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. I had been let in, the door locked behind me, and I had the place to myself. To research an article I was writing on birdsong, I planned to review some of the literature in the lab's private library, including materials that were available nowhere else, but within minutes I found myself drawn to another kind of archive. 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 had the sterile functionality of a hospital room, and it appeared, if anything, cleaner and more orderly. The only sound was air moving through the vents. 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 130,000 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 narrow aisles, I found boxes that held the sounds of crickets chirping, mountain gorillas thumping their chests, triggerfish squirting water, and prairie dogs barking. It was birdsong, though, that had drawn me there, and birdsong that dominated the Library of Natural Sounds. There, arranged taxonomically from ostrich to raven, were the songs of nearly six thousand of the world's nine thousand or so species of birds. 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, laughing kookaburra, black-and-gold cotinga, snowy-headed robin chat, and more. If I looked, I could find the sounds of my childhood, the ok-ka-leee of a red-winged blackbird and the squeaky-gate call of a blue jay. And somewhere, surely, was Keats's nightingale and Shelley's skylark. The room was brimming with sound. But of course I heard nothing. The silence was profound.
This archive of sounds is invaluable. A second set of recordings made from these originals is kept in a vault in a limestone cave for safekeeping. Several recordings hold the voices of birds now extinct. 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.
Today, many ornithologists are listening to bird vocalizations and studying their intricacies in ways that were beyond the grasp of their predecessors only a generation ago. Avian bioacoustics has flourished in just the last few decades, a result of two inventions from the mid-twentieth century: the tape recorder and the audiospectrograph, or sonograph. The latter, which produces a visual representation of sound, allows ornithologists to measure the details of a bird's song as concretely as Darwin measured the beak of a finch. These tools make it possible to look for answers to some ancient questions: Why do birds sing? What do their songs mean?
Bird "songs" — typically an elaborate series of notes, often musical to our ear — are delivered almost exclusively by the male of the species in the breeding season and sung repeatedly for prolonged periods; in contrast, bird "calls" — relatively simple, brief vocalizations — are made by both males and females to influence behavior in particular contexts (nestlings begging for food or geese honking in flight to coordinate the movement of the flock). Naturalists have long recognized that birds in temperate zones begin singing each spring when they are forming pairs, mating, and rearing young, so the common thinking was that the function of a bird's song was to romance a mate. In the 1930s a series of studies by a British naturalist, H. Eliot Howard, confirmed that birds are territorial. It's now known that a release of hormones, triggered by the lengthening of days during spring, spurs male birds to begin singing to announce their presence to other males. The dueling arias, sometimes punctuated with physical skirmishes, establish territorial boundaries. To females in the vicinity, the same songs (in most cases) brim with that most essential lust: the desire to reproduce oneself. Thus, male birds sing both to claim territory and attract a mate.
But the question is larger than this. Why do birds sing? Why have they come to rely on this particular means of communication? One line of thinking connects song with flight. Flying takes a great deal of energy. 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? Furthermore, if the functions of birdsong are only to claim a territory and attract a mate, why do chipping sparrows sing one song but marsh wrens sing fifty or more? If both species are equally successful in defending territories and reproducing, what good are those extra forty-nine songs for the marsh wren? Why do mockingbirds continue learning new songs throughout their lives and imitate the songs of other birds? Why in some species — cardinals, Baltimore orioles, rose-breasted grosbeaks, among others — do females sing as well as males?
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 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. University of Massachusetts professor of biology Don Kroodsma had rented it for the weekend and invited me to join him and several biologist friends for what he called, more seriously than not, a party. With Kroodsma that evening were Jan Ortiz, a naturalist from Amherst; Linda Macaulay, an experienced natural sound recordist; Sylvia Halkin, a professor of biology from Central Connecticut State University; Curtis Marantz, one of Kroodsma's graduate students; and Bruce Byers, who had recently completed a Ph.D. at the University of Massachusetts. Still more of Kroodsma's friends were to arrive the next day.
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. With an efficiency that suggested he was accustomed to getting up at this hour, he quickly loaded up his car with a tape recorder, a parabolic microphone, and a bag filled with cable connectors, batteries, and other electronic gear. We drove off, Kroodsma intent on his mission: to explore the mystery of the song of the black-capped chickadee.
