Have you ever lain awake at night wondering how, when, and why birds sing? If so, Bird Song may be the book for you. But no one should imagine that it is about the charming and euphonious (or sometimes cacophonous) things birds say to one another. Catchpole and Slater present a competent and logically organized synthesis of the vast scientific literature on bird song, concentrating almost entirely on experimental investigations by researchers. “Bird song” is defined here in the narrowest sense as “long, complex vocalizations produced by males in the breeding season.” All avian utterances that do not conform to this narrow definition, and there are many, are excluded from consideration. The text has citations to nearly seven hundred scientific sources, and its dry prose makes liberal use of technical jargon. It begins with a concise review of the neurophysiology of sound production and sound reception in birds, discusses how songs are learned, and concludes with an inquiry into the evolutionary function of song and its variability in nature. It tells, in fact, a story of remarkable scientific advances.
Surprisingly little was known about the functional and acoustic complexity of bird song until the second half of this century. Although birds are eminently accessible to study, being found even in the centers of our busiest cities, rigorous scientific inquiry into their vocalizations awaited the invention of high-fidelity tape recorders and speakers. These are the basic tools that both field and laboratory workers use to record bird sounds and later to play them back to the birds used in tests. Since the earliest playback experiments, conducted in the 1950s, continuing technological advances in both sound and information processing have stimulated ever deeper inquiries into the structure of song and its function.
Birds have been the main object of studies of animal vocal communication because they are both ubiquitous and relatively cheap and easy to maintain in captivity. The birds under discussion here are members of the great “passerine” order, comprising nearly half of all birds, and distinguished anatomically from non-passerine birds by their grasping feet, with the first toe turned backward. But birds are by no means the only animals to “sing.” Myriad frogs and insects produce sounds to attract mates or to proclaim territories. There are even singing mammals, including a number of monkeys, the South American bamboo rat, and, of course, whales.
Birds produce sound in a structure called the syrinx, located deep in the throat at the juncture of the bronchi, the two main branches of the windpipe. A membrane on each side regulates the passage of air. It was first thought that sound emanated from vibrations of the membranes, but it is now acknowledged that the source is air turbulence, produced much in the manner of a whistle. The truly remarkable feature of sound production by birds is that the two sides of the syrinx can act independently, so that some birds are literally able to produce a chord. The song of the common cat-bird, for example, consists of a sequence of phrases, some of which arise on the left side of the syrinx, some on the right, and some on both sides working in concert. It took some very clever experimentation to discover this. Tiny thermistors sensitive to pressure had to be implanted in the middle of each bronchial passage and monitored simultaneously on a time scale of milliseconds.
Tones, once produced, are then modified by changes in the length and shape of the vocal tract, just as an opera singer is able to vary the quality of sound from mellow to edgy by relaxing or tightening the throat, and varying the size and shape of the cheeks and mouth (the “buccal cavity”). Birds perform analogous operations by lengthening and shortening the neck and opening and closing the bill. The highly coordinated activity that generates song does not, however, originate in the syrinx itself, but in as many as nine neural centers in the brain. Separate but interconnected neural pathways are involved in producing sounds and in learning to make sounds.
With many songbirds the males sing and the females don’t. Such marked asymmetry of behavior led Rocke-feller University scientist Fernando Nottebohm and others to examine the avian brain, a significant portion of which is devoted to sound production and processing. Nottebohm’s investigations led to the first demonstration of a distinct difference in vertebrate brains between males and females of the same species; the neurological centers for sound production were revealed to be three times as large in male canaries as in females. Since Nottebohm published his results, individual differences in the structure of human brains (as between males and females or homosexuals and heterosexuals) have become one of the hottest topics in human neurobiology.
Nottebohm’s studies of the neurological basis of song in the canary yielded another major discovery, one that may also have important medical repercussions. Song in canaries is seasonal, as it is for most other birds that breed in the temperate regions of the world. It has long been known that seasonal variations in hormone levels, mediated by the changing length of days, were associated with differences in vocal activity and reproductive behavior. What had not been appreciated was the extent to which the regulation of seasonal behavior was reflected in the structure of the brain itself. It seemed reasonable to think that varying hormonal levels could have an effect on preexisting neurological circuits so as to turn on and off seasonally appropriate behavior.
