Acoustic features within a forest bird community of native and introduced species in New Zealand

Habitat-dependent selection pressures may result in convergence of the characteristics of vocalisation of species living in the same habitat, favouring optimal transmission features. However, competitive interactions for acoustic space between different species may lead to divergence of acoustic signals mediated by character displacement. Here, the vocalisations of 16 bird species were examined within a forest bird community in New Zealand, and the acoustic space that each species occupied was quantified using principal component analysis. The vocalisations of species were over-dispersed within the acoustic space. The vocalisations of introduced species were distributed closer to the spatial centre, whereas the vocalisations of native species were distributed towards the outer edge. The frequency of vocalisations was correlated with height from which birds vocalised and body weight. For native species, the results support the acoustic niche hypothesis, in that avian vocalisations appear to have diverged evolutionarily. This may represent acoustic adaptation to different transmission properties associated with different elevations (song-post heights), or is perhaps a by-product of physiological parameters such as body weight, as well as representing phylogenetic differences. A relationship between environmental selection pressure and body size on signal design is supported, and the study provides new insights into the importance of partitioning of acoustic space in the evolution of acoustic signals in a mixed community of native and introduced birds.