Correlated divergence of ecology, morphology, and fine-scale vocal motor performance among sparrow subspecies

Abstract Animal structures often evolve for multiple functions, such as limbs used for both climbing and fighting. Adaptations that optimize one function may limit performance of others. Here, we compare songs of inland (freshwater) and coastal (saltmarsh) subspecies of swamp sparrows (Melospiza georgiana) for which adaptation to distinct habitats has driven divergence in beak size and shape. Prior studies have shown that these subspecies’ songs differ in a broad-scale metric of vocal performance—vocal deviation—in a direction consistent with the hypothesis that vocal performance is disproportionately constrained in larger-beaked birds. We hypothesize that beak divergence has also driven divergence in additional, more fine-scale phonological and temporal measures of song including note types and the acoustic structure of notes and inter-note gaps. Consistent with expectations, coastal birds’ songs show greater proportions of low-performance notes and note transitions, notes and inter-note gaps that span narrower bandwidths, and notes situated farther from presumed performance maxima. Yet in contrast to expectations, coastal birds’ songs show shorter average inter-note gaps, suggesting a compensatory strategy to maintain baseline levels of vocal performance. We also find that, for each subspecies, males maximized performance of the parameters they sang with the least consistency, a finding that highlights a need to consider vocal consistency not in isolation, but rather in relation to other performance metrics. This study enhances our understanding of a previously developed example of how ecologically driven changes in morphology may cause divergence in display performance for sexually selected traits, which in theory could drive further divergence among lineages.