Supplementary material from "The social origins of vocal sequences in songbirds and human infants"

From birdsong to human language, acoustic communication by vocal learners involves concatenation of sounds into sequences. Sequences are more efficient for the producer and more accommodating to the capacities of receivers. Over development, the compression of syllables into rapid sequences (in terms of more syllables per second) may reflect both social learning and motor maturation. We tested whether sequence compression could be predicted uniquely by exogenous (i.e. social) and endogenous (i.e. motor) sources. In human infants, we found (1) that vocal sequences strongly engage adults, (2) from 5 to 10 months sequences compress, (3) social feedback to 5-month-old’s sequences predicted sequence compression over development, and (4) sequence compression over development predicted infants’ vocabulary development. We next examined the extent to which compression develops in a paradigm in which we could separate social feedback from motor practice. In zebra finch (Taeniopygia guttata), we found that (1) motor development predicted sequence compression, but (2) compression only predicted song maturity (similarity to tutor) when birds received contingent social feedback to their immature vocalizing. In addition, (3) social feedback predicted finches’ sequence compression. These findings demonstrate the potency of social feedback, across two vocal learning species, in the emergence of vocal efficiency.