Supplementary material from "Ontogenetic evidence of socially learned call sequences in Western Australian magpies"

Combinatoriality is the capacity to combine discrete vocal elements into larger structures. Previously thought unique to human language, combinatoriality has now been documented in several non-human taxa. This capacity is believed to be related to social complexity and the need to transmit complex information, as predicted by the Social Complexity Hypothesis for Communicative Complexity (SCHCC). Research into combinatoriality is predominantly limited to studies on adult repertoires, limiting the ability to determine social influences on the emergence of vocal sequences. We used longitudinal observations of magpie fledglings to document the emergence of the non-song repertoire and investigate the effect of social complexity on repertoire size and the rate at which call sequences are acquired. We found that fledglings produce the sequences of their social group, not the broader population, and acquire their sequence repertoires over time, suggesting they are socially learnt. Furthermore, individual sociability was found to modulate this process: fledglings who spent more time with others produced sequences sooner, and those with more social contacts produced more sequence types. This study is the first to document the emergence of combinatoriality in an open-ended vocal learner and provide individual-level ontogenetic evidence that the SCHCC applies to combinatoriality.