Young parrots, songbirds and hummingbirds learn a repertoire of songs, just as human infants learn to talk. But why is this ability to learn a vocal communication system something we share with birds, but not with our closer relatives, such as the nonhuman primates?
For over 30 years, Donald Kroodsma has worked to unravel such mysteries of avian communication. Through field studies and laboratory experiments, he's studied the ecological and social forces that may have contributed to the evolution of vocal learning.
Kroodsma has paid particular attention to local variation in song types, known as dialects. The Black-capped Chickadees (Parus atricapillus) on Martha's Vineyard, for example, have an entirely different song than their counterparts on the Massachusetts mainland, he says. Birds that live on the boundary between two dialects or that spend time in different areas can become "bilingual," learning the songs of more than one group of neighbors. Recently, Kroodsma discovered that the Three-wattled Bellbird (Procnias tricarunculata) is constantly changing its song, creating what he calls a "rapid cultural evolution within each generation." This kind of song evolution is found in whales but, up until now, rarely in birds.
A professor of biology at the University of Massachusetts at Amherst, Kroodsma is also co-editor of the book Ecology and Evolution of Acoustic Communication in Birds (Cornell University Press, 1996). Though he plans to continue his field studies, he says that one of his most important goals now is to help people understand how to listen to birdsong. "Many people can identify a Wood Thrush (Hylocichla mustelina) when they hear it¿it's one of the most beautiful songs in the world," he says. "Little do they realize they could hear the things that Wood Thrush is communicating if they just knew how to listen."
SA: Can you make any comparison between how a baby bird learns to sing and how a young human learns to speak?
DK: On the surface, it¿s remarkably similar. I often play a tape of my daughter, recorded when she was about a year and a half old. She is taking all the sounds she knows¿"bow-wow, kitty, no, down"¿and randomly piecing them together in a nonsensical babbling sequence. Then I play a tape of a young bird and dissect what it¿s doing in what we call its "subsong," and it¿s exactly the same thing. It¿s taking all the sounds it has memorized, all the sounds it has been exposed to, and singing them in a random sequence. It looks like what the baby human and the baby bird are doing is identical. Some might say that¿s a crass comparison, but it¿s very intriguing.
SA: Why do the song repertoires and dialects of some birds vary from place to place?
DK: For the species of birds that do not learn their songs, I like to think of it simplistically as the song being encoded right in their DNA. With these birds, if we find differences in their songs from place to place, it means that the DNA has changed too, that the populations are genetically different.
But there are species in which the songs are not encoded in the DNA. Then we have something very similar to humans, in which speech is learned and varies from place to place. If you were raised in Germany, for example, you¿d be speaking German rather than English with no change in your genes. So with the birds that learn their songs, you get these striking differences from place to place because the birds have learned the local dialect.
For some audio examples of birdsong dialects, click here.
SA: How is this affected by whether a bird is nomadic?
DK: If you know the rest of your life you¿re going to be speaking English, you work hard at learning English. But what if you know that you¿ll be repeatedly thrown in with people speaking different languages from all over the world? You start to see the enormous challenge it would be to learn the language or dialect of all these different locations. So I think for nomadic birds like Sedge Wrens [Cistothorus platensis] , because they are thrown together with different birds every few months from all over the geographic range, they don¿t bother to imitate the songs of their immediate neighbors. They make up some kind of generalized song, or rather the instructions in their DNA allow them to improvise this very Sedge Wren-y song.
The contrast for the Sedge Wren is the closely related Marsh Wren [Cistothorus palustris]. The western Marsh Wrens, in the Seattle area or California, stay on their territory year-round. Once a male settles on a territory he learns the songs of his neighbors. They live within this very stable community, and I think that gives them the impetus to imitate each other. Why should they imitate each other and all have the same songs? I wish I knew the answer to that.