Instead, Nottebohm discovered that the principal center for song production in the canary brain dramatically increases in size as the birds enter the spring breeding period. Precise analysis of brain tissue with radioactive tracers provided evidence that enlargement of the song center was accompanied by cell divisions and an increase in the number of neurons making up the center. Until then, it had been accepted among neurobiologists that new neurons could not form in the adults of higher vertebrates. Nottebohm’s discovery has led to a flurry of new research into the factors that regulate the proliferation of neurons in brains and other neurological structures.
For a bird to sing, it has to know what to sing. There are two ways that birds acquire such knowledge, and the distinction divides songbirds of the passerine order into two major divisions. In the so-called suboscines, including particularly the New World flycatchers, song is genetically encoded, so that birds raised in acoustic isolation are capable, when mature, of emitting the characteristic vocalizations of the species. Among the oscines (including all other songbirds, such as warblers, finches, wrens, and many more), song must be learned. Auditorily deprived birds are only able to generate disorganized sounds or, at most, slurred and garbled versions of the typical song.
Much research has been invested in studying how oscine birds learn to sing. When do they learn, and what instructs them? Although the precise answers to these questions vary with the species, one universal finding is that birds are not indiscriminate learners. Birds exposed to inappropriate sounds, including the songs of closely related species, do not learn how to sing. Nor do they imitate irrelevant sounds. They must hear members of their own species, which shows that song learning involves a complex interaction between experience and instinct. Instinct guides what is and what isn’t learned, but in oscine birds instinct alone does not suffice to pass on the appropriate behavior.
Most species have a “learning window,” a time in their first year of life when they acquire the necessary experience. Often this takes place during the fledgling and post-fledgling months when the birds themselves are silent. The details of the song are memorized, forming an “auditory template” that then serves as a basic reference the following spring when rising hormone levels induce the first attempts at song.
The first utterances, termed subsong, may come months after the crucial learning experience. Subsong tends to be weak and variable, often containing only elements or rudiments of the final product. With practice, the efforts become more structured and less variable, until “crystallization” occurs. By this time the bird is singing in full voice, and producing a highly structured and precisely repeated rendition of the song typical of its species.
Learning may or may not continue after crystallization of the song. Some species gradually develop elaborate repertoires of dozens or even hundreds of variants of a basic song, in which elements are added, deleted, or recombined. A familiar example is the mockingbird, renowned for its ability to imitate other species. Mockingbirds borrow bits and pieces of the songs of many other species, and weave them into an exuberant and highly varied outpouring. But mockingbirds living in urban or agricultural environments, where there are few models to imitate, sing relatively barren and uninteresting songs.
The world record holder for the largest variety of songs may be the brown thrasher, a relative of the mockingbird, whose song consists of repeated phrases, A-A, B-B, C-C, etc. What is remarkable is that some brown thrashers have been found to include well over one thousand different phrases in their repertoires. In variability and individual flavor, the songs of most passerine birds fall somewhere in between the highly stereotyped utterances of suboscines and the nearly limitless versatility of the brown thrasher.
Why mimics such as the mocking-bird of North America, the lyrebird of Australia, and the nightingale of Europe imitate other species is still an unanswered question. But the related question of why individual birds elaborate their own distinctive variants of the species’s basic song has received a great deal of attention, and some clear answers have emerged.
The basic answer is that songs serve multiple purposes. But this now widely accepted conclusion was a long time in coming. Romantics and poets held that birds sang for the sheer joy of being alive, or in rapture over the long-awaited arrival of spring. But Darwin, in keeping with his theory of natural selection, proposed the more practical view that males sing to attract females. It took fifty years before E. H. Howard put forward, in 1920, the alternative theory that male birds sing to advertise their dominion over a patch of space, a territory. The two views are really quite different but in practice they are difficult to disentangle. In the first case, males direct their exertions at females; in the second, at rival males. Which is it? In reality, it is both—and more.
It has taken decades of experiment to show that there is no simple answer to the question “What is the function of bird song?” Birds have several things to communicate—their species, territory location, sex, marital status (whether paired or not), and even individual identity. All this information can be, and often is, coded into the songs we are so pleased to hear each spring.
Early experiments were directed to supporting or refuting the hypotheses of Darwin or Howard. Redwing blackbirds that had been experimentally muted by puncturing the interclavicular air sac were vulnerable to increased rates of territorial incursion by other males; they engaged in more fights, and, in some cases, lost their territories. Muted birds partly compensated by resorting more to visual displays of their red epaulets. When the muted birds recovered their ability to sing, moreover, they rapidly regained lost territory, clearly demonstrating that song was effective in competition with neighboring males.