A morning in spring would not be complete without birdsong, ranging from long and complex, such as the winter wren’s high-pitched, multi-note melody, to the Townsend’s solitaire’s single plaintive whistle. To study song learning, researchers isolate growing male birds from all singing males. In classic studies, English ethologist W.H. Thorpe raised European chaffinches in complete isolation. These birds sang only the skeleton of a chaffinch song when they matured the following spring. By contrast, a chaffinch exposed to adult male “tutors” singing during the first few weeks of life grew up to sing several fleshed-out variations on the basic song. He presumably could compare his simple first attempts with a model remembered from the previous summer.
Raising chicks in complete isolation, however, can produce maladjusted birds, according to Meredith J. West and Andrew P. King, of the University of North Carolina and Duke University, respectively. So they raised hatchling male cowbirds from North Carolina with adult females from Texas. To their surprise, the males grew up singing with a distinct Texas twang. But how had the silent females “taught” the dialect?
European starling (Sturnus vulgaris)
Photo © 2006 Daniel Baleckaitis
Courtesy Margoliash Lab
Researchers use sound recordings and spectrograms to study bird song.
Here, we’ve combined one-second pieces of a spectrogram with an audio track to produce a video.
audio and spectrogram MarthaLeah Chaiken, of Hofstra University
Video © 2003, Stephen Hart
By careful observation of cowbirds in the wild, West and King solved the puzzle. Males sing four to seven variations built on a basic gurgle, gurgle whistle theme. Each male sings a unique set of variations. When a female hears one she likes, she signals her approval by flipping one wing up and out, batting her eyelashes, as it were. The behavior represents a rare example of an expression of approval among non-primate animals. The signal takes place in an eye blink, only a few thousandths of a second, during the male’s one-second song. But that’s enough for the males. Once they know what a female likes, they’ll concentrate on just that variation. The researchers draw parallels to humans. Babies show approval of adult vocalizations with smiles and alert looks. Their caretakers learn by repeating the kind of vocalizationssoothing baby talk, for examplethat triggered the approval . As babies grow, however, their preferences change. By the age of a year and a half, they prefer a more adult-sounding vocal style, and the easy-to-train adults comply.
Dialects among birds probably have no more significance than human dialects, says the University of Washington’s Michael D. Beecher. “Birds learn songs, presumably, because of benefits to the individual, and these benefits appear to relate to learning songs from (and thus sharing song types with) your immediate neighbors (following dispersal from the nest and natal area). If you learn songs from your immediate neighbors, one larger-scale consequence is area dialects.” Matching songs with your neighbors, he says, may play a crucial role in the social structure of birds.
Bird songs send two related signals, as in most male animal vocalizations; first, to announce the male’s breeding status to females and second, to post his territory with auditory “Keep Out” signs. Song sparrows sing eight or nine distinct songs, several of which they share with close neighbors. And they’re not alone. About 70 percent of all songbird species sing more than one song type. Evolutionary explanations of these repertoires remain a topic of debate among bird behaviorists, many seeing the song variants as interchangeable. Beecher, however, sees a more complex communication going on. A young male learns songs in his repertoire from three or four older males. Then when he sets up and begins to defend his own territory, he can reply to songs of neighborsboth his former tutors and his young classmatesbecause he shares song variants with them.
Hearing a neighbor’s song from near or inside his territory, a song sparrow could defend it in a couple of ways. He could repeat the same song type or reply with a different song variation, one both males share in their repertoires. Song sparrows Beecher has studied typically share about 40% of their song types, and only rarely share no song types. Beecher played recorded songs to defending song sparrows in a field study. Sparrows hearing a neighbor’s song responded most often with a song shared by the two, but not with the song they’d just heard. Presented with a strange song or their own song, they rarely used a shared song to respond, instead matching the song they heard. Beecher suggests that using a song from a shared repertoire helps to maintain a status quo. It’s a reply of low intensity and represents a mild challenge, more like a hard stare than a shouted “Get lost!” Matching a song more precisely represents a direct challenge, which can escalate into an energy-sapping chase.
Most birds, like Thorpe’s chaffinches, learn their songs during a critical period of growth. For chaffinches, it’s any time before they mature sexually. For the white-crowned sparrow of the American west, it’s the first summer of life. After that they don’t learn. Humans likewise learn language best at a young age. After their teens, few people can learn to speak a foreign language without a distinct accent. A final parallel lies in the genetic component of learning communication. Experiments on songbirds show they can learn variations on their species-specific songseven variations never seen in nature, such as songs with the first and second halves switched. But they can’t learn songs of other species. (Of course some species specialize in imitating other species’ songs.) Humans, for their part, seem genetically programmed to learn human language. They sponge it up by the quart. They don’t, however, learn to imitate the myriad other sounds surrounding them as they grow, even when raised in isolation from human speech.