Researchers from Arizona State University, the University of Colorado-Boulder and Princeton University conducted an experiment that involved artificially coloring the breast feathers of male barn swallows. That coloration resulted in the birds' testosterone levels significantly increasing within one week of the coloring. The scientists said that was unexpected because it was observed at the time in the breeding cycle when levels of sex steroids such as testosterone are typically declining.

"The traditional view is that internal processes of birds determine their external features -- in other words, physiology forms the feathers," said ASU Assistant Professor Kevin McGraw. "But our results indicate that a perceived change in the color of an animal can directly affect its internal physiological state. A barn swallow's hormonal profile is influenced by its outward appearance."

The research that also included Rebecca Safran, James Adelman and Michaela Hau is reported in the journal Current Biology.