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 itit'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, its 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 its doing in what we call its "subsong," and its exactly the same thing. Its 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 thats a crass comparison, but its 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, youd 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 youre going to be speaking English, you work hard at learning English. But what if you know that youll 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 dont 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.
SA: One of the ways you were able to prove that song knowledge is innaterather than learnedfor certain species was by depriving young Phoebes of their ability to hear.
DK: Wed done a bunch of experiments, but we knew that the final step before we could declare that they do not learn was to prevent them from hearing themselves practice. So we removed the cochlea from the ears of a few Eastern Phoebes [Sayornis phoebe], and they still perfectly produced these beautiful songs. They should not have been able to develop normal songs after being deafened if there was any learning component at all.
SA: Youve compared the Three-wattled Bellbird of Costa Rica to the Humpback whale [Megaptera novaeangliae] because their songs evolve rapidly within each generation. How have the bellbirds songs changed since people started recording them?
DK: We have a series of recordings going back to the mid-1970s, giving us nice documentation of their songs in three dialects. In two of the dialects, the songs of the Seventies are drastically different from the songs today. In the third dialect, the one were working with more carefully, we can plot many of the microchanges over time.
One change is a very loud whistle that has been declining in frequency [pitch] since the 1970s. The frequency has gone from about 5,500 Hz, or cycles per second, down to about 3,700 Hz. Thats an enormous drop, and its an average drop of 70 Hz per year from the 1970s to 2001.
SA: Is the Bellbird unique among birds in that its songs evolve this way?
DK: These birds are relearning their songs probably all the time as they monitor what other birds are singing. This kind of change has been found in only a couple of other birds, including the Yellow-rumped Cacique [Cacicus cela] in Panama. Its a blackbird and it lives in colonies. The songs within colonies change within a generation.
In birds that have fairly short lives, such as Indigo Buntings [Passerina cyanea], which live a couple of years, once the male develops his song he sticks with it throughout life. Each individual male is not constantly relearning his songs over time.
SA: Why do you think the Bellbirds songs change?
DK: Probably like in most lekking systems, relatively few males are successful. The males display [before an audience of females], and many of the females agree as to which is the best male. The females are probably in charge of this system that enables males to show how long theyve been around, whether theyre singing the songs of the local dialects and keeping up with the changes. So the successful males could be changing their songs, forcing the other males, especially the younger birds, to keep up with them. It may be a way for the females to be able to identify the dominant males or the ones who have been in the population the longest.
SA: And one of the ways you were able to prove that Bellbirds learn their songs is that you found they could mimic other birds.
DK: A friend told me about a city called Arapongas in Brazil. If you say "Arapongas" and emphasize the "pong," more or less theyre describing the song of the Bare-throated bellbird that lives in southern Brazil. The town is named after the bird.
People keep Bellbirds in cages in this town. My friend heard a caged bellbird making sounds like a Chopi blackbird [Gnorimopsar chopi] there. He found out that it had been raised with Chopi blackbirds and had learned elementsthe whistles and the purrof their songs. This was a nice experiment done by bird fanciers, and we were able to obtain what I see as unequivocal proof that this one bellbird learned its sounds from the blackbirds.
SA: Why do you find the Bellbird project so compelling?
DK: Its difficult to think objectively once you see these birds because they are so charismatic. They hop around on their perches, they square off, they push each other off perches, they scream in each others ears, they stick their heads into other birds' mouths. Theyre just extraordinary.
The reason that as a scientist I find it exciting is that this is only the fourth group of birds in which we have documented any type of vocal learning. I think they provide a window on the conditions under which vocal learning might have evolved in other groups.
SA: What mysteries of birdsong would you most want to solve in your lifetime?
DK: Why do birds acquire sounds the way they do? Why do some birds learn and some dont? Neighboring robins seem to have dissimilar songs, and that suggests that they probably make them up. There must be some grand evolutionary blueprint by which all of these birds are operating, and if we just knew enough about their life histories, my gut feeling is that all this variety we see among birds would start to make sense.
Jennifer Uscher, a freelance science writer in New York, specializes in writing about birds.