INTRODUCTION

Striking parallels exist between the development of speech in human infants and the development of song in birds. Many sparrows, for example, learn their songs more readily during a sensitive period than at other times during development, require practice, and must hear themselves sing for normal song to develop (Baptista & Petrinovich 1986). These same features characterize both the earliest speech of human infants (see e.g., Ferguson et al. 1992) and second language learning, whether spoken or signed, among older individuals (Johnson & Newport 1989). Song production in zebra finches and canaries, like speech production in humans, is under lateralized neural control (Arnold & Bottjer 1985; Nottebohm 1991). Damage to any one of these areas, like damage to Broca's or Wernicke's area in (usually) the left temporal cortex of the human brain (for reviews, see Caplan 1987, 1992), produces highly specific deficits in the production or processing of communicative sounds.

As a result of these parallels in both behavior and neurobiology, studies of avian song development currently provide the best animal model for research on the mechanisms underlying speech development (Marler 1987). In contrast, while nonhuman primates are our closest living relatives and have often been used as animal models for the study of human social development (see e.g., Hinde 1984), their vocal communication is generally thought to provide no useful parallels with the development of human speech.