A Song for Every Occasion
New technology is revealing that the notes, warbles and chirps of birdsong say more than anyone had guessed
At 4:30 a.m. in an upstate New York meadow, a 1980 Pontiac rolls along the grass and comes to a halt in the dark. Overhead, stars are beginning to fade from the night sky. Dawn approaches. The car door pops open and out steps John Bower, a shaggy-haired, bearded graduate student from Cornell University. A former farm boy with an affinity for cross-country skiing and ice hockey, Bower has come to this meadow to pursue ornithological studies.
He walks to the trunk, opens it and, with a flashlight in one hand, pulls out eight microphones and carries them to microphone stands already arranged in two rows 50 yards apart and about 200 yards long. As he mounts a microphone on each stand, the sky lightens to gray, then turns pink. Scattered trees grow visible among the rolling meadows, and the first chirps of birds greet the sun. By the time Bower's done hooking up cables to the microphones, the birds sing in full chorus.
The birds and their songs are the source of Bower's interest in the meadow. When the morning is bright enough, he begins monitoring the comings and goings of song sparrows, a common, brown bird with a streaked buff breast. As Bower wanders the field taking notes, his microphones are collecting data of their own. A transmission of meadow sounds travels down the cables back to Bower's Pontiac, where they feed into $15,000 worth of digital eight-track recording equipment. "The whole front seat of the car is turned into a little recording studio," says Bower. Most spring mornings, Bower stays out until about ten, recording five hours of bird song on high-resolution tapes.
The birds in Bower's field are not just twittering blithely for lack of something better to do--they are creating a complex musical conversation fraught with subtleties that biologists are only now beginning to decode. Dedicated birders have been able to recognize bird species by their distinctive songs since time immemorial, but the story behind the songs lay hidden until 40 years ago, when ornithologists began using then-new portable tape recorders and other equipment to capture songs for analysis. Now a younger generation of scientists like Bower is going into new technological realms, using personal computers, digital audiotape and other cutting-edge gear to make quantum leaps in the study of bird song.
Fewer than half of the 8,700 known bird species actually sing, creating musical sounds through a kind of double voicebox that lies at the base of the windpipe, where it branches into the lungs. There, two sets of membranes and muscles vibrate at high frequencies as air is exhaled from each lung. While singing, the bird can alternate between the two lungs, even singing in harmony with itself.
While the structures that make bird song possible have long been known to anatomists, the structure of the songs themselves remained a mystery until the 1950s. During that decade, biologists started playing taped bird songs into sonographs, devices that analyze sound frequencies and create a picture of them. "Until we could actually produce a picture that looked like musical notes, we had no way to publish an accurate description of the songs," says Sandra Vehrencamp, a biologist at the University of California at San Diego. "We gave them names like phoe-be-be-be. But you can't render a more complex song that way."
By poring over sonograms in the decades that followed, ornithologists discovered that bird songs have some similarities to human language. Each song is composed of phrases that, in turn, are made up of smaller units called syllables. Each species follows its own rules for using phrases and syllables, but individual birds bend those rules slightly to create their own distinct songs. These uniquely modified songs become the individual bird's repertoire.
The number of song types in an individual bird's repertoire varies from species to species. Each song sparrow has a repertoire of only eight songs on average, a number similar to the repertories of individual juncos, cardinals and western meadowlarks. In other species, individual birds have much longer lists from which to choose and can even learn new songs as adults. A starling's repertoire may include as many as 67 song types, a mockingbird's as many as 150, a brown thrasher's more than 2,000.
In spring, male birds perch on high points and break into song. Their usually bright colors help to accentuate their presence. In an effort to stake out territories and attract females, the birds engage in true song wars, threatening competitors with often melodious calls. The battles are tough: Researchers recently discovered that when blackbirds hear another blackbird sing, their pulse rates shoot up.
While determining that songs are used to lay claim to land came easy and early, biologists found it more difficult to understand why birds need a whole repertoire of songs for this task. Some ornithologists have suggested that the birds, by issuing an array of songs, are trying to deceive male newcomers into imagining that the neighborhood is overcrowded and not worth moving into. Biologists call this the Beau Geste hypothesis after the story of a lone French legionnaire who defended a fort by propping up dead soldiers on the ramparts and firing their guns in quick succession.
