"Because if they don't have a territory, they don't have a partner, they can't breed, and they don't have a spot to find food," she said.
The study appeared today in the journal Current Biology.
A previous survey has suggested that about 200 birds species croon harmonious duets to defend territory, Slater said. The phenomenon is almost entirely restricted to the tropics, he added.
There, birds tend to stay put year-round and are almost entirely socially monogamous.
This way pairs are more mutually invested, and perhaps sing together to show it.
"It's in both of their interests to defend their shared territory and often their pair-bond as well," said David Logue, a postdoctoral fellow in the department of biological science at the University of Lethbridge in Canada, who was not involved in the study.
Dueting birds are more cooperative in their territory defense than non-dueting birds, said Logue, who has done research in the area.
The most impressive example comes from the plain-tailed wren, which sings in choruses of four parts.
(See related: "Tropical Wrens Sing Complex Tunes, Researchers Find" [August 8, 2006].)
"[Wrens] all join together and produce these extremely stylized and rather extraordinary songs," said Slater of St. Andrews University.
"It's like a war chant, really."
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