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 call — a whistled fee-bee-ee and a buzzy chick-a-dee-dee-dee respectively — are known by even the most casual observers of birds. Yet, many experienced bird-watchers 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. When I first called him to introduce myself and ask about his work, he quickly invited me to join him on Martha's Vineyard. The mystery of the chickadee's song, he said, was irresistible. I knew the song well, or thought I did, having grown up with it in the Midwest. Of the two whistled notes (fee followed by bee-ee), the first is delivered at a higher pitch than the second, and the second note is interrupted with a slight hesitation. 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." Dialect differences may also occur in two populations only a few hundred yards apart. The northern parula warbler's song rises rapidly in pitch — zeeeeeeeeeeup. In eastern Daniel Boone National Forest of Kentucky, the ending 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 conformity is remarkable. Why, Kroodsma wanted to know, don't chickadees develop dialects like other songbirds? He knew of no other species so widespread in its range for which this was the case. The study of song dialects has been one of the most active areas of research in avian bioacoustics. Enough reports have been written on it to fill several volumes, but few had stopped to consider the opposite phenomenon, the lack of variation in the chickadee's song. "Sometimes you can learn a lot about the rule by looking at an exception to it," Kroodsma said.
To put this mystery in perspective, it is important to understand something of avian taxonomy. There are thirty orders of living birds (according to Frank Gill's widely respected text Ornithology). While some orders, such as the Sphenisciformes (the penguins of the world), are composed of only a few species, others have many. There are 150 species of Anseriformes (ducks, geese, swans), 288 species of Falconiformes (hawks, eagles, etc.), and 340 species of Psittaciformes (parrots, macaws, lories, and cockatoos). The order of Passeriformes, however, dominates the world, accounting for roughly 5,500 of the planet's 9,000 species. The Passeriformes (small land birds with feet adapted for perching) are divided into two suborders: the oscines and suboscines. The latter, which number about 1,000 species, are found primarily in the New World tropics and are represented in North America by only a few species of flycatchers. But the oscines, speaking in evolutionary terms, are the most successful of all avian taxa, having spread worldwide into a splendid profusion of 4,500-plus species that include jays, crows, chickadees, titmice, wrens, nuthatches, warblers, thrushes, vireos, sparrows, blackbirds, orioles, tanagers, and finches — to name some of the more well-known oscines in North America. It is the oscines we refer to when we speak of "songbirds."
Songbirds are special, Kroodsma told me during our first phone conversation. In the world of avian bioacoustics, songbirds are what all the fuss is about. Although nearly all birds use some form of vocal communication, the widespread development of complex, often musical vocalization has occurred only among the songbirds. Why such singing behavior developed in the oscines and not in the closely related suboscines is one of the great unknowns in ornithology. This is only half the story. Something else separates songbirds from suboscines and the other orders. Nearly all other birds, and every other animal on the planet, are born with their vocalizations genetically encoded, which is to say that they would grow to adulthood and vocalize as others of their species do even if they were born deaf. But baby songbirds learn their songs in much the same way children learn to speak. They listen to an adult, then practice what they hear until they can repeat it. So far as we know, no other land animal — not even our closest relatives, the primates — passes on learned vocalizations this way from generation to generation. Of all other animals, only some cetaceans (whales, dolphins, and other marine mammals in the order Cetacea) appear to learn their "songs" — though the process is not well understood.
Learning increases the possibilities for variation. For nearly every songbird species studied, geographic variation — dialects — exists. Thus, 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. The study actually involved a series of experiments performed by Kroodsma and several colleagues in the late 1980s and early 1990s that proved that male black-capped chickadees exposed to a variety of songs and song-learning situations will learn more than one song. Moreover, young chickadees that were isolated from each other developed different dialects.
So the black-capped chickadee was able to learn more than one song and even appeared predisposed to do so. 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. Budney knew that a Martha's Vineyard resident, Dolly Minis, had donated recordings of many of the island's birds to the library, and the recordings spanned more than two decades. "When Greg and I listened to the recordings of chickadees from Martha's Vineyard, we looked up at each other in disbelief. 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 the mainland chickadees. Islands make good outdoor laboratories. Their boundaries are sharply defined, and an island the size of Martha's Vineyard could be investigated virtually in its entirety. More important, if the chickadees on the island were isolated from the mainland chickadees, never intermingling, then differences in song might be connected to evolutionary changes in the two separate populations of the species. The scientific literature on chickadees indicates that they are reluctant to cross open water. Perhaps long ago one chickadee on Martha's Vineyard sang the song backward, and others picked up the variation until generation after generation of the island chickadees had learned the backward island song, never having heard a mainland song.
