Structural and functional complexity of vocalizations in a cooperatively breeding passerine, Jungle Babbler

Animal vocal communication ranges from simple to complex based on repertoire size, structure, and composition of calls and the information encoded in them. According to the social complexity hypothesis, communication complexity tends to increase with an increase in social complexity. While several studies on mammalian systems exist supporting this, evidence from avian systems is comparatively limited. Towards this, we present evidence for complex acoustic communication in a cooperatively breeding passerine, Jungle Babbler, based on three aspects of complexity: an extensive repertoire of acoustically-distinct calls, within-call structural complexity and the diverse behavioural contexts in which these calls are used. Jungle Babblers were found to possess a structurally and functionally diverse vocal repertoire comprising 15 different calls. Detailed acoustic analyses of multisyllabic calls revealed that these calls are composed of different notes. Further, despite a large number of notes present in the repertoire, the number of calls were limited to 15. This implies that there may be underlying rules that determine call composition to give rise to functional calls to which receivers respond. We also found that these calls were produced in a variety of affiliative and agonistic contexts and were employed towards coordination of diverse social behaviours including group movement, foraging, brood care, aggression and vigilance. Yet, 7 out of 15 vocalizations were produced in the context of vigilance. This disproportionate investment of vocalizations towards co-ordinated acoustic vigilance is characteristic of many cooperatively breeding birds. Our study extends support for the social complexity hypothesis and also lays the foundation for future investigations on combinatorial and syntactical rules underlying call structure and function in bird vocalizations. Significance statementStudies on vocal complexity in birds have focussed mainly on repertoire size, structure and function. However, fine temporal and spectral features of elements that constitute a call/song are rarely examined to evaluate vocal complexity. We examined complex communication in a cooperatively breeding social passerine, Jungle Babbler for which we assessed repertoire size, function, acoustic features of calls and of their constituent elements. Jungle Babblers were found to possess a structurally and functionally diverse vocal repertoire comprising of 15 calls, 46% of which were in the context of vigilance, thereby extending support to the social complexity hypothesis. We also found that several calls were composed of multiple, acoustically distinct notes. These findings will be foundational in understanding the interrelations between sociality and communicative complexity and underlying combinatorial rules that determine call structure and function.