In an experiment that unlocked insights into the learning process, scientists said Wednesday they had seen brain cells in a adolescent finch change as it listened for the first time to the warble of an adult bird.

Just a single experience can rapidly shape a juvenile brain and alter the way it functions, the research suggests.

Whether in birds or humans, acquiring the ability to perform certain acts is critically important for survival.

"Many skills, including communication skills, require great precision if you want to stay in the gene pool," said Richard Mooney, a professor at Duke University in Durham, North Carolina.

"A male songbird has to learn to sing precisely or he won't attract a mate."

Previous studies have pointed to a link between structural changes in the brain and sensory input.

But whether these changes lead to learning remained uncertain.

To find out, Mooney and other researchers peered directly into the brain of an anesthestised immature bird with a laser-powered microscope.

As it heard the song of a mature male from the same species, they witnessed a dramatic transformation in connective tissue, called dendritic spines, that link nerve cells in the brain.

Yet the change was not the one they had anticipated.

"We expected to see the building of new spines and the loss of old spines accelerate," Mooney said.

This is because it can take weeks or months for a juvenile to master the adult song. As a result, the scientists assumed that the brain would remain highly malleable, or "plastic," during that period.

Instead they saw exactly the opposite: hearing a tutor song rapidly stabilised previously dynamic synapses, according to the study, published in the journal Nature.

The findings also suggest that the window of opportunity for picking up the all-important mating song slammed shut after a certain age.

"Juveniles in which spines were already highly stable weren't able to learn from their tutors," said lead author Todd Roberts, a neurobiologist at Duke.

The work could help efforts to restore plasticity to cerebral nerves, called synapses, after a stroke or other brain damage, he said.