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The Sound of Music

Down through the ages, birdsong in all its variations has captivated everyone from musicians to poets. In one of their newest fields, scientists are now seeking to unlock the mysteries of how chickadees and other birds use sound to communicate.

By Don Stap
Photograph by John Goodman

Wired for sound: Don Kroodsma, parabola microphone in one hand and reel-to-reel tape recorder strapped to his waist, takes aim at a singing bird, whose song travels directly to his ears via headphones.

Copyright © 2005 by Don Stap. From the forthcoming book Birdsong, by Don Stap, to be published in March by Scribner, an imprint of Simon & Schuster, Inc., New York. Printed by permission.

Several years ago on a Sunday afternoon, I wandered through the one-story cinder block building at one of the most famous addresses in bird studies—159 Sapsucker Woods Road: Cornell University's Laboratory of Ornithology. Passing through an unlit hallway hung with the paintings of Louis Agassiz Fuertes, the greatest of bird artists and an early associate of the lab, I made my way to the southeast wing of the building and opened the gray metal door to Room 125. Stepping inside, I felt a rush of cool, dry air. The windowless room, tightly packed with rows of metal shelves, was austere: white walls, a cement floor, exposed ductwork and girders, and bare lightbulbs hanging from the ceiling. It would have been difficult to imagine a more lifeless space, yet all around me, stored on wall-to-wall shelves, was the aural life of the planet. This was the archive of the Macaulay Library of Natural Sounds, the largest collection of its kind in the world. The shelves, which rose above my head, contained more than 160,000 individual recordings, some in neatly labeled boxes containing seven-inch reels of tape, others on standard cassettes in makeshift containers with hand-scribbled labels.

Walking down the aisles, I found boxes that held the sounds of crickets chirping, mountain gorillas thumping their chests, and prairie dogs barking. It was birdsong, though, that had drawn me here, and birdsong that dominated the Library of Natural Sounds. There, arranged taxonomically from ostrich to raven, were the songs of more than 6,000 of the world's 9,000 or so bird species.

On one shelf were the babbling-brook arias of mockingbirds; on another, the flutelike ee-oo-lays of wood thrushes; and on others, the wistful melodies of white-throated sparrows, the caroling of robins, and the songs of birds I had never seen nor heard: the superb lyrebird, the laughing kookaburra, the black-and-gold cotinga, the snowy-headed robin chat, and more. The room was brimming with sound. But of course I heard nothing. The silence was profound.

This archive of sounds is invaluable. Several recordings hold the voices of birds now extinct (the Guam flycatcher and Bachman's warbler) or most likely extinct (the ivory-billed woodpecker). Many are the only known aural records of rare and elusive species. Recordings like these are critical in one of the newest fields in zoology: bioacoustics, the study of how animals use sound to communicate. In recent years scientists have discovered that elephants use infrasonic sounds to send messages across great distances; that hippos are able to communicate simultaneously in water and air; that small insects known as treehoppers send vibrations through the stems of plants to communicate with other treehoppers; that vervet monkeys use one kind of alarm call to signal that a leopard is nearby and a different one to signal the presence of a snake. But it is birds that have attracted the most attention. It has always been birdsong that has most enthralled and mystified us. Frogs croak, crickets chirp, wolves howl, and lions roar, but birds sing.

But why do birds sing? Why have they come to rely on this particular means of communication? One line of thinking connects song with flight. Song is an energy-efficient way to advertise and defend a territory. A bird need not fly from boundary to boundary to ward off interlopers. It can sit in one spot and sing. There are other ways to ask the question. Why do some birds sing and others not? Why don't eagles sing like robins? Why does the chestnut-sided warbler sing one song before dawn and switch to another at sunrise? Why do mockingbirds imitate the songs of other birds?

