Song complexity in relation to repertoire size and phonological syntax in the breeding song of Purple Sunbird

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.