Though many animals have demonstrated the ability to learn individual words, it has long been thought that the cognitive capacity to grasp syntax – that is, the extraction of compound meanings from combinations of words – is uniquely human. However, in a new study that appears in the journal Nature Communications, researchers present evidence that Japanese great tits may in fact be capable of forming syntactic compositions.

Just as dogs are able to learn the meanings of terms such as ‘sit’ or ‘fetch’, so too are Japanese great tits capable of recognizing individual vocalizations carrying specific connotations. Previous research has indicated that birds related to the tits tend to use three different song notes to warn each other of nearby predators. Labeling these notes A, B, and C, the authors of the new study describe how they are rarely used individually, but virtually always in combinations of two or three, and invariably elicit a response in other birds which scan their surroundings upon hearing the call.

A further note type, D, is used to beckon other tits to approach. As such, researchers have concluded that the birds have a finite "vocabulary," consisting of individual call types that carry specific meanings. However, the fact that A, B, and C calls are always used in combinations also indicates that Japanese great tits may be able to convey more complex meanings by stringing individual "words" together, thereby creating a more diverse array of expressions from a limited number of vocal elements – just as humans do.

Observing the birds further, the researchers noted that when A, B, and C calls are immediately followed by a D call, receivers both scan their environment and approach the caller. Witnessing this, the study authors wondered whether this occurred because the birds perceived the two instructions separately, or the ABC-D combination as a single meaningful unit.

To test this, the researchers used loudspeakers to play ABC-D calls, followed by reversed D-ABC calls. Upon doing so, they noted that, while birds executed the expected scan-and-approach response to the initial ABC-D recording, no such response was elicited by D-ABC vocalizations.

This would appear to suggest that the tits did not respond to both the ABC and D instructions separately, but to the ABC-D call as a compound structure. Conversely, D-ABC represents a meaningless phrase within the birds’ "language," and therefore did not generate a corresponding action.

Summing up, the study authors suggest that “these findings support the hypothesis that the communication system of tits represents semantically compositional syntax,” indicating the ability to generate compound meanings from individual elements may not be uniquely human after all.