Sounds Wild and Broken, page 16
Then, south to Oregon. Eee-diddle-buzz-tew. Another remix. The buzz comes earlier in the song and a quick sweep is added at the end. Other Oregon birds stick with the scheme but add more sweeps. A little north, near Seattle, they throw in a second buzz and some inflected sweeps, more ornamented than the Oregonians.
The white-crowned sparrow breeds all across the northernmost part of North America, in shrubby habitat at the edge of the boreal forest and tundra, and also farther south, in the mix of low vegetation and grasses in the western mountains and all along the Pacific coast. This is a vast range, roughly three million square kilometers, home to about eighty million individuals of the species. The diversity of the sparrows’ songs reveals some of the complexity of their lives, layers and textures within the multitude.
As I listen to sonic memories from my travels and the electronic gifts left by others from their wanderings, I sense that the diversity of human sound, so rich within our cultures and individual lives, is just one manifestation of sound’s creative workings within animal species.
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Winter is migratory sparrow season in the southern United States, bringing a taste of the tundra and the boreal forest to fields and gardens in Tennessee. In the brushy edges of fallow cotton and cornfields, white-crowned sparrows glean the remnants of last summer’s grass and herb seeds and pluck insects from the soil. These are migrant birds, here only for the darker months before they return to their breeding sites in the North. Their relatives, white-throated sparrows, also winter here, distinguishable by sight with their white bib, yellow daub above the eye, and head stripes that are less crisp than those on the white-crowned sparrow. White-crowns prefer fields, white-throats the denser vegetation of forest edges and rural gardens. These preferences mirror their breeding habitats: open, scrubby land, including treeless areas north of the Arctic Circle for white-crowns, and boreal thickets, swamps, and forest edges for white-throats. Winter in Tennessee is usually a half-hearted business, staying mostly above freezing, and so insect life seldom completely stills. The first flowers of henbit and bitter cress emerge in fields in late February, and edible seeds soon follow. For hardy northern birds, this is easy living.
As the days lengthen, song begins. Light penetrates the birds’ skulls, bathing receptors buried in the brain. Aglow, the receptors steep the blood in hormones and signal brain nodes to spark the lungs and syrinx. Birds feel springtime’s surging vigor, lift their heads, and sing. In winter, the sparrows communicate through at least nine different short calls, each suited to a different context—pink when perched alone or flying, brief trills when meeting other birds, rasps when chasing one another—and only occasionally with a burst of song. In spring, for males especially, song production surges.
The song that emerges from the sparrows’ beaks can be an amusing delight. I stop digging in the garden or pause my steps on a rural road and smile. Young sparrows are practicing their songs and, like the charming tumble of human infant voices, their jumbled experiments evoke a sense of newness and play.
A young white-crowned sparrow gives two whistles, like the introductory note of an adult, but each one wavers, seemingly unable to hold steady. Another bird gives a single whistle, also wobbly, then three rough sweeps. Te-e-rew, instead of the adults’ tew. A third has a steady opening whistle, then five sweeps, clear at first, then breaking up into stammers. The birds repeat their phrases, shifting the timing a little, pausing after the whistles or curtailing the concluding sweep notes. The voice of each bird is instantly recognizable as a white-crowned sparrow but, compared with adult songs, these youngsters’ sounds are disordered in their arrangement of elements, unsteady in tone, and inconsistent from one repeat to another.
The same muddle and teeter come from young white-throated sparrows. Their adult song is a ringing series of clear tones, two long, then three broken into triplets, ohhh-sweeeet-canada-canada. Usually the first note is lower, but some birds start high and work down. The second note varies from steady to slightly stuttered. The species nests in the northern forests of eastern North America and winters all across the southern states. This wide geographic range and the tonal purity of its song make the species one of the best-known avian singers in the region, a sonic mark of the end of winter in the South and the start of summer in the North.
Can-a cana ca. In their first spring, young white-throated sparrows sing shuffled and unsteady versions of the adult song. Their hesitations, innovations, and errors are strongly reminiscent of the babble of human infants. O-sw-swee. Sweet-cana. I hear learning, play, experimentation, and maturation. I delight in these sounds because they are the marks of present well-being and future possibility. Ohhh-swee-ee-eet.
In Tennessee, we hear only the later stages of the young sparrows’ vocal development. While they are still on the northern breeding grounds, shortly after leaving the nest, they give highly fragmented whispers, barely audible even at close range. A trance overcomes the birds as they murmur. Their eyes droop and bodies slump, as if in deep sleep. In this state they may be blissed out on the feel-good hormone oxytocin, a chemical that has been shown to motivate and regulate vocal learning in both birds and mammals. Over months, these first stirrings of song get louder and more organized, and the trance fades from their lives, perhaps briefly reappearing during adult slumber as a sweet memory of childhood.
Like humans, sparrows learn the form of their vocalizations by listening to others. Their songs travel through the generations not as coded strands of DNA but through the attentive ears of young birds listening to the songs of their elders. Sparrows in the Rocky Mountains sound different from those in California primarily not because their genes have evolved different songs, but because the form of the song passed down by learning has diverged.
