Darwin's finches are well known for their remarkable diversity in beak form and function. Field studies have shown that beaks evolve by natural selection in response to variation in local ecological conditions. We posit a new hypothesis: As a consequence of beak evolution, there have been changes in the structure of finch vocal signals. We base this hypothesis on the discovery that beaks play a functional role in song production in songbirds. Recent field studies provide support for a link between beak morphology and song structure in Darwin's finches, although much remains to be learned. Because song plays a significant role in finch mating dynamics, we suggest that the functional link between beaks and song may have contributed to the process of speciation and adaptive radiation in these birds.