Beak Size Matters for Finches' Song, Scientists Suggest

John Roach
for National Geographic News

August 27, 2004

Darwin's finches in Ecuador's Galápagos Islands are cornerstones to the late British naturalist Charles Darwin's theory of natural selection: the size and shape of the finches' beaks are adapted to take advantage of their individual ecologic niches.

Some of the sparrow-sized songbirds have large beaks which are able to crush hard seeds—an especially useful trait in drought-prone regions. Other finches have short, sharp beaks which are good for eating insects.

Now, Jeff Podos, a biologist at the University of Massachusetts, Amherst, says that the variation in the beaks also influences the finches' musical sound, which may play a role in how a female selects a mate.

"It's the males who sing and there are other studies that show that females really pay attention to the songs males are singing. There could be a link there," Podos said.

Finding a definitive link requires a better understanding of how a bird sings his song so that scientists can be sure that when a female chooses a mate she is doing so for the quality of his song that is determined by the properties of his beak.

"A beak affects certain parts of the sound, it doesn't affect all the song," Podos said.

Like a Trombone

By using video of singing birds played back in super slow motion, Podos and colleagues are teasing apart the mechanics of the beak's role in birdsong.

To get low frequency sounds, a small-beaked tree finch, one of the 13 species that inhabit the Galápagos Islands, keeps its beak almost closed and stretches its head forward. To produce a high frequency sound, it opens its mouth wide and pulls its head back.

These movements—the opening and closing of the beak, stretching and pulling back the neck—alter the volume of the finch's vocal tract, which in turn alters the frequency of the sound coming from the bird.

The vocal tract of the finch—and all other birds—consists of the beak and trachea, which is also known as a windpipe. The syrinx, a sound-producing organ unique to birds, is located in the trachea.

According to Podos, the vocal tract in a songbird works in a way similar to musical instruments such as the trombone. The sounds produced by the syrinx are filtered and dampened as they pass through the trachea and beak, coming out as sweet sounding birdsong.

"Think of the vocal tract in a bird as a resonance chamber," Podos said. "That's what a tube in a trombone is like."

As a trombone player pulls in the slide to make a higher frequency sound by reducing the volume of the tube, so does a bird open its beak and pull back its head to reduce the volume of its vocal tract.

Podos and his colleagues have also found that smaller beaked finches, such as the tree finch, are able to produce more complex songs because their beaks are more versatile than larger-beaked finches, such as the seed-eating ground finches, who sing slow, simple songs.

"Because all of these birds—finches and other songbirds—have evolved songs that require precise matching between vocal tract movements and song structure, the birds with bigger beaks will be forced to evolve songs with simpler structure to allow the matching to happen in spite of their poor versatility," Podos said.

Partial Support

Peter Grant, a professor of ecology and evolutionary biology at Princeton University in New Jersey and expert on Darwin's finches, said studies he has done "provide only partial support" for the theory that beak size influences birdsong.

For example, Grant noted that finch species with different beak sizes sometimes learn to sing each other's songs and when they do they mate across species and produce hybrids.

"Song plays an important role in mate choice in the birds we have studied, but not because beak size has determined the song," Grant said. "Learning can override any tendency there might be for songs to be governed by beak size within broad limits."

In attempt to test the theory that beak size can influence mate choice, Podos and his colleagues are trying to determine which part of a bird's song is influenced by the size of his beak.

"Ideally, in a few years, we'll be able to take songs that only vary in features influenced by the beak and then give [the females] a choice and see how they respond to those," Podos said.

A female indicates her choice by what Podos refers to as a mating display: beak pointed to the sky, wings quivering, and tail lifted.

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