A Little Bit of Color Turns Barn Swallows into Macho Males

Feather darkening boosts testosterone levels in barn swallows

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 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 sexier, 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.  

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.   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.