 

I left the Lab of Ornithology that Sunday afternoon with more questions than I'd arrived with. A few weeks later I flew to Cape Cod, boarded the 6:15 p.m. ferry to Martha's Vineyard, and followed the winding, hilly roads to Gay Head, on the west edge of the island. I arrived shortly before dusk at a summerhouse overlooking an inlet. Don Kroodsma, a professor of biology at the University of Massachusetts, had rented it for the weekend and had invited me to join him and several biologist friends for what he called, more seriously than not, a party.

The “party” began about the time others might be ending—3:30 a.m. I rose, startled by the alarm clock, and turned on the bedside lamp. The house was silent except for a rustling in the kitchen, where I found Kroodsma. There was no time for breakfast. Kroodsma was already headed for the front door, carrying a backpack over his shoulder. We drove off, Kroodsma intent on his mission: to explore the mystery of the song of the black-capped chickadee.

“The mainland chickadee sings only one song, hey-sweetie, but these Vineyard birds have at least three different songs: sweetie-hey, sweetie-sweetie, and sosweetie-sweetie. They're trying to tell us something. I just don't know what.”

The chickadee is one of the best known and most common birds in North America, a tiny puff of white and gray with a black cap and chin. Its range encompasses the northern half of North America, from Nova Scotia to British Columbia, and it is a frequent visitor to bird feeders. Consequently, its song and most common calls—a whistled fee-bee-ee and a buzzy chicka-dee-dee-dee, respectively—are known by even the most casual observers of birds. Yet many experienced birdwatchers and more than a few ornithologists would have been surprised to know there was any mystery regarding the chickadee's song. Clearly, they hadn't listened to it as closely as Kroodsma had.

Kroodsma, a tall, fit man in his fifties, has been eavesdropping on birds all of his adult life. He is widely regarded as one of the world's experts on birdsong. The mystery of the chickadee's song, he said, was irresistible. The tone of the whistled notes and the descending pitch give it a wistful quality. Kroodsma, as hard-nosed as they come when he's thinking as a scientist, becomes wistful himself when he talks of the beauty of a bird's song. Under the spell of the chickadee's modest melody, and mindful that one function of the male chickadee's song is to attract a female, Kroodsma prefers to refer to the song as hey-sweetie rather than fee-bee-ee, as it is often described in field guides.

Nearly every black-capped chickadee in North America, Kroodsma explained to me, sings his simple hey-sweetie in exactly the same way. This is just what one would expect, I thought. In fact, however, it is highly unusual for a songbird to sing its song precisely the same way across such a wide range. Songbirds, like people, have dialects, Kroodsma said. The song of the common yellowthroat, a rapid witchity-witchity-witchity-witch , changes from North to South, each witchity containing more “syllables.” The northern parula warbler's song rises rapidly in pitch— zeeeeeeeeeup. In the eastern part of Kentucky's Daniel Boone National Forest the song drops off sharply at the end, while in the west it fades out on a high note. But black-capped chickadees in Montana sing hey-sweetie just the same as those in Minnesota and Vermont.

The chickadee's lack of dialects was intriguing. And it was all the more interesting because Kroodsma knew it wasn't the result of some kind of restriction in the song-learning center of the brain. He knew because he had taken a nest of baby chickadees home, raised them, exposed them to more than one song, and watched as they developed several songs. Not long after he had completed his experiments with the baby chickadees, Kroodsma came across a paper, published in 1958, that reported chickadees on Martha's Vineyard sing both notes of hey-sweetie on the same frequency. Mainland chickadees sing the second note, the sweetie, on a lower frequency. Kroodsma was intrigued by this peculiarity, and on a trip to the Library of Natural Sounds in the fall of 1993, he asked Greg Budney, curator of the library, if he could listen to the library's collection of chickadee songs. “The songs were backwards,” Kroodsma told me. The Martha's Vineyard chickadees sang not hey-sweetie but sweetie-hey.