Social learning of vocalization is rare among animals. In many insects, adults have stopped singing or are long dead before their offspring mature. In other species—fish that spawn in open water or insects that lay their eggs in soil—the young develop away from where adults sing. But even among species whose generations overlap, sound is mostly shaped by genes. Toadfish hatch and spend their first weeks in their fathers’ nests, enveloped in his croaking sounds, yet toadfish eggs raised in a lab without a dad grow into adults with normal songs. A cockerel raised with no teachers crows just as well as one raised with adults nearby. Captive flycatchers exposed by human experimenters only to the songs of other species give perfect renditions of their own, unheard species’ song. They do the same even if their captors puncture the birds’ eardrums. Deaf squirrel monkeys also vocalize normally. Periodic cicadas sing without tutelage, seventeen years after their parents last filled the air with song, an extreme example of the genetic inheritance that characterizes all known sound making in insects. Some species can learn to distinguish the songs of others—frogs, for example, identify rivals by ear, and primates are experts at learning the meanings of sounds—but few learn their own sounds by listening and imitation.
All the exceptions known to date are birds and mammals. Hummingbirds, parrots, and some songbirds learn their songs. These branches of the bird family tree are separated from one another by tens of millions of years, and so represent three separate inventions of vocal learning. Among the majority of mammal species, social learning is focused on predator avoidance, foraging, mediating social dynamics, and mate choice. Sound making in these species is mostly innate, although many species learn to modify their use of these inborn calls in different social contexts. The exceptions include bats, elephants, some seals, whales, and one great ape: humans. Our close relatives, chimpanzees, bonobos, and gorillas, have sophisticated cultures, but these are not founded on vocal communication. These groups of vocal learning mammals are not closely related, and each group likely independently evolved vocal learning. Because birds are easier to study in the field and manipulate in the laboratory than whales, elephants, and seals, it is from species like the white-crowned sparrow that we have learned the most about vocal learning in nonhuman animals.
It is a puzzle why many birds and a few mammals are champion vocal learners, but their close kin and most other animal species communicate with mostly innate, unlearned sounds, even among species that have extensive abilities to learn other behaviors. It is possible that learning is only favored when the information imparted by vocalization varies considerably from one generation to the next, and that this is only true in a few species with complex social networks. In such situations—sounds revealing the identity of individuals and the ever-shifting nature of clans and other social groups—learned vocalizations might allow animals to more effectively navigate social dynamics. Among other species, where sounds’ meanings are relatively fixed—territorial songs, calls to signal the discovery of food, alarm at the sight of a predator—vocal learning offers no advantage and may instead merely impose a costly delay as youngsters learn.
The kinship I feel with the youthful palaver of birds is therefore a bond of analogy, and not of direct shared ancestry. Differences in the details of how learning works in birds and humans underscore the divergent evolutionary paths that we took to arrive at vocal learning. Yet there are also some surprising parallels, a unity of process despite our divergent histories.
The birds I hear in the gardens and fields are experimenting with sounds that they first heard last summer, more than six months ago. As nestlings and recent fledglings, they listened to the songs of parents and neighbors. They sang only in their whisper trances, although they also made squalling begging sounds and diverse forms of cheep notes and trills. The memories of the adult songs they heard last summer now serve as a standard against which to judge their own attempts. Over weeks, the birds try different combinations, converging on a final version, the song that they will henceforth use as their own. This is a feat of memory alien to how humans learn sound. We hear and vocalize in the moment, a back-and-forth that refines sound as it is made, although infants also comprehend many sounds before they are able to repeat them.
Listen to human parents and their toddlers: The child makes a cute attempt. Parents smile and repeat in adult form. The child insists. The parent repeats. A duet that, over months and years, eases the sounds of infant speech into adult form. For sparrows, hearing and vocal production are largely separated in time and space. A song heard in June in northern Quebec lives through the winter in a young sparrow’s brain, meeting the bird’s hesitant attempts to vocalize later that year in Tennessee. Months-long memory is the sparrow’s primary teacher as it matures its song.
Among white-crowned sparrows, there are exceptions to this wide separation of hearing and singing. On the California coast, the sparrows do not migrate but instead live in dense, stable communities where they defend territories year-round. In these populations, a young sparrow setting up its first territory will learn the songs of neighbors, aligning its song to those of this new home rather than to those it heard where it hatched. This extension of learning into early adulthood is common among songbirds that live in permanent settlements and allows young birds to more closely fit into the acoustic milieu of the neighborhood. Neighbors often negotiate territorial disputes by matching phrase to phrase in back-and-forth singing contests that seem to boast of each bird’s local knowledge of song variants. If you sound different, you cannot compete.
In all vocal learners, genes guide the process indirectly, producing brains eager to and able to learn, and predisposing each species to the sounds of its own kind. These predispositions are activated by social connection. Sparrows isolated in the lab and fed sound via speakers learn their songs, but they can do so only for a few weeks after hatching. Sparrows embedded in the rich social life of a flock keep listening and learning for months.
