May 10, 2000 ITHACA, N.Y. -- While ornithologists consider cowbirds the parasites of the bird world -- commandeering the nests of other birds, hoarding their food and causing starvation -- Cornell University behavioral researchers know these songbirds have a redeeming quality: They carry an important, evolutionary tune.
Like the Mick Jaggers of music, male cowbirds attract females by strutting their masculine feather colors or by their singing ability. But the Cornell scientists have found that both songs and mating rituals correlate with the size of the cowbird brain.
"The female cowbirds are looking only for your genes, so that if you're a male cowbird, you had better look darn good," says Mark E. Hauber, a Cornell doctoral student in neurobiology and behavior. "You had better advertise yourself."
Before this latest research, data that showed a connection between an animal's brain size and mating behavior relied mostly on guesswork, says Timothy J. DeVoogd, Cornell associate professor of psychology. "This is the first time this information has been correlated in a parasitic bird species. No one has ever done this before. Surprisingly there is a close correlation between brain size and a cowbird's abilities and behavior."
The research, "Sexual Dimorphism and Species Differences in HVC Volumes of Cowbirds," appeared in Behavioral Neuroscience, a journal of the American Psychological Association (Vol. 113, Number 5.)
Cowbirds, which are found in North, Central and South America, are medium-sized black birds, that, in some cases, such as shiny cowbirds, also have brown feathers. The birds' songs sweep a large range of frequencies, sounding "like a gargoyle, very bubbly, not very melodious -- like water dripping from a faucet," says Hauber.
The researchers note that the cowbird appears to have brought Darwinian theory clearly into focus. "We think that visual and sexual selection factors seem to go hand-in-hand," says
Hauber. "The better the song, the better the feathers, the better the mate. In a way, we can tell from the brain size who gets mates and who doesn't."
Inside the cowbird brain is a high vocal center, or HVC, and the scientists have correlated HVC size with sexual differences and sexual preferences. To do this, Hauber and DeVoogd studied the shiny cowbird, the screaming cowbird and the bay-winged cowbird, all native to South America.
They learned that the promiscuous, male shiny cowbirds have large brains and sing forcefully, while the females, whose brains are smaller, do not. While the screaming and bay-winged male cowbirds also sing for sex, these birds are monogamous. Interestingly, both male and female bay-winged cowbirds have a similar HVC size -- and the females sing like the males.
Hauber says that cowbirds are a great scientific model. "Through them we do a lot of learning about their reproductive and social behavior, signals in mating and male competition."
In addition to Hauber and DeVoogd, the research was authored by Nicola Clayton, University of California, Davis; Alex Kacelnik, University of Oxford, United Kingdom; and Juan C. Reboreda, Universidad de Buenos Aires, Argentina. The research was supported by funding from the National Institutes of Health, a Wellcome Trust grant, the Whitehall Foundation, the British Council and Conicet. Hauber also received a Howard Hughes Medical Institute Predoctoral Fellowship.
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