Animal vocal communication ranges from simple to complex based on repertoire size, structure, and 16 composition of calls and the information encoded in them. According to the social complexity hypothesis, 17 communication complexity tends to increase with an increase in social complexity. While several studies on 18 mammalian systems exist supporting this, evidence from avian systems is comparatively limited. Towards this, 19 we present evidence for complex acoustic communication in a cooperatively breeding passerine, Jungle Babbler, 20 based on three aspects of complexity: an extensive repertoire of acoustically-distinct calls, within-call structural 21 complexity and the diverse behavioural contexts in which these calls are used. Jungle Babblers were found to 22 possess a structurally and functionally diverse vocal repertoire comprising 15 different calls. Detailed acoustic 23 analyses of multisyllabic calls revealed that these calls are composed of different notes. Further, despite a large 24 number of notes present in the repertoire, the number of calls were limited to 15. This implies that there may be 25 underlying rules that determine call composition to give rise to functional calls to which receivers respond. We 26 also found that these calls were produced in a variety of affiliative and agonistic contexts and were employed 27 towards coordination of diverse social behaviours including group movement, foraging, brood care, aggression 28 and vigilance. Yet, 7 out of 15 vocalizations were produced in the context of vigilance. This disproportionate 29 investment of vocalizations towards co-ordinated acoustic vigilance is characteristic of many cooperatively 30 2 breeding birds. Our study extends support for the social complexity hypothesis and also lays the foundation for 31 future investigations on combinatorial and syntactical rules underlying call structure and function in bird 32 vocalizations. 33 34 Significance statement 35 Studies on vocal complexity in birds have focussed mainly on repertoire size, structure and function. However, 36 fine temporal and spectral features of elements that constitute a call/song are rarely examined to evaluate vocal 37 complexity. We examined complex communication in a cooperatively breeding social passerine, Jungle Babbler 38 for which we assessed repertoire size, function, acoustic features of calls and of their constituent elements. 39Jungle Babblers were found to possess a structurally and functionally diverse vocal repertoire comprising of 15 40 calls, 46% of which were in the context of vigilance, thereby extending support to the social complexity 41 hypothesis. We also found that several calls were composed of multiple, acoustically distinct notes. These 42 findings will be foundational in understanding the interrelations between sociality and communicative 43 complexity and underlying combinatorial rules that determine call structure and function. 44 45
There are multiple measures for bird song complexity such as repertoire size, phonological or compositional syntax and complex vocal mechanism (CVM). We examined these in an old-world passerine, Purple Sunbird. First, we identified and acoustically characterised the repertoire size (of notes and phrases). We then assessed positional fidelity and ordering of notes within phrases. We found 23 distinct notes by aural-visual inspection of spectrograms which was validated using a Classification and Regression Tree based on 5 acoustic parameters. These notes combined in different iterations to form 30 different phrases. Phrases comprised of an overall structure with an introductory note (prefix) at the onset, followed by notes occurring at specific positions within the phrase body, and terminated with a trill (suffix syllable(s)). Prefix was present in 93% of phrases whereas suffix syllable(s) occurred in 27% of phrases only. We found that notes exhibited positional fidelity and combined in specific order to form a phrase. This is indicative of underlying phonological syntax that limits the ways in which notes combine to form phrase and offers insights to song complexity. Finally, we found that suffix syllables exhibit the presence of mini-breath (very short inter-note interval) which are known to be produced by CVM.
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