People have the remarkable ability to generate meaning out of meaningless sounds by arranging them in a, well, meaningful way. That’s how we form words. We used to think that this was a uniquely human talent, but according to a new PLOS Biology study, babbler birds communicate this way too.
"Although previous studies indicate that animals, particularly birds, are capable of stringing different sounds together as part of a complex song, these songs generally lack a specific meaning, and changing the arrangement of sounds within a song does not seem to alter its overall message," Sabrina Engesser from the University of Zurich says in a statement.
To see if animals do convey new meaning by rearranging combinations of sounds, Engesser and colleagues turned to chestnut-crowned babblers (Pomatostomus ruficeps) who, unlike most songbirds, don’t sing. Instead, the extensive vocal repertoire of these highly social birds living in the Australian Outback are characterized by discrete calls made up of smaller, individual sounds, she explains.
After observing wild babblers at the Fowlers Gap Arid Zone Research Station in far western New South Wales, the team found that babblers reuse the acoustically distinct "A" and "B" in various arrangements when they’re performing certain behaviors. Whey they’re flying, for example, they’ll emit "AB" flight call; when they’re feeding their baby chicks in a nest, they’ll let out "BAB" prompt calls. Then, to see if these different arrangements are functionally distinct, the team played back recordings through speakers in aviaries arranged like the schematic below.
The back was made of metal meshing to give them a view of the outside, and the two sides were made of aluminum. The front allowed a one-way view from the outside to the inside. The compartments also contained a perch, feeding station, nest, and sleeping box. Single birds used compartment 3, pairs of birds used compartments 1 and 3, and trios used compartments 1, 3, and 6. 2015 Engesser et al., PLoS Biol
The listener birds showed that they could discriminate between different call types. They looked at the nests when a feeding prompt call was played back to them, and they looked out for incoming birds when they heard a flight call. When the team switched the elements between the two calls -- generating flight calls from prompt elements and prompt calls from flight elements -- they found that the two calls were indeed generated from rearrangements of the same sounds.
"Although the two babbler bird calls are structurally very similar, they are produced in totally different behavioral contexts, and listening birds are capable of picking up on this," study co-author Simon Townsend from the University of Zurich says. "This is the first time that the capacity to generate new meaning from rearranging meaningless elements has been shown to exist outside of humans."
The first sound element "B" appears to be what differentiates between flight and prompt vocalizations. In English, that’s like the "C" that changes "at" to "cat," the differentiating element is called a phoneme. "Although this so-called phoneme structuring is of a very simple kind, it might help us understand how the ability to generate new meaning initially evolved in humans," Townsend adds. "It could be that when phoneme structuring first got off the ground in our hominid ancestors, this is the form it initially took."
Babbler birds may have rearranged existing sounds because that’s quicker than evolving a new sound altogether.