Testosterone helps birds to live fast, but die young

Reuters:

For dark-eyed junco males, having a little extra testosterone made them more attractive to females -- especially older ones -- helped them fly farther and sing more sweetly, scientists said on Tuesday.

And the hormone-laden males produced more offspring, North Dakota State University assistant biology professor Wendy Reed found. But the chicks were smaller and less likely to survive, she reported in The American Naturalist.

Extra testosterone made the dark-eyed juncos more susceptible to disease and shortened their life spans. "They had lower immune function and paid a cost with lower survival rates," Reed said in a statement.

The higher testosterone levels also made the songbirds worse fathers, Reed found. They made fewer nest visits, resulting in less food delivered and less time spent at the nest.

Reed's team watched more than 400 junco nests in the Appalachian Mountains of Virginia for nine breeding seasons. Juncos are little finch-like birds with light-colored breasts, small beaks and a darker head and wings.

One group of dark-eyed juncos in the study had tubes of testosterone implanted under the skin and the control birds got implants that were left empty. The researchers gave some of the birds jolts of testosterone to match what they had found were the maximum levels seen in the songbirds naturally.

"Typically, young male juncos court and sing less vigorously and have lighter plumage than old males and also have lower circulating levels of testosterone," the researchers wrote in their report.

Adding testosterone made the males fidgety -- they were more active over a wider area, but also had higher levels of stress hormones. This could suppress the immune system, the researchers said, and the higher activity could attract predators.

"Although young testosterone-treated males act like old males, they do not realize the same reproductive performance as old males. Young males have decreased nest success, clutch sizes, and season durations compared with older males," the researchers added.

Birds Gone Wild: Extra Testosterone Makes Males Irresistible