Bird Duets Are "Aggressive Audio Warfare"

Ker Than
for National Geographic News

September 4, 2008

The intertwining songs of tropical wrens are their weapons of choice in turf wars, says a new study that could also shed light on other duetting species. The duets may also help the birds locate one another in dense foliage.

(Related: "Tropical Wrens Sing Complex Tunes, Researchers Find" [August 8, 2006].)

The new discoveries were made using a new multi-microphone technology, which triangulates the positions of singing birds in trees even when they aren't visible.

"With this technique, we can find exactly where breeding pairs are while they perform duets, and where males and females move between subsequent duets," said study leader Daniel Mennill, a biologist at the University of Winsdor in Canada whose work is partially funded by the National Geographic Committee for Research and Exploration.

Mennill and his team recorded the duets of tropical rufous-and-white wrens in the forests of Santa Rosa National Park in northwestern Costa Rica.

(Hear the duets.)

Male and female wrens sing flutelike songs that are so tightly synchronized that human listeners often mistake the duets for the songs of a single animal.

Like many duetting animals, the wrens are highly secretive, and they live in dense tropical forests that shield them from view.

Using the new microphone technology, Mennill and his team could track the positions of the birds in the foliage without having to first capture and tag them.

Among the team's revelations was that male and female wrens use duets to find one another.

One bird sings, listens for the songs of its partner, and moves toward its partner after hearing a response, Mennill said.

Battle Songs

But duets are not always sung in the name of love. The study revealed the wrens also performed together in response to territorial invasions by rival duetting pairs.

The researchers used two loudspeakers to simulate the voices of a pair of duetting wrens to other birds.

"As soon as I played the duets of a rival pair, the resident birds duet rate shot through the roof," Mennill said. "Their voices are beautiful harmonies, but they're also aggressive audio warfare.

"If left unchecked, the battle of songs can escalate into actual physical violence," Mennill said. "Fortunately, encounters between rival wren pairs in the wild are infrequent."

The new findings support the idea that the purposes of wren duets can change depending on the situation.

"They use these beautiful sounds to find each other in thick forest, and they use their duets to defend territories against rivals," Mennill said.

The research is detailed in the September 4 issue of the journal Current Biology.

Towards a Common Purpose

The new findings challenge many current theories about the purpose of duets, according to Herman Mays, an evolutionary biologist at the Cincinnati Museum Center in Ohio who was not involved in the study.

In recent years, Mays said, researchers have focused on duets in conflict situations—"the so-called battle of the sexes, where duetting behavior reflects a struggle between males and females to follow two-mutually exclusive strategies."

But these new findings support theories from the 1990s and earlier—that duets are essentially cooperative, helping to bring males and females together for a common purpose, he added.

The new microphone technique could help scientists decipher the purposes of duets in other species, such as gibbons, frogs, and insects, commented Haven Wiley, a biologist at the University of North Carolina, Chapel Hill.

"Perhaps it will turn out as these authors suggest, that there is no single answer for why animals produce these spectacular duets," Wiley said.