Testosterone shuts down the learning process. In the sparrows’ first springtime, the percolation of the hormone through their blood causes the ebullient experimentation of youth to solidify into a final adult song. Artificially removing testosterone, through either physical castration or chemical neutralization, extends the learning period. Testosterone-fueled territorial display is a yoke whose weight seems to expunge creativity.
Individual white-crowned and white-throated sparrows sing only one song variant, repeated tens of thousands of times throughout their lives. These repeats differ somewhat in the emphasis placed on different parts of the song and are embedded in a repertoire of call notes that also vary by context, but the basic form of the song is, to human ways of classifying sounds, singular. This consistency helps with communication. Every bird knows all their neighbors’ sounds. If everyone is singing from their patches, then all is well. If an unknown song pipes up or if a familiar song comes from a different territory, then the birds rise in an aggressive fury.
Other bird species learn not just one but multiple song variants. Song sparrows, common suburban and rural birds across North America, sing eight to ten different variants of their jaunty song of accented notes and trills, repeating each variant several times before switching to another. Every bird has its own repertoire. By listening carefully, we humans can build a sparrow-sound map of the neighborhood, drawn in air by the ephemeral ink of birdsong. Their repertoires are just rich enough to challenge human memory. From one spot in a garden in Tennessee I can hear five males and about forty song variants, a delight as I try to notice and keep each songster’s collection in mind.
The brown thrasher, though, defeats human ears. Each singer has up to two thousand phrases in its quiver. These it shoots out for hours in volleyed pairs. The thrasher also croons a softer, whispered song when close to its mate or newly fledged young. Some of the sonic variants are mimicked versions of other species, suggesting that learning continues throughout life, but most are the birds’ creations. Lifelong vocal learning is well known among parrots and starlings. Such flexibility presumably helps to mediate complex social lives among long-lived species, but despite centuries of imposing human words on caged birds, we know little about the many nuances and meanings of these birds’ learned sounds in the wild.
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Social learning—animals listening and observing others, then using this knowledge to shape their own behavior—is the gateway to culture. Genetic inheritance passes from parents to offspring at a tempo set by the length of generations in each species, the time it takes for an embryo to develop into a reproductive adult. Cultural inheritance can flow in directions unconstrained by kinship, and its speed is limited only by the time it takes animals to notice and copy one another. Animal species that learn their sounds, then, have opened new creative possibilities for sonic elaboration, refinement, and diversification, free from the rigidity and sluggishness of genetic inheritance.
When a white-crowned sparrow settles on its final adult song, it does not produce an exact replica of what it heard from its elders. Rather, the bird finds a song that fits within the norms of the neighborhood but also bears its own mark, perhaps a distinctive inflection or opening frequency. This balance between individuality and conformity is essential to the function of the sparrow’s songs. A song that diverges from local custom is unattractive to potential mates and a weak counter to territorial rivals. But complete mimicry of another bird invites confusion within the social order.
Changes, even minor ones, to any inheritance open the door to evolution. In genetic evolution, changes are introduced through mutations and the reshuffling of DNA in the coordinated dances of sexual cell division and union. These genetic variants then rise or fall in abundance within a population, either by chance or by Darwinian selection. When white-crowned sparrows listen, remember, then re-create songs that are not-quite-faithful replicas of what they heard, the birds fuel cultural evolution.
The speed of any cultural change depends on how conformist or innovative the learning is. White-crowned sparrows can be traditionalists. In some of the year-round resident populations in coastal California, the birds have been singing the same kinds of songs for at least sixty years. If current rates of cultural change hold into the future, some of the song variants of North American swamp sparrows will last for hundreds of years. In contrast, the songs of indigo buntings, birds of forest edges in the eastern United States, are more labile, always on the move. Indigo buntings sing by cycling through a repertoire of about six different song types. Young males establishing territories pick up song types from established local males, then sing them back to these older birds. But the newcomers also add new embellishments to their songs. Year by year, these novelties add up as the old guard dies off and new cohorts arrive. In just ten years, the song types in any given place are entirely replaced. The speed of cultural change is faster yet among yellow-rumped caciques in Panama. These garrulous, black-and-sulfur birds with daggerlike ivory beaks nest in colonies, dozens of nests in a single tree. Within a colony, birds sing from a repertoire of five to eight songs, copying from one another but also inventing novel variants. Three-quarters of the song varieties that were popular at the start of the breeding season are gone within a year. Yellow-rumped caciques invent their own sounds—whistles, clanks, and toots—and also mimic the sounds of frogs, insects, and other bird species. Rapid cultural evolution in this species is driven by minds that listen carefully to social context, copying songs of colony members and the many voices of the surroundings. Careful listening is then fuel for sonic innovation.
Vocal learning not only allows sound to change over time, independent of changes in DNA, it also creates geographic diversity. Every colony of yellow-rumped caciques has its own collection of sound, actively curated by the tastes of its members. Colony members mediate their alliances and disputes through noisy, squabbling interchanges of a shared repertoire. When a bird disperses from its natal colony and joins a neighboring group, it tosses its old lingo and quickly adopts the sounds of its new home. For these birds, every nesting tree is a distinctive cultural unit, its boundaries created by the imperative for all colony members to learn and use the same sounds.
