Compositional Differences, Functional Similarities: A Linguistic Analysis of Private Speech from a Young Child and a Home-Reared African Grey Parrot (Psittacus erithacus erithacus)

Humans and other vocal-learning species engage in solo vocalizations. Such vocalizations in the human literature are referred to as private speech and have been hypothesized to play a role in vocal repertoire development both in humans and nonhumans, alike. The current study used corpus linguistics techniques to quantitatively and qualitatively compare the private speech of a two-year-old child and an enculturated, home-reared African Grey parrot to identify similarities and differences in the composition and function of their private speech. Both speakers exhibited previously documented human private speech functions, including sound- and word-play and repetitive practice of new words. The composition of the utterances, however, was different between the two speakers. The child’s private speech contained a larger vocabulary, more frequent use of function words (i.e., prepositions, articles, pronouns, conjunctions) to string together ideas, and the use of multiple verb tenses. The cross-species finding that a home-reared parrot’s private speech is similar in function—including evidence of play and possible intrinsically-motivated practice—to that of a language-learning child offers unique insights into the evolution of human language.