No one is about to converse with this three-ounce bird, a European starling. But it proved itself an adept student in a study published recently in the journal Nature.The finding could upend linguists' view that only humans have the ability to learn recursion, a hallmark of complex language in which words or phrases are inserted into sentences to add nuance and meaning.
In his study, University of California San Diego psychologist Timothy Gentner showed that starlings could conquer at least basic recursion. He trained nine of 11 starlings to distinguish simple bird songs from ones that had phrases embedded within longer tunes.
It's as if the starlings were trained to differentiate the sentence, "My dog is black" from "My dog, who likes to chase cats, is black."
"This study has tremendous importance for our understanding of language - what makes it special and why only humans have it," said Jeff Elman, a UCSD researcher not connected to the study.
Scientists have wondered why only humans have highly sophisticated language, and many of them have searched for a single skill that only humans possess to make such language possible.
Until now, no other species had demonstrated an ability to grasp recursion.
In the starling study, Gentner's birds were rewarded with food if they pecked at a button when they heard a bird song that displayed recursion "grammar."
Gentner conducted 10,000 to 50,000 trials over 2 1/2 months, depending on how quickly each starling mastered recursion. Nine of the birds eventually learned to distinguish the patterns.
To make sure the starlings didn't merely memorize the songs, Gentner and his colleagues played recordings of bird songs with different combinations of "warble" and "rattle" sounds that followed the same rules of recursion. Even then the starlings performed well, suggesting that they had learned abstract patterns and not just remembered specific tunes.
"They're amazing little birds, and they're prodigious singers," Gentner said. "These guys are learning vast amounts of acoustic stuff ... and they string them together in these patterned bouts (of bird song)."
Gentner, who performed the experiments while at the University of Chicago, said he launched the study after researchers on the East Coast found that a type of monkey called the cotton-top tamarin could not learn a grammar similar to recursion.
Gentner said he isn't surprised that the monkey cannot learn recursion while the starling can.
Recursion arises from vocal learning, and only a small number of animals are vocal learners. They include humans, songbirds, a few species of bats and possibly some marine mammals such as dolphins and whales.
"Cotton-top tamarins don't have as rich a vocal, auditory world as songbirds do, and if there's any species of songbird that should be good at this sort of patterned learning, it's the (European) starling," Gentner said.
European starlings communicate with elaborate songs that contain different patterns and structures. Over their lifetimes, the males acquire a large repertoire of tunes.
Male starlings have plenty of motivation to become proficient singers. The longer and more complex their songs, the more attractive they are to females. And with a song that's distinctly theirs, they can more clearly mark their territory.
With the discovery, scientists can now analyze the European starling to better understand brain connections that enable it to recognize recursion, Gentner said.
His lab team at UCSD is exploring this subject. Later studies may try to identify what a given song means to the starlings, he said. One starting point might be to see what behaviors are associated with different types of bird songs.
"We're not entirely sure what the experiment would look like," Gentner said.
Much further down the road, scientists might build on starling studies to learn how human toddlers acquire language and how language disorders arise, he said.
The starling study reaffirms the value of studying vocal learning in animals, said Patricia Kuhl, co-director of the Institute for Learning and Brain Sciences at the University of Washington in Seattle.
"Theorists used to think that studies examining other species' abilities to learn aspects of language were silly and irrelevant," Kuhl said. "How could animals inform our understanding of language, perhaps the Holy Grail of human capacity?"
"But time and time again, we've been surprised by the degree to which nonhuman species ... can handle various language tasks."
If recursion is not exclusive to humans, it can't be the single defining feature of human language, scientists said.
It appears that language cobbles together a complex set of skills, Kuhl said.
"All the pieces - cognitive facility, computational wizardry, vocal learning, and ... even social skills play a role. Human infants put all these skills together to crack the language code."