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Developmental changes in song production in free-living male and female New Zealand bellbirds

Roper, Michelle M., Harmer, Aaron M.T., Brunton, Dianne H.
Animal behaviour 2018 v.140 pp. 57-71
acoustic properties, adulthood, adults, breeding season, cross-sectional studies, crystallization, evolution, females, fledglings, learning, males, sexual dimorphism, songbirds, vocalization, wild birds, New Zealand
Song development research has been dominated by studies of northern hemisphere species where typically only males sing. However, female song is present in many species and recent research shows that female song is in fact the ancestral trait for songbirds. Here we present results from a field-based cross-sectional study comparing song development in both sexes of New Zealand bellbirds, Anthornis melanura. We asked whether both sexes develop song at a similar rate, and how the components of a song differ with age. The motor phases of each sex began at similar ages, with subsong starting at 3 weeks posthatching and song types crystallizing by 24 weeks. Song components were compared between three age groups: learning phase, first breeding season and adults. Song structure and acoustic properties were similar between sexes within the learning phase (except for mean fundamental frequency) but differed in adults. The variety of syllable types produced was more widespread in the learning phase and differed significantly between age groups and sexes. Individual syllable production varied in consistency between age groups for both sexes and we suggest that more complex syllables may require more practice to develop to maturity. The findings support the consensus that female and male bellbirds learn song at similar rates; however, differences between the sexes in the learned song components result in sexually dimorphic songs. This study is novel in that we used a field-based approach in which the complex song of male and female wild birds was compared between age groups ranging from fledging to adulthood. Our study contributes to current knowledge of female song development, a topic important for further understanding the selection pressures driving song evolution in songbirds.