Recursive syntactic pattern learning by songbirds

The language of birdsong Noam Chomsky's work on ‘generative grammar’ led to the concept of a set of rules that can generate a natural language with a hierarchical grammar, and the idea that this represents a uniquely human ability. In a series of experiments with European starlings, in which several types of ‘warble’ and ‘rattle’ took the place of words in a human language, the birds learnt to classify phrase structure grammars in a way that met the same criteria. Their performance can be said to be almost human on this yardstick. So if there are language processing capabilities that are uniquely human, they may be more context-free or at a higher level in the Chomsky hierarchy. Or perhaps there is no single property or processing capacity that differentiates human language from non-human communication systems. Humans regularly produce new utterances that are understood by other members of the same language community 1 . Linguistic theories account for this ability through the use of syntactic rules (or generative grammars) that describe the acceptable structure of utterances 2 . The recursive, hierarchical embedding of language units (for example, words or phrases within shorter sentences) that is part of the ability to construct new utterances minimally requires a ‘context-free’ grammar 2 , 3 that is more complex than the ‘finite-state’ grammars thought sufficient to specify the structure of all non-human communication signals. Recent hypotheses make the central claim that the capacity for syntactic recursion forms the computational core of a uniquely human language faculty 4 , 5 . Here we show that European starlings ( Sturnus vulgaris ) accurately recognize acoustic patterns defined by a recursive, self-embedding, context-free grammar. They are also able to classify new patterns defined by the grammar and reliably exclude agrammatical patterns. Thus, the capacity to classify sequences from recursive, centre-embedded grammars is not uniquely human. This finding opens a new range of complex syntactic processing mechanisms to physiological investigation.