors than the duller females. But, per the Times, what surprised the researchers was finding roughly equivalent concentrations of carotenoid pigments in the plumage of males and females.

The explanation for the blacker blacks and more vibrant colors of the males revealed itself under the microscope.

The structure of feathers from the female tanagers was pretty standard fare: cylindrical barbs extending off of the feather’s shaft and smaller filaments extending straight off of the barbs. Under magnification, male feathers showed elaborate structures and abnormally shaped elements. Some had barbules protruding from barbs at rakish angles and others featured wide, oblong rather than cylindrical barbs.

To figure out how these different structures might alter a feather’s appearance the researchers used optical modelling software to simulate how light played across their surfaces, according to the Times.

male silver-beaked tanager
A male silver-beaked tanager perched on a branch. Doug Greenberg via Flickr under CC BY-NC 2.0

Per Wired, male scarlet-rumped tanager feathers had wide, oblong barbs that produced a brighter red, while male silver-beaked tanagers had angled barbules that scatter light and produce a velvety-looking maroon that females find particularly pleasing. Some species also used these angled barbules to produce patches of “super black” feathers to make their colorful plumage pop.

In terms of conserving energy, these microstructures might have evolved as a “cheap way” for male tanagers to make themselves more attractive, McCoy tells the Times. But she says more research is needed to make sure the microstructures aren’t themselves some kind of honest signal of evolutionary fitness.

While these findings may only apply to birds, McCoy tells the Times, “we all have probably had experiences where, whether it’s in the game of love, or getting grades, or playing a game or trying to pass fuel standards with your car as a company—people are constantly gaming the system and trying to appear a little bit better than they are.”

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