From seemingly simple statements to the sophistications of poetry, human language is incredibly complex. There is enormous unconscious cognitive effort behind extracting precise information from speech, which is ultimately a series of sounds varying in frequency, intensity and timing. Meaning thus depends not only on the sounds we make, but the grammar and structure built up by these individual noises. Understanding how complex vocalizations work within the larger context of animal life is an important step in getting a clear picture of what made language the incredibly diverse, and often beautiful, mode of communication it is today.
The intricacies of bird song provide enormous amounts of information. Passerines (songbirds) have a large repertoire of available songs and calls for communicating between themselves and amongst other species; the common raven (Corvus corax) has demonstrated at least 79 unique calls for territorial interactions alone. Studies have shown that not only groups of ravens but individuals display unique calls, allowing others to identify them from a long distance. As they display cognitive intelligence on par with some primates, exploring the actual components of calls ravens interpret and respond to provides fertile ground for acoustic research.
Researchers have known that songbirds differentially interpret the patterns within songs in addition to the tone and loudness of each song syllable. However, all previous studies have depended upon conditioning the birds’ response over time to differently structured songs. In this study, Reber et al. explored how ravens would react to novel songs composed of different component patterns than they were accustomed to. They hoped to determine if birds were able to distinguish a spontaneously heard new song structure composed of individual syllables to which they had been habituated, as primates have been shown to do.
They recorded long-distance calls used by raven mating pairs (one female, one male) in marking territory. With computer programs, they broke these songs into individual components and rearranged them into an (AB)n pattern, in which A is a male syllable, B is female, and n is the number of times the calls were repeated. Ravens were allowed to grow accustomed to these calls, after which the researchers rearranged the pattern, either by lengthening the overall call (increasing n), changing the pattern of component A to B (for instance, AABB versus ABAB), or dropping the final syllable (ABA versus ABAB). In order to determine whether the birds were interpreting and responding to the sounds, they recorded the time the ravens spent oriented towards a speaker from which the sound was played.
The birds were clearly more interested when the pattern was spontaneously changed than when a control was played through the speaker. Although dropping the last syllable and lengthening the call did not result in a behavioral change, the birds may have been processing the information and simply been uninterested in responding. Considering that ravens vocalize as pairs, the actual order of syllables within the call seems to provide relevant information about the two birds (for example, a pair invading another’s territory) together. Further research among not only other birds in different lineages but among primates as well may provide a clearer picture of the development of complex vocalizations across evolutionary time.
Source: Stephan A. Reber, Markus Boeckle, Georgine Szipl, Judith Janisch, Thomas Bugnyar, W. Tecumseh Fitch (2016). Territorial raven pairs are sensitive to structural changes in simulated acoustic displays of conspecifics. Animal Behaviour 116 153-162
