Richard Mooney records the songs of birds who are learning to sing and compares their progress to activity in the young birds' brains-research that could unlock the mechanics of human auditory learning.The sound of songbirds in the morning can be an impromptu serenade. But listen closely, and it’s clear the birds aren’t improvising. They’re reciting and repeating a signature tune they learned in adolescence from the dominant male in their lives.

Those melodious tweets are entrancing, but why study how birds learn their music? According to Richard Mooney, PhD, a neurobiology professor and investigator at the Duke Institute for Brain Sciences, understanding what happens inside a bird’s brain when it hears and memorizes a certain song could lay a foundation to improving speech in humans with auditory disabilities.

“Birds use auditory experiences to guide behavior just like humans use hearing to guide speech development,” Mooney says. “If a young bird doesn’t hear a tutor song or can’t hear itself sing, it doesn’t develop a normal song.”

According to Mooney, who has spent the last 25 years studying the brain circuitry and neural pathways that control singing, a bird has a finite amount of time to be exposed to and learn a tutor’s song.

The juvenile bird needs to hear a tutor song during a developmentally sensitive period, similar to a human child’s need to hear language consistently in the first years of life in order to develop fluent speech. If a songbird does not hear the tutor song before two months of age, its brain becomes committed to producing a simple “isolate” song. The tutor’s song lasts for a few seconds, and adolescent birds only require a few minutes of exposure to the same song to memorize it. However, to produce an accurate copy of the tutor song, they must practice the song thousands of times over a month or more.

What makes some juveniles better song learners than others? Looking inside the bird’s brain can reveal the presence of dendritic spines, doorknob-shaped protrusions on a nerve cell that receive and process electrical signals from other nerve cells, at specialized junctions known as synapses.

By looking into the brains of naïve juvenile songbirds, Mooney and his colleagues found that the rate at which these spines come and go (spine turnover) could predict how well a juvenile would learn from a tutor. Juveniles with the highest levels of spine turnover were the best learners, while birds with stable spines learned little or nothing from their tutors.

To visualize living neurons in juvenile birds as they learn to sing, Mooney’s team first injects a brain area in the bird analogous to Broca’s area in humans with a fluorescent green protein. Then, using a scanning laser microscope, they peer through a small surgically implanted window in the anesthetized bird’s skull.

Cells expressing the protein glow green when struck by the laser light, allowing them to be visualized under the microscope. After obtaining a baseline measure of spine turnover, the bird is exposed to the tutor song. The imaging process can be repeated over many days and weeks as the bird slowly copies the tutor song. This approach allows spine changes to be monitored as the juvenile memorizes and copies its tutor’s song.

The effect of hearing and internalizing the tutor song was counterintuitive, Mooney says. “In those juveniles with high spine turnover, hearing a tutor song immediately stabilized spines, even though the copying process had hardly even begun,” he says. “It appears that in receptive juveniles, hearing a tutor song rapidly stabilizes and strengthens the synaptic network. One intriguing possibility is that we are watching the formation of a memory that sets the stage for motor learning.”

Mooney says the findings of this work ultimately will help explain how the human brain harnesses auditory information to guide learning of complex skills, such as speech and music. It could also help to explain how, as we age, our brains become less receptive to learning new skills, including foreign languages.