The study used a simple marker pen costing $6 (£3) to darken the rust-coloured breast feathers of the New Jersey barn swallow -- making lighter coloured birds look like those naturally darkest.

As a result, their biochemistry began to change -- they started producing more testosterone,

"The experimental manipulation didn't just improve their looks in the eyes of the females, it actually changed their body chemistry," said Professsor Rebecca Safran, lead author and an evolutionary biologist based at the University of Colorado in Boulder.

"A male barn swallow can't look in a mirror and assess his social status."

"But if he flies into a group of other swallows, the birds will quickly assess it for him and give him a sense of where he fits in."

What surprised researchers was that the changes in testosterone manifested one week after the birds were marked -- a surprisingly short period of time for such a drastic change.

"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 Professor Kevin McGraw, co-author and a fellow evolutionary biologist based at Arizona State University.

Safran paraphrased Shakespeare to draw a parallel between avian and human behaviour.

"It's the 'clothes make the man' idea," she said.

"It's like you walk down the street and you're driving a Rolls Royce and people notice."

"And your physiology accommodates this."

But shr thinks that we should be wary of making too many comparisons between birds and humans.

Barn swallows are 'socially monogamous and genetically promiscuous, same as humans,' she said.

"There are some interesting parallels, but we do need to be careful about making them."