Humans and songbirds are both vocal learners. Although many features were pointed out to be shared between them, there were no common indexes to compare their vocal development. Vocal learning requires the control of respiration. We analyzed the temporal features of successive vocalizations in human infants and juvenile Bengalese finches, one of the songbird species. We used three temporal parameters; Note duration (ND; vocalizations with expirations), Inter-onset-interval (IOI; between the onset of the preceding note and the onset of the following note) and Inter-note-interval (INI; silent period without any vocalizations). These temporal parameters might be related to the intentional vocalization with respiratory control. It should be useful to compare different vocalizations across the developmental stages and across the species. The aim of this study was to show whether developmental changes of the temporal parameters were common with human infants and birds.

Human infants produced unstable IOIs before babblings or at the beginning of babblings. Their frequency distributions diverged, though the distribution patterns in the early phase (1 month old) and that in the later phase (12 months old) were similar. We found that the distribution of IOI became diverse before babblings and then later the distribution converged into the same range of the early phase. On the other hand, Bengalese finches had unstable duration of temporal parameters in the early phase. Through the development, these distributions gradually converged. Developmental pattern of temporal parameters in finches showed a different trajectory from that in humans.

The babbling in human infants and the subsong in birds were considered to be analogues as the unstable vocalizations in the beginning of sensorimotor learning phase. However, our results suggested that the babbling might be the vocalization with the controlled respiration while the subsong might be the vocalization in which respiration control is still under development.