Animal mating signals evolve in part through indirect natural selection on anatomical traits that influence signal
expression. In songbirds, for example, natural selection on beak form and function can influence the evolution of song
features, because of the role of the beak in song production. In this study we characterize the relationship between
beak morphology and song features within a bimodal population of
Geospiza fortis
on Santa Cruz Island, Galápagos.
This is the only extant population of Darwin’s finches that is known to possess a bimodal distribution in beak size.
We test the hypothesis that birds with larger beaks are constrained to produce songs with lower frequencies and
decreased vocal performance. We find that birds with longer, deeper, and wider beaks produce songs with significantly
lower minimum frequencies, maximum frequencies and frequency bandwidths. Results from the analysis of
the relationship between beak morphology and trill rate are mixed. Measures of beak morphology correlated positively
with ‘vocal deviation’, a composite index of vocal performance. Overall these results support a resonance model
of vocal tract function, and suggest that beak morphology, a primary target of ecological selection in Darwin’s finches,
affects the evolution of mating signals. We suggest that differences in song between the two modes of the distribution
may influence mate recognition and perhaps facilitate assortative mating by beak size and population
divergence. © 2006 The Linnean Society of London,
Biological Journal of the Linnean Society
, 2006,
88
, 489–498.
ADDITIONAL KEYWORDS:
bird song – Galápagos – Geospizinae – vocal performance.