Bird speech perception and vocal production: a comparison with humans.

Research into speech perception by nonhuman animals can be crucially informative in assessing whether specific perceptual phenomena in humans have evolved to decode speech, or reflect more general traits. Birds share with humans not only the capacity to use complex vocalizations for communication but also many characteristics of its underlying developmental and mechanistic processes; thus, birds are a particularly interesting group for comparative study. This review first discusses commonalities between birds and humans in perception of speech sounds. Several psychoacoustic studies have shown striking parallels in seemingly speech-specific perceptual phenomena, such as categorical perception of voice-onset-time variation, categorization of consonants that lack phonetic invariance, and compensation for coarticulation. Such findings are often regarded as evidence for the idea that the objects of human speech perception are auditory or acoustic events rather than articulations. Next, I highlight recent research on the production side of avian communication that has revealed the existence of vocal tract filtering and articulation in bird species-specific vocalization, which has traditionally been considered a hallmark of human speech production. Together, findings in birds show that many of characteristics of human speech perception are not uniquely human but also that a comparative approach to the question of what are the objects of perception--articulatory or auditory events--requires careful consideration of species-specific vocal production mechanisms.