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A Natural History
by Don Stap
Scribners, 2005, Hardcover
Following Donald Kroodsma, one of the world's experts on birdsong, from the woods of Martha's Vineyard to the tropical forests of Central America, Don Stap brings to life the quest to unravel an ancient mystery: Why do birds sing and what do their songs mean?
Praise for Birdsong
"Birdsong is a wonderful journey through the science, history, and biology of how and why birds sing. We highly recommend it for all bird lovers, nature lovers, and science enthusiasts." -- Don and Lillian Stokes, authors of Stokes Field Guide to Birds
From Chapter One
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?
. . .
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.
. . .
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.
. . .
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.
. . .
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."
Don Stap's first work of nonfiction was A Parrot Without a Name. Currently professor of English at the University of Central Florida, he is a frequent contributor to Audubon magazine and has also written for Smithsonian, Travel & Leisure, and The New York Times.
Three recent books about bird song