The ABCs of Birdsong
A is for Auditory Memory
Song appears to be a learned behavior for songbirds. As nestlings, young male songbirds listen to the vocalizations of their fathers and other males of the species, then commit these songs to their auditory memories. This "sensory" phase appears to be critical for subsequent song production for some species: Young songbirds raised in isolation produce abnormal songs never heard in nature.
B is for Babbling
When male songbirds reach puberty they begin practicing their vocal skills with soft, unstructured tweets and trills, comparable to the prelinguistic babbling of human infants. Next they shift to louder, more stereotyped vocalizations ("plastic" song) and finally to "crystallized" songs that match the auditory memory formed months earlier.
C is for Courtship
Most male songbirds rely heavily on their singing to woo members of the fairer sex--and to repel competing males from the territory. What makes one male's song more attractive than another's? It seems that size does matter--of the repertoire, that is. In experiments with mockingbirds and red-winged blackbirds, females were more impressed by males who had a larger variety of songs in their arsenal.
D is for Dialects
Some songbird species appear to have their own version of the Texas twang and the Boston broad R. In California, trained listeners can distinguish white-crowned sparrows living in Berkeley from those living around Sunset Beach from those in Marin County. Though all these birds produce songs that are recognizably those of the white-crowned sparrow, the end phrasing of their song differs by region.
E is for Environment
Social interactions seem to affect song learning. During the early sensory phase, birds of many species memorize more songs than they'll later crystallize. So which songs stick and which don't? Studies show that those most similar to their neighbors' songs are retained, and those that are different are rejected. Ball compares this to the way human infants are thought to "tune" their babbling on the basis of the adult language environment they experience.
F is for Feedback
Even as adults, some songbirds must continue to hear the songs of others within their species in order to maintain song. The evidence: Adult zebra finches who were deafened after they had developed adult crystallized speech could no longer sing correctly.
G is for Gender Differences
Though the brain weight of male and female songbirds is similar, there are major differences in the structure of the brain's song control circuit. (In the zebra finch, a species in which the female has never been observed to sing, the nucleus known as the high vocal center is seven times larger in the male.) This difference in brain volume, first discovered by Fernando Nottebohm and Art Arnold in 1976, stunned neurobiologists and set the stage for work that ultimately uncovered marked sex differences in brain structure in mammalian species.
H is for High Vocal Center
This portion of the songbird's brain is key to vocal learning and song production. Lesions to this area of the brain, experiments show, cause problems in vocal production reminiscent of Broca's aphasia in humans. (People with this condition can comprehend speech but have great difficulty producing it. Often articles, auxiliaries, and other non-essential parts of speech are omitted; "the cat can run" becomes simply, "cat run.")
I is for Identification
Even without an identifying, "Hi, it's your mom," most of us would immediately recognize the voice of our mothers (spouses, kids, etc.) over a phone line. After all, humans are adept at recognizing individuals on the basis of voice cues. What about songbirds? Recent experiments with the great tit show that a female can pick out her mate by the way he sings. Starlings also can distinguish the song of one starling from another, based on recent studies done at Hopkins by Hulse and colleagues.
J, K, L is for Linguist Noam Chomsky
His oft-debated view that humans are "prewired" to learn language has its parallel support among some bird researchers. When songbirds are raised in acoustic isolation, they later produce abnormal song--yet this "isolate" song still retains species-typical attributes, such as the number of notes per song and trilled syllables per song. Such data led researcher Peter Marler to make the controversial suggestion in 1992 that much information about birdsong is "pre-encoded" in innately specified brain chemistry.
O is for Ornithologists
Field workers who study birds out in "the wild" were the first to identify variations in birdsong. In the early part of this century, ornithologists used musical scores to represent the songs they heard. Record keeping became much more objective with the advent of the tape recorder and, later, the sonograph, which generates sound spectrographs--physical representations of complex acoustic signals.
P is for Perfect Pitch
In humans, it's a rare gift to be able to "name" a pitch like C or G-sharp--an attribute known as absolute pitch. Songbirds, it turns out, rely extensively on absolute pitch to perceive and classify sound. Recent experiments with European starlings, done by Hulse and other researchers, show that birds also use relative pitch ("higher than" or "lower than"), the strategy most often employed by humans.
Q is for Quick Learner
It doesn't take long for some young songbirds to memorize, and then later imitate, the songs they first heard as nestlings. A baby song sparrow need only hear 30 repetitions of a song, on a single day, in order to later produce the song. Nightingales are even faster learners--studies show they need only hear 10 to 20 presentations of a song to learn it.
R is for Recognition
If you've ever conversed over the din of a baby wailing, or a radio blaring, you know that people can communicate remarkably well in noisy environments. So, too, can birds. In nature, all kinds of things conspire to distort birdsong--environmental noises, acoustic "filters" like trees and bushes, the long distance it must carry. When researchers used a computer to distort the song of field sparrows and then played the distorted song out in the field, the sparrows tolerated the distortion very well, vocalizing back in the expected ways. Interestingly, pitch proved to be the most salient factor; distort that, and the field sparrows had a much harder time recognizing the song.
S is for Syllable
Human speech has a finite set of units called phonemes. Does birdsong have similarly identifiable units? The "processing unit" used by songbirds appears to be the syllable, as an ingenious experiment shows. A male and female zebra finch were placed in side-by-side cages to stimulate his singing. Researchers then flashed a strobe light at various intervals to interrupt his singing. The male almost always stopped singing between syllables (a place of natural pause); only rarely was a syllable interrupted without being completed.
T is for Turkey
Known for its distinctive "gobble-gobble," rather than melodious songs, the turkey does not belong to the order of songbirds (Passeriformes--which accounts for about half of all living bird species). Neither does the chicken, quail, pigeon, or dove. For these birds, and many others not in the songbird suborder, vocal behavior appears to be inborn, not learned. When deafened as nestlings, birds like these go on to develop normal "gobbles," "clucks," and "coos" as adults.
U is for Use It or Lose It
Humans have a "critical period" for learning our native language, studies show; by adolescence, our ability to learn new languages closes off, in part because we lose our sensitivity to phonetic contrasts from nonnative languages. (While an American 6-month-old can easily differentiate the English "da" from the softer Hindi "da," an adult finds this task more difficult.) So, too, for song sparrows and swamp sparrows, birds that look very much alike yet have very different songs. Though often raised within earshot of one another, they tend to form early memories only for their own species' song.
V is for Volume
Brain volume that is. Just as birdsong varies with the season (male songbirds sing most frequently in the spring, during mating season), so does the volume of song nucleii. In one study of canaries, the high vocal center in the brain was 99 percent larger in the spring than in the fall. Do seasonal changes in the brain involve the formation of new nerve cells? While it's long been thought that new neurons are not produced in adulthood, recent studies suggest that new neurons do form, on a seasonal basis; in a study of canaries, new neuron development was highest in October, a time when these birds are learning new songs.
W is for Wingstroking
When female cowbirds are particularly pleased by the song of a male, they move their wings rapidly to and fro in appreciation-- the avian equivalent to batting the eyelashes. Such feminine wiles have an effect on male cowbirds, who are more likely to repeat the songs that receive such a reaction. This is further evidence that songbirds are "action-based" learners--their social experiences influence which innate songs will be crystallized and produced in adulthood.
X, Y, Z is for Zebra Finch
While male zebra finches sing up a storm, their female counterparts are essentially mute when it comes to song. This striking difference between the sexes makes the zebra finch a natural choice for researchers looking at the relationship between brain and behavior.
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