But a recent experiment suggests that this hypothesis may be as fictitious as the novel after which it is named. John Krebs, an ornithologist at Oxford University in Britain, and his colleagues played to captive birds a single song from an individual that the captives had never heard sing before. Later, the test birds could pick out that individual bird even when it sang other songs they had never heard before--suggesting that birds can recognize each other's voices. "The Beau Geste hypothesis cannot be true if this mechanism works," explains Andre Dhondt, an ornithologist at Cornell University.
Dhondt suspects that males blast away at one another with large repertories as a way of showing off--not just for each other, but for females. "The way a male sings reflects his quality," says Dhondt, and clues to quality are of critical interest to female birds looking for mates likely to produce healthy, numerous young.
Experimental research backs up Dhondt's speculation. Studies have shown that the larger a male bird's repertoire, the greater his reproductive success. And since a female has to spend a lot of energy producing and hatching an egg, she is going to listen carefully to vocalizing males. For instance, male red-winged blackbirds can be easily fooled by a mockingbird's imitation of red-wing songs, while red-wing females are four times more likely to make courtship displays when hearing real male redwings than when hearing a mimic.
Birds can use their specific choice of song type to signal subtle messages, such as changes in level of aggression, to neighboring birds. For example, they can match the song a neighboring bird is singing or avoid matching it. They also can vary the rate at which they switch between song types. The only way a biologist can investigate the meaning of such singing strategies is to experiment with an interactive playback technique.
Vehrencamp's own program, SingIt!, allows her to store a repertoire of songs and play back any song type with a stroke of a key. She takes the program into the field via a laptop computer, then chooses a bird that she has been studying and plays songs to it, using various strategies. Sometimes, she matches the song the bird is singing and sometimes she does not. She then records the bird's response to the various strategies.
In this way she has broken part of the code of songster combatants: Her research on song sparrows suggests that this species repeats a particular song type several times before switching to another type. Vehrencamp refers to each round of repetition as a bout. The most confrontational involves one bird continually matching the song of another. "They get locked into these matching bouts, which persist for unusually long periods of time," says Vehrencamp. "The bouts seem like games of chicken to see who will change first, but perhaps the birds are communicating more subtle information than we can measure."
One of the newest techniques for exploring the song wars began with research on whales. When John Bower first came to Cornell, he worked with bioacoustician Christopher Clark, who was trying to figure out how to track whales in the Arctic Ocean by their calls and songs. With the help of students like Bower, Clark set up a network of microphones under the arctic ice, with each microphone registering the songs of a given whale. Since it took different periods of time for the songs to reach different microphones, Clark could compare the signals and pin down the location of any singing whale.
"I realized it would be a great technique to use on land," says Bower. Which is why he now spends spring mornings setting up two chains of microphones linked to his mobile recording lab. At the end of each morning, Bower takes his taped songs to the lab and downloads them into a large computer that can display sonograms of the sounds recorded at each microphone. The displays are packed with all the sounds of the field, including the songs of many different bird species. But Bower, with practiced expertise, can pick out the songs of individual species by eye. When he indicates a particular bird's song on the screen, the computer calculates the bird's location on a map of the field. The system is so precise that it can track a flying bird simply by its shifting song.
Now Bower has a full record not of what just one bird is doing and singing, but of what a dozen birds are doing all at once. That has allowed him to reveal further complexities in avian vocal combat. He has discovered, for example, that early in the spring, males tend to ignore the euphonious threats of distant competitors. "During periods of high conflict, with lots of fighting, neighbors will match each other and not match others," he says. "They're sending the message, I'm talking to you, not to him over there, and you better get out of my territory.' That's the first tantalizing stuff that I have."
And more tantalizing stuff is certainly on the way, since the new technologies have researchers drowning in information. Although he will trudge into a New York meadow to record birds most mornings during the season of song, Bower says, "I could spend the rest of my life analyzing what I've already recorded."
New York journalist Bill Rankin wrote about snowshoe hares and sunspots in the June/July 1995 issue.
The best way to understand a bird song is not to hear it but to look at it. To do so, ornithologists create what's known as a sonogram. Using a computer, they break a song into intervals a few thousandths of a second long. Each interval contains many different frequencies of sound, which the computer teases out. The data from all the intervals are then laid out on a graph, with time charted on the horizontal axis and frequencies on the vertical. The resulting pattern is almost as distinctive to a species as a fingerprint is to a person.