Song divergence in the duetting Western Whipbird

Geographical divergence in bird song can lead to the evolution of premating barriers
and subsequent reproductive isolation and speciation. Bird song can diverge through
the processes of sexual selection (intrasexual competition and intersexual choice),
cultural drift, genetic drift, or ecological selection. Geographical divergence has been
studied extensively in male song, but not in female song. I investigated geographical
divergence in male and female song in the duetting western whipbird (Psophodes
nigrogularis) to determine the relative importance of evolutionary processes in
shaping the patterns of variation among their populations. To investigate the relative
importance of sexual selection, I compared differences in the responses of both sexes
to intra- and interspecific song. Females were more discerning than males in their
responses to geographical variants of male and female song, with females
preferentially responding to local dialects compared to foreign dialects in each case.
To investigate the potential influence of ecological selection in shaping patterns of
male and female song variation, I examined correlations between vegetation
characteristics and the geographical song differences of both sexes. Vegetation
characteristics differed significantly across the populations but did not correlate as
predicted with the song differences in either of the sexes. I conclude that ecological
selection has not had a significant effect in shaping male and female song variation
in the western whipbird. My results indicate that female response could be driving
song divergence in male songs, and that female-female competition could be driving
song divergence in female songs. Alternatively, the songs of both sexes may have
diverged through the random processes of cultural drift. My work contributes to our
understanding of the selective forces shaping male and, more importantly, female
song divergence among populations, and of how populations speciate.