Sexual dimorphism of song and life history trade-offs in the New Zealand bellbird : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Albany, New Zealand

Abstract

Birdsong and its function is well studied in terms of male-male competition and female-mate choice. This has generated a male bias in the song literature and the dilemma that little is known about female song. However, recent research posits that female song is not only common but is also the ancestral state of songbirds. Therefore, it is timely that I investigate the ontogeny, structure and production of female song within the context of the life history of female songbirds in order to increase our current understanding of the function and evolution of birdsong. In this thesis, I use a wild population of New Zealand bellbirds (Anthornis melanura) as a model species. The New Zealand bellbird is ideal for this research as they produce complex but sexually dimorphic song. With a cross-sectional approach, I found the songs of each sex diverged and became more consistent as the song developed from juveniles to adults, and that their sexually dimorphic songs developed over similar timeframes, suggesting potentially related functions. I also compared how the adult song repertoire of each sex varied over time, and found that males had larger repertoires at both the population and individual levels. The syllable repertoire of each sex changed at a similar rate due to shifts in relative abundance over time, suggesting both sexes may have analogous song functions and are potentially under similar selection pressures. Sexual variation in song could theoretically be explained by differences in the syrinx structure but there is a lack of comparative research in this field. I found that bellbirds had greater sexual dimorphism in the size of their bronchial half rings compared to species both with and without female song. This suggests syrinx size alone cannot explain sexual dimorphism in repertoire size, but may have a stronger influence on sex- and species-specific song frequencies. Long term studies provide insights to life history and my study population on Tiritiri Matangi Island has breeding data available as far back as 1977. The island’s history of ecological restoration has resulted in exponential growth of the bellbird population, and I found correlated reproductive trade-offs with a reduction in clutch size over time, likely owing to increasing competition for resources. My research demonstrates how female songbirds develop and change their song over time and that they have flexible life-history traits that enable them to cope with changing breeding conditions. My research is significant in that it is one of the first to study female song in a wild population and provides important insights into male and female song development, structure and role.