“The mystery deepened,” Kroodsma said. Understanding how the Martha's Vineyard chickadees sang might, by way of contrast, illuminate the singing habits of mainland chickadees. A year after Kroodsma's visit to the Library of Natural Sounds, he traveled to Martha's Vineyard, got up before dawn the next day, and drove to the airport in the center of the island. He parked his car, got on his bicycle, and rode along a route that circled much of the island, stopping to tape chickadees along the way. By the end of the day Kroodsma had recordings of not only the sweetie-hey song he and Budney had listened to but also two more songs. Some birds sang sweetie-sweetie , and still others sang sosweetie-sweetie. How could the birds on this island have developed three songs when the chickadees all across mainland North America sang only one song, without any variations? He would need to gather hundreds of recordings from the island chickadees, as well as recordings of mainland chickadees. He began planning his party.

 

On the first morning of the party Kroodsma chose to record at Correllus State Forest, near the center of the island. With only the dome light to guide him, he rummaged through the recording equipment in the back of his car. Within minutes he outfitted himself. Over his left shoulder, on a thick, heavily padded strap, he carried a bulky Nagra reel-to-reel tape recorder, nearly the size of a small suitcase. A second strap, tied to Kroodsma's waist, kept the recorder from swinging too much as he moved. A cable led from the recorder to an 18-inch parabolic microphone—a “microphone system” to be exact, made up of a saucer-shaped dish with a microphone mounted in the center at exactly the point the sound waves converged when reflected from the concave sides of the dish. It functioned like a hand cupped to an ear, gathering sound and funneling it to a receiver. A second cable ran to a pair of head-phones, which rested askew on Kroodsma's head, like earmuffs. All in all, he looked like a walking satellite dish.

We walked a few paces from the car and stood near the edge of the pine forest. I stood quietly, shifting from foot to foot and rubbing my hands to keep warm in the cold night air. Finally, just after first light, a hundred feet or so back in the pines, a chickadee sang. Kroodsma listened, moving the parabolic reflector back and forth, trying to get a fix on the bird's position. But after four repetitions of his song, the bird fell silent. Five minutes later a second bird sang briefly, then stopped. A third bird started up 50 feet down the path. Kroodsma took off quickly in the direction of the singing chickadee, his long, purposeful strides leaving me behind momentarily. The bird sang loudly and clearly for a couple of minutes, enough time for Kroodsma to zero in on him with the parabolic microphone and tape the song. But what Kroodsma heard surprised him. Last year the birds in the center of the island were all singing sweetie-hey , leading him to think there might be a geographic pattern to the three songs. But this chickadee's song sounded like sosweetie-sweetie. “I don't understand what's going on here,” Kroodsma said softly.

On the ride back, Kroodsma pondered the island chickadee's song. “What do we know?” he asked. “The mainland chickadee sings only one song, hey-sweetie, but these Vineyard birds have at least three different songs. There's sweetie-hey, sweetie-sweetie, and sosweetie-sweetie. And then there's this interesting thing the mainland birds do—they sing hey-sweetie on different frequencies. But these birds on the island do something else. I've been hearing them sing their song on two different frequencies, a high and low, but nothing in between.” Kroodsma paused. “They're trying to tell us something. I just don't know what.”

When the party was over, Kroodsma had nearly 200 representative samples of chickadee songs from various parts of the island. By the following summer, he had most of what he'd hoped to get, including recordings of birds from Chappaquiddick and Nantucket, both of which provided surprises. Kroodsma spent 18 months analyzing the new data and writing up the study. In 1999 he published “Geographic Variation in Black-capped Chickadee Songs and Singing Behavior” in The Auk, the journal of the American Ornithologists' Union. The paper details the results of analyzing the recordings, noting the different island songs, their distribution, the songs on Nantucket and Chappaquiddick, and how different this is from the mainland black-capped chickadee's song. At the end of the paper, where one might expect some conclusions, Kroodsma lists half a page of questions he would like to see answered in subsequent studies.

But when I called to ask him about the paper, Kroodsma seemed happy regardless. “What I find so fascinating is when you look at the maps, you see that the low-frequency version of the sweetie-hey song covers much of the island,” he said. “You see it in the west, on Gay Head, throughout the center of the island, and even far to the east in Edgartown. There are a few pockets of the high-frequency sweetie-hey within the distribution of the low-frequency version. At one point they had to have occurred together, they must have arisen together, and then one was just more successful than the other, for whatever reason.”

