A new American research highlights the costs and benefits of testosterone in birds.
Individual male birds can differ dramatically in their behaviour, and this difference is often due in part to how much testosterone they produce.
In many species, some males produce high testosterone and are more aggressive, while others produce lower levels and are more parental.
Testosterone and the behaviours it mediates may predict how well a male succeeds.
For example, an aggressive male may be more likely to obtain high-quality territories that attract females.
At the same time, aggression might pose a survival risk, because aggressive males might be more likely to engage in costly fights.
These considerations suggest that hormones like testosterone might be under strong natural selection in the wild.
To test this idea, a team of researchers from Indiana University studied a common songbird, the dark-eyed junco in the Appalachian Mountains of Virginia.
They tested how much testosterone a male could produce by using an injection of a hormone produced in the brain that causes the bird to increase its testosterone levels temporarily, mimicking what they do naturally when fighting with other males.
The researchers then followed the birds, measuring their survival and success at reproduction, both in their own nest and those of their neighbours.
They found strong relationships between testosterone and both reproduction and survival, demonstrating that natural selection is currently acting on testosterone production in this population of juncos.
The exact pattern of selection they found was surprising, however.
Lead author Joel McGlothlin, a postdoctoral fellow at the University of Virginia, said: "The males that did the best at both survival and reproduction had testosterone production very close to average.
"It was bad to produce either really high or really low levels of testosterone."
High-testosterone males did have one universal advantage - they were more likely to be the genetic father of the offspring raised in their nests.
McGlothlin added: "It's not as simple as saying testosterone is good for reproduction and bad for survival.
"Testosterone seems to underlie this delicate balance between competing traits and behaviours, and the right balance might be different for different males."
The study has appeared in The American Naturalist.