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

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.