By darkening the breast feathers of male New Jersey barn swallows with a common marker, researchers from the Arizona State University showed that birds with naturally darker feathers attract more females than birds with lighter feather coloring. The experiment further proved that dark feathers can also boost the testosterone levels and may even help the birds lose weight. In a previous study carried out three years ago by the same team, it was proven that male birds that have been darkened were more attractive to females and mated more than others.

The new study reveals that in birds, attractive appearance can determine changes in the chemistry of the body, by increasing the testosterone levels.

"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 Kevin McGraw, an evolutionary biology professor at Arizona State University and participant in the study.

In male birds that got their feathers darkened, the testosterone levels increased by up to 36 percent in just a week, regardless of the fact that at that time of the year the hormonal changes would trigger a decrease in testosterone. However, in the control group of 33 birds that did not get their feathers darkened, the testosterone level dropped to half during the same period.

"It's the 'clothes make the man'. It's like you walk down the street and you're driving a Rolls Royce and people notice. And your physiology accommodates this", said Rebecca Safran from the University of Colorado, the leader of the study. Safran added that human mating system is relatively similar to that of these birds, whether we like to admit it or not. "Barn swallows [are] socially monogamous and genetically promiscuous, same as humans. There are some interesting parallels, but we do need to be careful about making them", she said.

However, in humans hormonal changes usually take place when behavior changes occur, according to a study carried out in 1998 which revealed that testosterone levels rise in sports fans by up to 20 percent when their favorite team wins, although the exact mechanism of how this happens remains mostly unknown. It could be due to more frequent mating, the researchers speculate.

Alternatively, the feather darkening could lead males into thinking that the pecking order has changed, thus boosting the hormone levels, but it could just as well be both. The loss in weight can be accounted through more frequent mating than eating.

Princeton University researcher James Adelman said that usually the hormonal changes determine behavior and appearance, but there may be a way to reverse that.

"It certainly is a very new and interesting finding", said Elizabeth Adkins-Regan professor of psychology and neurobiology at Cornell.