A comparison of vocalisations between mainland tui (Prosthemadera novaeseelandiae novaeseelandiae) and Chatham Island tui (P. n. chathamensis)

Vocalisations are important for territorial defence, mate attraction, and species recognition in many songbirds. Comparative studies on the songs of birds between islands and mainland populations provide insight into the evolution of vocal communication in terms of both ecological and social factors. We compared the vocalisations of tui (Prosthemadera novaeseelandiae novaeseelandiae), an endemic honeyeater from New Zealand’s mainland, with those of a subspecies from the remote Chatham Islands (P. n. chathamensis). Song spectral variables of male long-range ‘broadcasting’ songs differed between mainland and Chatham Island tui populations. Songs recorded from five mainland populations had significantly greater syllable diversity and produced a higher percentage of trills than those from the Chatham Islands. These characteristics have been revealed in past studies as being sexually selected traits positively correlated with male genetic diversity. The acoustic adaptation hypothesis could also help to elucidate the presence of a significantly higher percentage of trill components in the songs of the mainland population, as the mainland study sites in general contained more areas of open vegetation, where trills transmit more effectively than in dense forest, which dominated the sampling sites on the Chatham Islands. Future research into the variation of tui song complexity in relation to population size, geographic isolation, and habitat structure between different mainland sites would provide further insight into links between selective pressures and vocal complexity.