Deciphering Cowbirds' Complex Song and Dance
for National Geographic News
|January 22, 2004|
In the bird world, male brown-headed cowbirds (Molothrus ater)
perform a choreographed song and dance routine that would be the envy of
any American Idol contestant.
Using sophisticated monitoring devices and high-speed video cameras, two biologists have revealed just how complex a routine it is. Their research is part of a broad effort to understand how the birds' song and dance works as a communication tool and how songbirds communicate in general.
While female cowbirds don't sing, male cowbirds belt out a small repertory of songs, each one less than a second long. The short riffs begin with a low frequency note complex and almost all end with a high-frequency whistle.
Intriguingly, while all male cowbirds can sing without dancing, they never dance without carrying a tune. Precisely what the birds are trying to communicate, however, remains a mystery.
The research by Franz Goller and Brenton Cooper, biologists at the University of Utah in Salt Lake City, is described in tomorrow's issue of the journal Science.
Song and Dance
Employing video cameras and other monitoring equipment, Goller and Cooper correlated the songbirds' breathing and muscle patterns with their song and dance routine.
The pair found that some of the songbirds' wingor dancemovements appear to make their singing flow more easily. However, during the most complex wing movements, the songbirds went silent.
While silence is expected when the birds breathe in, it's unusual when they exhale. "Cowbirds are the first species to show a long silent period during expiration. It actually looks like they are holding their breath and pushing to exhale at the same time," said Cooper.
The biologists suggest that this silence avoids the most severe biological and mechanical conflicts between singing and dancing. It may also give the cowbirds a musical rest that helps "keep the beat," said Cooper.
Stephen Rothstein, a zoologist and cowbird expert at the University of California Santa Barbara (UCSB), said he has always considered the visual display component of the routine important. But that he and other researchers didn't appreciate how complex it was until looking at Goller and Cooper's paper, he said.
So what, exactly, does all the showmanship mean?
"Specifically, we don't know what the cowbirds are trying to communicate," said Cooper. "But generally speaking, it is thought to be critical for mate selection and territory defense," said Cooper.
Steve Nowicki, a biologist at Duke University in Durham, North Carolina, said that competition for females might be the thrust behind the routine. "Songs that are harder to produce actually may be more attractive to females," he said. "Males that can produce them demonstrate something about their quality . The relationship between movement and the song adds another dimension that females can evaluate."
Rothstein, the UCSB cowbird expert, said research he conducted with Adrian O'Loghlen, a psychologist at the University of Washington in Seattle, suggests that songbirds use singing ability as a quality indicator.
"We have done pilot experiments that show that females are more responsive to song playbacks when they are accompanied by video of a singing male," he said. "It appears that the quality of a male's song spread display and his ability to coordinate it with his singing may also be important."
Which Came First?
Goller and Cooper suggest that the evolution of the cowbirds' singing and dancing abilities are closely linked, making it difficult to conclude which came first, the song or the dance.
Rothstein said that since most songbirds only sing, he suspects the cowbirds learned to sing before they started to dance. "This is what most songbirds show," he said. "One thing is certain, though, and that is that the combination of visual and acoustic signals is an ancient one in the cowbird lineage."
The cowbird group's closest relative, which includes the highly common red-winged blackbird (Agelaius phoeniceus), also combines visual and acoustic signals. Cowbirds branched off from the red-winged blackbird group about four million years ago, suggesting the association goes back later than then, said Rothstein.
The next step for the researchers is to decipher specifically what the cowbirds' song and dance is meant to communicate. Cooper said he is particularly curious to learn what song and dance lines female cowbirds prefer.
"We are hoping to set the stage, for future work along those lines," he said.
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