A year after Kroodsma's visit to the Library of Natural Sounds, he drove 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 not only of 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? Kroodsma's original question — why didn't mainland chickadees have dialects? — led to new questions: Why were there several songs on Martha's Vineyard? How many songs were there altogether? Was there a geographic pattern to them? Kroodsma decided that looking for answers to these questions was a way to approach the mystery of the mainland chickadee's lack of dialects. 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. The night before, he had spread out a map of Martha's Vineyard on the kitchen table and, with his recordist friends gathered around, divvied up the island. Golf courses, city parks, and various public land were accessible in most areas, so it was possible to cover much of the island. Residents were warned of the predawn invasion. The island's newspaper carried an article headlined "He's baaaack." The year before, Kroodsma had alarmed residents as he rode his bicycle past their homes in the gray of first light waving what looked like a gun. The police had tracked him down, but discovered he was armed with nothing but a "shotgun" microphone, a wand-shaped mike with a short handle.
Now Kroodsma parked alongside a dirt road at the edge of the forest and shut off the engine. 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 it from swinging too much as he moved. Small digital recorders were becoming increasingly reliable, and several models of high-fidelity analog machines used cassettes, but Kroodsma preferred the Nagra because it was dependable and generally unaffected by temperature, humidity, and dust. A cable led from the recorder to an eighteen-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 headphones, which rested askew on Kroodsma's head like earmuffs. Kroodsma had tucked his pant legs into his socks (ticks carrying Lyme disease had been reported on the island), and he wore several layers of T-shirts and sweatshirts against the cold morning air of early May, and a glove on his right hand, the hand that would hold the cold grip of the parabolic mike. 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. By my internal clock it was the middle of the night. By any clock on Martha's Vineyard it was the middle of the night. There was not a hint of light in the sky. I had enough experience with birds and bird people to expect to get up early, but didn't realize what early meant to Kroodsma. Kroodsma took it as a source of pride that we were out of the house before the others had gotten out of bed. As we stood in the darkness, barely able to see the trees in front of us, we heard only a whip-poor-will, that insistent voice of the night. I stood quietly, shifting from foot to foot and rubbing my hands to keep warm in the cold night air. Twenty minutes went by without another note of birdsong. Then a towhee sang a few songs — drink-your-teeee — and minutes later we heard a few other birds waking up. Swallows twittered, a woodcock emitted his nasal peent, several mourning doves cooed. Then a crow called from overhead, and a catbird mewed in the underbrush just as first light began to creep into the sky. Kroodsma was listening carefully, ready to swing his microphone in the direction of the first chickadee song.
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. Last year by this time of the morning the chickadees were singing nearly nonstop. Unfortunately, two days ago a cold front had come through Martha's Vineyard and dumped sleet across the countryside. Dolly Minis, the birder who had recorded Vineyard birds for decades, warned Kroodsma that birds on the island had nearly stopped singing. Moments after the second chickadee fell silent, a third bird started up fifty 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. He looked at the ground. We waited. Kroodsma hoped the bird would resume singing, but after ten minutes, he made three scuff marks in the dirt with the toe of his boot so he could later return and listen again to bird number three. Though the chickadees weren't singing much, they had most likely established territories by now and so this bird would probably remain here. We walked on. The sun was up. A few crows were calling, but the forest was otherwise silent. It was 6:15 A.M. and forty-eight degrees, details that would become part of the data Kroodsma included with his recording of the bird that sang sosweetie-sweetie.
For a short while we walked slowly down a lane along the edge of the forest, then retraced our steps. Back at the car, Kroodsma stood quietly for a moment. Perhaps the chickadees were more vocal elsewhere on the island, he said. We got back in the car and drove a few miles east to a Massachusetts Audubon Society preserve near Felix Neck. Here, the forest was hardwoods. Leaves were beginning to bud on some trees, but most were still completely bare. The scene, absent of color, looked like a pencil sketch. We walked a couple of hundred yards down a trail, then Kroodsma stopped at the edge of a clearing. "It's time to cheat a little," he said, taking out a tape recorder small enough to hold in the palm of his hand. In this recorder he had a tape of a chickadee's song. Seconds after Kroodsma played a few songs, four chickadees appeared in the bare branches of saplings a few feet away. The birds flitted about, agitated and curious, looking for their rival. For several minutes the only sound was the buzzy chick-a-dee-dee-dee the birds are named for. Then one bird came particularly close and sang one sweetie hey, the reversed chickadee song Kroodsma expected. Moments later, all the birds were gone. Chickadees are gregarious, and despite the cold weather it was a bit odd that the birds we'd come across this morning were so reticent. We walked for another half an hour, listening, waiting for chickadees to announce their presence, but with no luck. This first morning of recording was beginning to look bleak.
