Bill morphology and neutral genetic structure both predict variation in acoustic signals within a bird population

Adaptive evolutionary divergence within a population can be facilitated by associated divergence in mating signals. Acoustic signals are often involved in mate choice and are also known to diverge spatially in response to a variety of processes. In birds, for instance, variation in bill size and shape can result in correlated changes in vocalizations due to functional constraints on sound production. Acoustic signals can also vary spatially in relation to neutral genetic structure (due to cultural drift) and/or habitat structure (due to acoustic adaptation for optimal sound transmission). Here, we test these alternative hypotheses as causes of variation in acoustic signal structure in the Island Scrub-Jay (Aphelocoma insularis), a species that is restricted to one small island (Santa Cruz Island, CA) and exhibits spatial genetic structure and microgeographic divergence in bill morphology across short distances and habitat types. We find that bill morphology is related to the structure of the female "rattle" call, a vocalization associated with territorial disputes and male-female interactions. Females with longer, shallower bills produced calls that were more rapid, and those with shallower bills also produced calls that were lower in frequency. In addition, rattle rapidity varied across the island in accordance with neutral genetic structure. Vocal characteristics were not related to habitat structure, suggesting that variation in rattle calls is unlikely to reflect optimization for sound transmission. Our findings indicate that selection on bill morphology and cultural drift can jointly shape variation in acoustic signal structure, even at fine spatial scales within populations.