In any case, "Female barn swallows seem to be running the show by generating the physical differences we see in the male barn swallows around the world, from the color of their breast feathers to the appearance of their tail feathers," Safran said.
"Since the currency of evolution is successfully raised offspring, the message from our work is that darker males, at least in North American populations of barn swallows, are favored over duller ones," she said. "The fact that darker males have naturally higher testosterone levels might be a clue as to why they are more successful."
A paper on the subject is published in the June 3 issue of Current Biology. The 2-year study was funded primarily by the National Science Foundation.
—By Jim Scott, CU-Boulder
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.