"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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