I asked about the other songs. “Remember what the island looked like?” Kroodsma said. “If you stood on the ridge at Gay Head and looked across the island toward the east, you could see how forested the island is. But there's a picture in a book from over a hundred years ago that looks across the island from Gay Head and there's not a tree in sight. When I saw that picture, I could imagine that there would have been isolated pockets of trees here and there—chickadee habitat—where groups of chickadees were isolated, and so different dialects could have developed.”

Of course, the original mystery remained: Why did the mainland chickadees sing one song, without any dialects? Kroodsma seemed no closer to an answer than when he'd started. “What we now know about the island birds adds to the evidence that the black-capped chickadee is capable of singing in larger repertoires. But the mainland birds restrict themselves to . . .” Kroodsma paused a moment. “Here's where you have to choose your words very carefully: They restrict themselves to what looks like a single song that they sing on different frequencies. But are our words—what we mean by song —limiting how we think about these birds? Maybe every one of these different frequencies on the mainland is—to a chickadee—a different song.”

Kroodsma's leap of thought left me blank for a moment. Singing the same song on different frequencies is not what ornithologists think of as singing different songs. Kroodsma was pushing beyond the customary definition of song. This was sheer speculation of course, Kroodsma said. “Who knows what all this means to a chickadee?” All along I had misunderstood the business of the mystery of the chickadee's song. For Kroodsma the birdsong party was as much about enjoying the mystery as solving it.

 

Don Stap, who teaches creative writing at the University of Central Florida, wrote about redstarts in the June 2003 Audubon.

 

 

© 2005 National Audubon Society


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The Shape of Sound

Naturalists have been sketching birds since the 16th century, but until the 20th century, they had no adequate way to represent the subtleties of what they heard when birds sang. Two inventions changed all that: the tape recorder and the audio spectrograph, or sonograph. The latter, developed by Bell Laboratories, produces a sonogram, a visual representation of sound that enables scientists to analyze a bird's song as concretely as Darwin measured the beak of a finch.

A sonogram displays time on the horizontal axis, and frequency in hertz (cycles per second) on the vertical axis; a sonogram typically displays something like 1 to 8 kilohertz. Typical bird sounds translate into specific shapes. A pure whistle creates a solid line that rises or falls if the frequency changes. A trill—with evenly spaced individual "syllables"—often looks roughly like the folds of an accordion. A buzz usually looks like a very compressed trill.

Thus, the typical whistled two-note song of the North American black-capped chickadee ( hey-sweetie, usually noted in field guides as fee-bee-ee ) produces two distinct lines, because the frequency of the second whistle is lower. A small break in the lower whistle reveals the slight pause that the chickadee takes in singing the sweetie. Chickadees shift the pitch of their hey-sweetie song over a range of frequencies.

Songs of the Bewick's wren typically consist of several "phrases." In its sonogram, the song begins with a couple of harsh notes and two high-pitched notes that look a bit like cartoonish eyebrows. Next is a harsh-sounding phrase consisting of nine syllables, then four emphatic S-shaped notes. The entire three-second song goes up and down in pitch from 2 to 8 kilohertz.

If you understand these basic shapes, the sonogram of a fairly simple song, such as an eastern towhee's drink your teeeeeeeee, should be easy to imagine. It has two introductory notes and then a longer phrase of identical syllables. It may be more difficult to conjure a mental picture of a song sparrow's whistled notes and trills. But with practice the trick of transforming a sound to a visual image can be valuable in the field: Simply jot down what you hear and it will be easier to identify the song later .

—D.S.

The sonograms above are reprinted from The Singing Life of Birds: The Art and Science of Listening to Birdsong, by Donald Kroodsma, to be published by Houghton Mifflin in April 2005. To hear a collection of birdsongs, go to "The Oldest Music" at http://magazine.audubon.org/birdsongs.