Sex and the Dark-eyed Junco

Males with more testosterone are irresistible to females--but there's a downside

03-03-2010 // NWF Staff
Dark-eyed Junco


But when it comes to its social and breeding behavior, the bird is anything but ordinary.

Over the years, researchers have found that during winter the birds form rigid hierarchies led by dominant males and their seemingly closest adult male buddies. Younger males and females often are pushed to the periphery. Now a newer study reveals that another set of dynamics guides the birds when they pair up on their breeding grounds in early spring.

A team of scientists led by Indiana University behavioral ecologist Ellen Ketterson and graduate student Joel McGlothlin have discovered that males with higher levels of testosterone—no matter what their age—are irresistible mates for older, more experienced females. They also produce more offspring.

But there’s a catch: The scientists also discovered that these these testosterone-pumped studs are not particularly good fathers, and their offspring are born smaller and die at higher rates. The hormonally charged birds are so eager to show off and chase after new partners that they don’t visit or bring food to their nests as often as their less popular rivals.

In an earlier study, Ketterson found that the über-juncos tend to die at a younger age. Not only does elevated testosterone make them less wary of predators, it also increases their stress levels and promotes production of corticosterone, a hormone that mobilizes quick-release energy but also breaks down protein and leads to the atrophy of muscles, feathers and organs.


How to entice juncos to your yard




Adapted from "The High Price of Being a Hunk" by David Lukas, National Wildlife, February/March 2009.


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