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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