for National Geographic News
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.
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