Language and birdsong: Introduction to the special issue.

By most accounts, language is uniquely human. No other species has a communicative system with such expressive power, one that allows us to transmit a seemingly endless set of meanings to our conspecifics. Every sentence we produce is composed of smaller linguistic units put together in a rule-governed manner. The standard view is that sentences are put together using syntactic rules that define hierarchical relationships within each sentence. These rules are claimed to contain recursive elements that permit sentences to be combined to form ever-longer sentences. As a consequence, language has a structure and open-endedness that is unlike the communicative systems of any other species, as far as we know (Hauser, Chomsky, & Fitch, 2002). Historically, considerations such as these have discouraged efforts to identify potential animal models of language, even though a comparative approach has been essential in ascertaining the neurobiology of other cognitive functions (but this has started to change in recent years; cf. Holy & Guo, 2005). Even if language in its entirety is unique, however, certain aspects of it might not be. It is at least conceivable that useful homologies or analogies might exist between aspects of speech and language and the communicative systems of other species. For example, one essential characteristic of human language is that it is based on a capacity for vocal learning with reference to auditory feedback. Humans are not born knowing the sounds that are relevant to the language they will speak. Vocal learning is itself a rarity in the natural world, but it is not unique. The short list of known vocal learners among animals includes parrots, some hummingbirds, bats, elephants, marine mammals such as dolphins and whales, and humans. But by far the most numerous vocal learning species (at about 4000) are the oscine songbirds. From an ontogenetic perspective, the acquisition of speech and birdsong have compelling parallels (Doupe & Kuhl, 1999). Humans and songbirds learn their complex, sequenced vocalizations in early life. They similarly internalize sensory experience and use it to shape vocal outputs, through sensorimotor learning and integration. Auditory feedback from self-generated vocalizations is necessary for vocal learning, as it is in humans. Songbirds show similar innate dispositions for learning the correct sounds and sequences; as a result, humans and some species of songbird have similar sensitive periods for vocal learning, with a much greater ability to learn early in life. These behavioral parallels at least make it plausible that there might be some non-trivial similarities between birdsong and speech, with respect to underlying neurobiological mechanisms.