It's often been said that clothes make the man. This also holds true in the animal kingdom, where larger antlers or brighter plumage can increase a male's chance of attracting mates. Now, a team of scientists based in Boulder, Colorado, has documented that, simply by changing the appearance of a common songbird called the swallow, they can improve its mating success. Shelley Schlender reports.

Taking a tiny drop of blood from male swallows has helped researchers determine that males with a darker orange chest father more young

Barn swallows fly into a horse barn near Boulder, Colorado, carrying insects for their hungry hatchlings that wait eagerly in nests made from pellets of mud. With their swooping flight and dark blue plumage, swallows are elegant birds. The males are especially striking, thanks to long tail streamers and a brownish-orange chest. But according to biologist Rebecca Safran, those average looks won't guarantee a mate.

"Females really have the upper hand in the mating game," Safran says. "Males are kind of a dime a dozen, and females are really holding their cards close to their chest."

To find out how much appearance appeals to the lady birds, Safran and her research team captured male barn swallows. Using colored markers, they darkened the orange chest feathers on some of them. They also collected a tiny drop of blood to identify each male's DNA and track his hormone levels. In addition, they got DNA from baby swallows. In this way, they discovered that the males whose chest feathers had been enhanced fathered more young.

University of Colorado biologist Rebecca Safran leads a team of researchers investigating how male barn swallow "looks" influence mating success

"When we manipulate a male's color to look darker, he does better," Safran says. "He attracts more mates. He has more offspring. This is the currency of evolution. Having a number of babies."

Those males also had higher levels of the male sex hormone, testosterone.

"The surprising finding is that by simply changing the male's appearance, his physiology also changed," Safran says.

The darker orange chest feathers seem to attract more females. Safran and her student assistants speculate that these successful interactions may be what raised the birds' testosterone levels. She says that her study is the first to document that changing the outward appearance of an animal can change its inner chemistry.

Research assistant Connor Fitzhugh says the need to attract a mate is a big reason for the dramatic tail feathers on male peacocks and a lion's magnificent mane - especially when it's a darker orange.

Barn swallows raise their young in nests made from daubs of mud

"This whole notion that the clothes make the man, it's repeated throughout the animal kingdom," Fitzhugh says. "It's not unique to barn swallows."

In the wild, those looks are often a sign of fitness, since only a male in good health can produce strong antlers, furry ruffs or extra-colorful feathers.

Another team member, Ph.D. candidate Matt Wilkins, says that when females like what they see, they're probably judging with some accuracy a male's ability to father babies, feed them and defend the roost.

"It makes sense. It's kind of an honest signal of quality in a good male," Wilkins says.

Wilkins adds that the signal varies depending on the male's environment.

Researchers make individual swallows easy to identify in flight by coloring the underside of their tail feathers

"In North America, the males who have more young have darker feathers. That's in comparison to in Europe, where the males on average are lighter and have longer tail streamers," Wilkins says. "Because there, females don't care so much about the breast color, but they prefer males with longer streamers, so that's driving the difference in these species across the globe."

Those differences might be caused by subtle variations that determine swallow health in Europe and North America. It's an example of how species change over time, says Rebecca Safran. Her research is showing how much female choices about male looks can be part of the change.

"What we believe is happening is that we're actually able to record evolution in action," Safran says. "That is, we're studying a new formation of new species as they're evolving."

Researchers do many of their observations at horse barns, a popular site among nesting swallows

But while the researchers have learned a great deal by enhancing the feather color of male barn swallows, they don't recommend this subterfuge as a successful path of evolution. While their doctored males did father more young, they also lost weight, and this may indicate they had to work too hard in order to keep up with their new appearance. After all, unlike a male with naturally darker orange feathers, these counterfeits didn't get their better looks because they were actually healthier.

Still, Matt Wilkins says, the research has shown him how much clothes - or in this case, feathers - can make a man.

"I've started wearing orange a lot more, to some success," he says. "No. Just kidding."

Next summer, Wilkins says that as part Safran's research project, he's going to study how male barn swallow singing correlates with chest feather color.

Audio clips courtesy of University of Colorado at Boulder.