"Let's go see how Jan is doing," Kroodsma said. Jan Ortiz, the naturalist from Amherst, was recording at a town park in Edgartown on the eastern side of Martha's Vineyard. Kroodsma hoped the birds she recorded were singing sosweetie-sweetie since that was the song he'd recorded there last year. When we arrived, Ortiz was standing at the edge of the park, her microphone pointed at the ground. Not a good sign. Not many birds were singing here either, she told Kroodsma, but the birds she had recorded were singing sosweetie-sweetie. As we stood talking, Kroodsma rewound the tape on the Nagra. As he did, a strange noise came from the recorder. Opening the cover, he found several feet of tape bunched and tangled where it had slipped off a reel. Given the disappointing results so far this morning, one would expect some frustration to surface, but Kroodsma's face registered little reaction. Jan Ortiz, whose mouth dropped open, seemed more upset. Kroodsma stared at the tangled tape. "Well, I don't think this recorded anything," he said. He slowly rewound the tape by hand. When he was done, he played the reel from the beginning, discovering that he'd only lost a bit of the last recording. A chickadee, hearing the songs, flew into a nearby tree and sang in response. To me, its song sounded like sweetie-sweetie. "Hear that?" Kroodsma said. "There's a break in the first note. This is sosweetie-sweetie." I shook my head no. The bird sang again, and still I didn't hear the break in the first sweetie. It was becoming clearer by the hour that I had never listened closely to birdsong. Kroodsma pointed to the sand at our feet and with the toe of his boot drew what looked roughly like three waves to represent how this bird's song would appear on a sonogram. Seeing the slight break in the first note somehow made it easier for me to listen for it. When the bird came closer yet and sang again, I could now hear what Kroodsma had drawn. Sosweetie-sweetie, the chickadee sang.
Moments later, the bird was gone. Kroodsma looked at the cloudy sky and decided it was time for a break. We climbed into the car and ate granola. It was still early, a bit past 9 A.M., but Kroodsma was beginning to accept that he wasn't going to record many chickadees this morning. He consoled himself with the weather forcast, which predicted a warming trend over the next three days. Still, he wasn't ready to end the morning. Kroodsma decided to experiment with a mist net to see how hard it was to capture a Martha's Vineyard chickadee. A mist net, widely used in ornithological fieldwork, looks something like an oversize badminton net when strung between two poles. The fine, black thread blends in with the background, making it nearly invisible. A bird flies into it and gets its feet tangled in the mesh. Frank Gill, who was arriving the next day, had a permit to capture six chickadees and draw their blood to compare the DNA with those on the mainland. Since the island chickadees were singing differently from their mainland counterparts, they might have significant differences in genetic makeup, particularly if they were isolated from the mainland population. Kroodsma had the net up within a few minutes and had placed his small tape recorder near the base of the net. He stood back as the recorder played chickadee songs. Any male chickadee in the vicinity, hearing these songs, would likely come investigate what he took to be a rival threatening to take over his territory.
In less than a minute a chickadee flew in, landed in one of the saplings, and sat quietly. It flew once from the branch toward the net, but returned within seconds. When the recorder ran out of songs, Kroodsma approached it and rewound the tape to play them again. The chickadee, losing what little interest it appeared to have, disappeared. Nothing was going well. Kroodsma sighed. "In this business it's important to know when you're licked," he said. With that he dismantled the net and loaded everything back into the car. It was a few minutes past 10 A.M. Kroodsma had already spent six hours trying to record chickadees. He had only a few songs on tape, and the best part of the day was over. Given the weather — it was still only in the upper forties — it wasn't likely that Kroodsma would get any more recordings from the laconic chickadees today.
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. They'll sing the song several times on one frequency, then go up the scale a bit and sing more renditions on the new frequency for a while, then switch again to another frequency. 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. Basically we know a lot and we don't know a whole lot."
