The Australian outback can be a noisy place if you are in the midst of several chestnut-crowned babblers (Pomatostomus ruficeps). These gregarious birds are raucous year-round scrubland residents that live in groups during the non-breeding season and nest cooperatively from July to November. In their tight social system, communication is key. Babblers do not sing, but they produce at least fifteen different calls. A new study shows that chestnut-crowned babblers have a particular communicative ability previously thought to be uniquely human, called phonemic structuring.
Phonemes are irreducible speech units—like the /p/ or /t/ in “pat”—that are meaningless by themselves, but that can be used to make words. Structuring phonemes into different words, such as “tap” or “apt,” helps account for human language’s limitless power to generate meaning. Researchers had noted that babblers in the wild seemed to be recombining phonemes, in a rudimentary way, to generate messages in particular contexts.
To investigate the babbler phenomenon, Sabrina Engesser and Simon W. Townsend of the University of Zurich, along with colleagues from the United Kingdom and Australia, chose two context-based calls to study: a flight call used to alert babblers to incoming birds, and a prompt call given by a parent at the nest when bringing food to its young.
The team first measured properties of the phonemes, such as duration, using acoustic analysis. They designated two phonemes A and B, which combined as AB for the flight call and BAB for the prompt call. Then they observed calls and behavior in natural settings, finding that babblers used the two-element flight call 62% of the time when flying, and the three-element prompt call 70% of the time when provisioning young.
Finally, the researchers turned the babblers into listeners by playing back recordings of both types of calls while birds were housed with an outdoor view and a used nest. The birds confirmed they knew the code: AB calls caused them to look outside and sometimes to flap and hop, while BAB calls drew their attention to the nest.
What’s next? Playbacks to chicks are planned, says Engesser, to see if the ability to discriminate between the sounds is innate or must be learned. She adds that other social species “are likely to be capable of phonemic structuring. “ For now, chestnut-crowned babblers’ ability can provide new insight into how this stage of human language development evolved. (PLoS Biology)