It's complicated: the association between songbird extrapair paternity and within-song complexity

Many songbird species are socially monogamous while exhibiting varying extrapair paternity frequencies. Song complexity, often subject to sexual selection, similarly varies across songbird taxa. Some species form highly complex songs whereas others produce simple songs. The basis of this variability, however, is unresolved. Because selection pressures generally favour the evolution of sexually selected characteristics reflecting male quality, such as song complexity, it should be subject to extrapair mate selection. We therefore predicted a positive association between extrapair paternity frequency and songbird song complexity. In addition, we predicted that broadcast (long-range) rather than interactive songs (short-range) would be more likely to contain sexually selected characteristics, such as higher complexity, especially in species with high extrapair paternity frequencies. This was tested using tui, Prosthemadera novaeseelandiae, a species with high extrapair paternity frequency. First, analyses on 78 songbird species indicated a significant positive association between extrapair paternity frequency and within-song complexity (the level of complexity within a species-specific song) but not between-song complexity (size of species-specific song or syllable repertoire), while no phylogenetic trait conservation was found. Additionally, our results suggested tui broadcast songs had higher song complexity than interactive songs. The findings in this study indicate extrapair mate selection may play a role in the evolution of within-song complexity in songbirds.