Birds Gone Wild: Extra Testosterone Makes Males Irresistible

By Robert Roy Britt, LiveScience Managing Editor

posted: 29 May 2006 09:39 am ET

: A dark-eyed junco songbird. Credit: Will Clark via North Dakota State University

Given a boost of testosterone, young male songbirds sing more sweetly, fly farther, and are more attractive to the females.

And, of course, they mate more often. Yet these testosterone-laden males don't have to take care of all their young—older males with less testosterone seem plenty willing to hang around the nest and do the dirty work.

That's the strange world of the dark-eyed junco.

Faking the experience

Wendy Reed of North Dakota State University figured all this out by boosting testosterone levels in some wild young males to the highest levels typically found in nature, then observing the results of their natural breeding.

Some of the findings might seem counterintuitive.

The extra testosterone weakened the birds' immune systems, causing them to die sooner. They were lousy partners and terrible dads. Furthermore, the chicks they fathered were smaller and didn't survive as well. Yet the ladies couldn't resist them.

"One thing that testosterone does is to make young males act like older males," Reed explained. They "sing more and court females more arduously, and females really like older, more experienced males."

While dark-eyed juncos are monogamous during the main breeding season, they are known to entertain an off-season affair now and then. But the young males, for all their tricks, lack the experience needed to take care of the young.

"They can talk the talk, but can't walk the walk," Reed said.

Pinch-hit dads

You might think evolution would have weeded out high-testosterone males. But they do exist among juncos in the wild.

One key to their existence, Reed told LiveScience, is that females get help raising the extra chicks from older males with lower testosterone, which are "good dads and good partners."

Reed and her colleagues watched more than 400 junco nests in the Appalachian Mountains of Virginia for nine breeding seasons, boosting testosterone in some males but not others. The results are detailed in this month's issue of the journal The American Naturalist.