Back at the house, Kroodsma placed the sonograph he'd brought with him on a folding table and connected his tape recorder to it. He was anxious to see what this morning's songs looked like. The sonograms would reinforce what he felt he already knew about the songs' structure and their frequency. After more than thirty years of listening to birds, Kroodsma could not only hear the most subtle elements in a bird's vocalization, but could hold in his memory the slight differences in song between two birds, or two songs of one bird. As a scientist, though, he would not rely on this. The sonograph supplied objective data. It could produce a visual image on a computer screen in real time so that we could see the sonogram as we heard the song. As soon as I saw and heard a few songs simultaneously, I began to understand what Kroodsma had told me earlier — how important sonograms were to the study of birdsong. The song itself was fleeting, difficult to hold in memory, but I could keep an image in my mind, and that somehow helped me hold the song too. There is nothing original in the observation that we have lost much of our sense of hearing, as well as smell, in favor of our eyesight, but it had never before been so clear. Kroodsma, like a musician, had simply trained his hearing to be as acute as his eyesight.
By the time he began printing out sonograms, the others had returned and were huddled around the table. In the western parts of the island, Bruce Byers recorded birds singing sweetie-sweetie, he told Kroodsma, but a few sang sweetie hey. Linda Macaulay recorded in the west as well, with similar results. Moving eastward to the west-central area of the island, Sylvia Halkin recorded birds singing sweetie hey. In the northeast, Kroodsma's graduate student, Curtis Marantz, found birds singing sweetie-sweetie and sosweetie-sweetie, the same as Jan Ortiz's birds in the east. A pattern was beginning to appear, though there weren't as yet any defined boundaries, and some songs occurred in overlapping areas. Kroodsma was puzzled, but it was too early to think about results and conclusions. What he most wanted to do at the moment was just look at sonograms. They would conclusively show the structure of the songs. It was always possible for someone to believe they heard one thing, when in fact they heard something else.
For an hour, Kroodsma produced sonograms of the songs he and the others had recorded, and he reluctantly stopped only when everyone else wandered into the kitchen to make sandwiches. There would be no more recording today, so there was plenty of time during the afternoon and evening to look at sonograms and discuss the results, and with that in mind everyone headed off to take naps. When I got up ninety minutes later, Kroodsma was back in the living room producing sonograms. "I couldn't sleep," he said. "I had to go through these tapes while everything was fresh in my mind, while I could still remember what each bird was doing." Not long ago, as part of a study on the song repertoires of sedge wrens, Kroodsma spent four days of a visit to the Falkland Islands recording one individual sedge wren. When he was done, he had ten thousand songs on tape. He is, he admits, "a bit obsessive."
During the afternoon and evening of that first day, Kroodsma continued to produce sonograms. The following day, a Saturday, the weather improved and the chickadees sang far more than they had the first day. Everyone came back with good recordings, including some long samples from individual birds. In the afternoon, I had to leave, but I didn't miss much on the final two days. Both days were cold and windy, worse than the first day of recording, and generally unproductive. During the weekend, six birds were captured and had their blood drawn, which Frank Gill would take with him for a DNA analysis. When the party was over, Kroodsma had nearly two hundred representative samples of the chickadee songs from various parts of the island. The next month, he began analyzing the songs with the help of three students at the University of Massachusetts. He also recorded chickadees in the Amherst area, getting long samplings of several different birds so that he could determine how many different frequencies the mainland chickadees sang hey sweetie on. In the fall he sent out an update to all the participants in the project, noting that he had a big box on his desk with analyses of all the chickadee songs from Martha's Vineyard. Furthermore, he had plotted the songs of 199 of the representative songs on a map of Martha's Vineyard. From this, he discussed what he called an "oversimplified" distribution for the songs, which matched what he had expected. He had now settled firmly on the three basic songs — sweetie hey, sweetie-sweetie, and sosweetie-sweetie, tossing out a scattering of unusual songs that only one or two birds sang. Since each of the three songs was sung on both a high and a low frequency, he plotted the high-frequency and low-frequency songs separately as if they were different songs altogether. Kroodsma wasn't satisfied. He wanted more songs to strengthen the data, and now that he had thought about it, he wanted songs from the neighboring islands of Chappaquiddick and Nantucket too. Kroodsma made plans to return to Martha's Vineyard in May of 1996, and several of the participants agreed to do the same.
