It's a “clothes make the man” lesson that, with some caveats, also applies to human males, researchers say.
Using a $5.99 marker, scientists darkened the rust-colored breast feathers of male New Jersey barn swallows, turning lighter birds to the level of those naturally darkest.
They had found, in a test three years ago, that the marked-up males were more attractive to females and mated more often.
This time they learned the more attractive appearance, at least in the bird world, triggered changes to the animals' body chemistry, increasing testosterone.
“Other females might be looking at them as being a little more sexy, and the birds might be feeling better about themselves in response to that,” said study co-author Kevin McGraw, an evolutionary biology professor at Arizona State University.
McGraw said the findings are surprising, in part because the hormonal changes occurred after only one week.
The researchers aren't certain how the testosterone boost happens. The study was published in Tuesday's edition of the journal Current Biology.
“It's the ‘clothes make the man'” idea, lead author Rebecca Safran, an evolutionary biology professor at the University of Colorado in Boulder, said. “It's like you walk down the street and you're driving a Rolls Royce and people notice. And your physiology accommodates this.”
In people, hormonal changes have been observed after changes in behavior. A 1998 study found loyal male fans of sports teams experienced a 20 percent rise in testosterone when their teams won.