The discovery that young songbirds babble before they can mimic an adult's song adds to evidence that young animals are born with circuitry to make them explore the possibilities of their vocal apparatus and to make sense of the world.
Scientists at the Massachusetts Institute of Technology, Cambridge, report in the journal Science that immature and adult birdsongs are driven by two separate brain pathways, rather than one pathway that slowly matures, in work that offers insights into how birds - and perhaps people too - learn new behaviours, and what goes wrong when they have difficulty learning.
"The babbling during song learning exemplifies the ubiquitous exploratory behaviour that we often call play but that is essential for trial-and-error learning," comments Dr Michale Fee, senior author.
"The main point of our finding is that the child-like behaviour of young animals may not be just because they have an immature form of the circuitry that makes adult behaviours, but because they have special circuits in the brain that purposefully drive their exploratory and random-looking behaviours.
In order to learn we need to try out different things and to explore. I suspect that there is a similar process going on in the brain of the human infant as he learns how to speak and how to convey meaning."
Baby zebra finches produce a highly variable, babbling song - a series of squeaks and beeps ("if you slow it down it can sound quite beautiful, like a rhythmic jazz riff," says Dr Fee).
The little birds practice incessantly until they can produce the stereotyped, never-changing song of adults "This early variability is necessary for learning, so we wanted to determine whether it is produced by an immature adult motor pathway or by some other circuit," Dr Fee explains.
Past research has shown that the zebra finch has two distinct brain circuits dedicated to song, one for learning and another - known as the motor circuit - for producing the song.
Dr Fee's colleagues, Dmitriy Aronov and co-author Aaron Andalman, used a drug to temporarily disable parts of the brain. They recorded signals from neurons in the singing bird, and were surprised to find that a hitherto overlooked part of the brain was important.
When they disabled part of the learning circuit, called LMAN, the birds ceased babbling. "This tells us that singing is driven by two different motor circuits at different stages of development," explains Aronov.
"We've long known that these two pathways develop physiologically at different times, so there's an elegant parallel between our functional findings and what is already known about anatomy."
Contrary to the "use it or lose it" assumption, the authors found that LMAN retains its ability to produce babbling even in adulthood. "In zebra finches, the exploratory phase ends when learning is complete," he says.
"But we humans can always call upon our equivalent of LMAN, the prefrontal cortex, to be innovative and learn new things."
This idea that there are circuits in the brain that drive exploration is also exciting to me in the context of creativity," says Dr Fee. "When we think creatively, we are putting ideas, words and thoughts together in new and often surprising ways, the same way a young bird puts sounds together in new and surprising ways.
How does this happen? My guess is that there are circuits in our brains that drive the apparent randomness underlying that creativity."
Although it might be possible to exploit this understanding to stimulate our exploratory circuits more, "I wouldn't want to suggest that," he says.
"I think the biggest implications of the work are in understanding how the human brain learns. Many of the things we learn to do, like speaking, learning math, playing a musical instrument, or hitting a tennis serve, require a lot of practice.
We need to explore and try different things in order to perfect any of these behaviours. I would guess that there are similar circuits in the human brain involved in learning these things. Our work may eventually help us understand what goes wrong when people have difficulty learning."