By the following summer, Kroodsma had most of what he'd hoped to get, including recordings of birds from Chappaquiddick and Nantucket, both of which provided surprises. Although only a short stretch of water separates Chappaquiddick from the eastern tip of Martha's Vineyard, the birds sang differently on the smaller island. Most sang a monotonal version of the mainland song, hey sweetie. This didn't match anything the birds were doing on Martha's Vineyard. Nantucket, the most isolated of the islands, held birds that sang the typical mainland hey sweetie as well as a considerable hodgepodge of other songs. There seemed to be more variation on Nantucket than on Martha's Vineyard, though the latter was twice the size.
Kroodsma spent eighteen months analyzing the new data and writing up the study. In 1999, he published "Geographic Variation in Black-capped Chickadee Songs and Singing Behavior." He had seventeen coauthors, an unusually high number even for the sciences, where papers are frequently coauthored. 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 song. In addition, the report contains the results of the DNA analysis, which showed no substantial difference in the genetic makeup of the island and mainland birds. 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, among them: "From whom, where, and how do young males learn their songs?...What is the fate of the diverse songs on the offshore islands? How dynamic are their distributions? Will some dialects become extinct and others succeed?"
I had two reactions when I read the study. I thought of the cold mornings and the reluctant chickadees and also the dozen people gathered at the house on Gay Head looking over Kroodsma's shoulder as he reviewed sonograms. None of that was in "Geographic Variation in Black-capped Chickadee Songs and Singing Behavior." The prose of science is dry and to the point. The report was about methods and results, not the miscellaneous enterprise of the fieldwork itself. But the fieldwork was the science, I thought. Science was stirring as Kroodsma drove through the predawn darkness at 4 A.M. to a state forest where chickadees might or might not be singing. There was science in the air when he stood listening for chickadees and heard only wind in the pines. And science was tangible when Kroodsma marked the dirt with the toe of his boot at the spot where he recorded the first chickadee.
Most of all, I thought the results of the chickadee study seemed messy. The distribution of the different songs on Martha's Vineyard didn't present a clear pattern. I remembered another research project Kroodsma once told me about. A few years ago, he was interested in why two closely related wrens — the sedge wren and the marsh wren — had such different singing behaviors. Western marsh wrens sing as many as 150 different songs, and neighboring birds share many of the same songs and know each other's singing habits. They engage in what is known as matched countersinging, elaborate displays of one bird matching another's series of songs note for note. Sedge wrens, similar in many ways to their near cousins, do not share songs with their neighbors. Kroodsma theorized that countersinging suits the social fabric of the marsh wrens because they live in stable communities where birds remain neighbors from year to year. Sedge wrens, by contrast, are usually nomadic, moving to a different location each time they breed (typically twice during each breeding season). But what if sedge wrens were sedentary? Would they behave like marsh wrens? It seemed like a purely hypothetical question. "You could clothe and feed sedge wrens and make them comfortable so they would stay in one place for a million years, and that experiment might give you the answer," Kroodsma told me. "But the experiment had already been done for me. In the Falkland Islands, and in places in Costa Rica and Brazil, sedge wrens are not nomadic. I postulated that sedge wrens in those places would behave as marsh wrens do, developing repertoires of identical songs. I went there and found out I was right. It was a jaw-dropping experience!"
The chickadee study offered no such dramatic moment. But Kroodsma seemed happy regardless. He was enthusiastic when I called to ask him about it: "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. 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. So, messy? Yeah. But I guess I look at it and don't see the mess so much as the cultural transmission of the different songs. The low-frequency sweetie hey was successful, and the high-frequency sweetie-sweetie was pretty successful too."
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, when the forests had been logged, 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. It doesn't take much for dialects to develop in typical songbirds. Any kind of barrier can create dialects, just as long as the birds are far enough apart that they can't hear each other singing. Here's a very simple scenario. We see three different songs, so imagine there were three different pockets of forest that were isolated from each other during the time the island had very few trees on it. The three different dialects developed apart from each other in those pockets, eventually becoming distinctly different songs. When the forest regenerated, the birds from these areas came into contact with each other, and the low-frequency sweetie hey dialect swept the island and became the dominant dialect. There's nothing uniform about the distribution of the songs on the island now, but this is how it may have occurred."
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 saw on the islands," he said, "especially on Chappaquiddick, where the birds had a number of songs per individual, together with that earlier study in the lab showing that chickadees would develop three or four songs, made the mainland birds all the more interesting. 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. It was a human definition after all, not a chickadee's. 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. "I suppose that good research introduces more questions than it answers," he said. "So in that sense, maybe the wren study was a bust, and the chickadee project was something we should run up the flagpole."
Copyright © 2005 by Don Stap