Learning how baby sparrows learn to sing their songs could provide clues to how humans learn to speak languages.
University of Utah biologist Gary J. Rose and his colleagues have taught baby sparrows to sing a complete song, even though the birds were exposed only to overlapping segments of the tune rather than the full melody.
Their study, published in Wednesday's issue of the journal Nature, provides clues about how musical memories are stored in the brain and how those memories help birds learn to sing. Rose is the principal author of the study.
"There are strong parallels between song learning in birds and speech learning in humans," Rose said. "Like humans, songbirds learn particular regional dialects, so they represent excellent opportunities to study the physiological basis of language. If we can understand something about how song is represented in their brains, then maybe we can better understand how speech learning occurs in humans and, when it goes awry, how we might go about fixing it."
Co-author Stephanie Plamondon, a doctoral student in neuroscience, said the scientists gave the birds just pieces of the song and they were able to assemble a complete song.
"A full song or a complete sentence isn't required to learn the song, only an association between phrases of the song."
Songbirds must hear their species' songs when they are young or they fail to learn to sing them, Rose said. Such birds produce very simple songs, mostly repeated whistles.
Birds learn to sing in stages. First, there is a subsong phase in which they babble softly, almost like human infants. Then they undergo a phase when they practice singing for eight or nine months and the bird is producing a song and comparing it to the memory he has formed, Plamondon said.
After that, birds undergo "crystallization," which means their song is crystallized or essentially set in stone, at least until the next mating season, when some changes can occur.
The Utah biologists tested a theory dealing with the long-term "auditory memory" formed by young sparrows when they first hear other sparrows sing. Scientists want to know how that memory is stored in the brain and how that memory is used as the birds learn to sing weeks later.
The complete white-crowned sparrow song has five segments or snippets — researchers call them phrases — represented by the letters ABCDE. "A" is a characteristic opening whistle; "B" is a note complex or several musical notes in a specific sequence; "C" is a buzzing sound; "D" is a trilling sound; and "E" is another note complex.
Plamondon said song learning is unlikely to be completely genetic because white-crowned sparrows in different regions have different "dialects"; that is, they differ in how they assemble song segments. Rose said there is no evidence the birds use short-term memory to remember their song when they are tutored, and it's unlikely the sparrows carry some sort of internal instructions on how to assemble song segments into a complete song.
Instead, the study indicates the sparrows' characteristic song is imprinted on their brains like a long-term memory and not as a complete song, but in pieces. Rose and his colleagues propose that circuits of certain nerve cells only need to detect pairs of song segments (AB, BC, CD, DE) for the birds to learn to sing. That is because each pair of segments overlaps the next, allowing the birds to figure out how to string together the complete melody.
Rose said nerve circuits that detect pairs of song segments are shaped as the birds practice singing. "In many cases experience shapes the function of the brain. If humans don't have normal vision during the first few weeks of life, they become functionally blind. If infants don't hear speech, they obviously won't learn to produce a verbal language," he said.
"If experience early in life is essential for shaping the function of the brain, then we need to understand how that happens. And songbirds are one of the few cases other than humans that actually learn their verbal language and have to be tutored," Rose said.
The researchers first recorded songs from white-crowned sparrows in the Wasatch Mountains. They digitized the recordings so they could break them into five segments or snippets called "phrases." They obtained permits to capture sparrow nestlings, hand-feeding and raising them in a laboratory in soundproof cages so they didn't hear each other.
When the sparrows were 2 weeks old, researchers began trying to teach them to sing by playing segments of the complete song in different orders. Separate 90-minute tutoring sessions were conducted for each bird twice daily for 60 days.
In the first experiment, the scientists played one segment or phrase of the sparrow song at a time, separated by 2.5-second silences. They played the segments in reverse order to control the birds simply storing what they heard in short-term memory and repeating it. The nine birds in the experiment could not string the segments together in the correct order to sing the entire song.
Next, eight sparrows listened to two segments of their song at a time. Each pair of segments was in the correct order with the pairs of segments played backward. However, because each pair of song segments overlapped another, the birds were able to string the segments together in the correct order and sing the full song.
Plamondon said when birds hear two song segments at a time, they implicitly learn the rules for putting all five segments together.
In the final experiment, five sparrows heard pairs of song segments with each pair in reverse order. The birds learned to string the segments together, but because the segments were reversed, they sang with the segments strung together backward.
"The relevance of these findings is that this may be representative of how learned sequences of movements of various types work," Rose said. "A jazz musician, for example, learns the rules for making transition from one note to the next and can compose full songs by observing those rules."
E-mail: lweist@desnews.com