BIRD SONG: The Interface of Evolution and Mechanism

Abstract

▪ Abstract Bird song provides an unusually impressive illustration of vertebrate behavioral diversification. Research on bird song evolution traditionally focuses on factors that enhance song diversity, such as cultural transmission and sexual selection. Recent advances in the study of proximate mechanisms of vocal behavior, however, provide opportunities for studying mechanistic constraints on song evolution. The main goal of this review is to examine, from both conceptual and empirical perspectives, how proximate mechanisms might temper patterns of song evolution. We provide an overview of the two “substrates” of song evolution, memes and vocal mechanisms. We argue that properties of vocal mechanisms (control, production, and ontogeny) constrain vocal potential and may thus limit pathways of meme evolution. We then consider how vocal mechanisms may constrain song evolution under five scenarios of drift and selection and examine four specific song traits for which mechanistic constraints appear to counter the diversifying effects of sexual selection. These examples illustrate the interplay between meme evolution as a diversifying influence and proximate limitations as a barrier to song divergence. We conclude by suggesting that vocal mechanisms not only constrain song evolution but also can facilitate the evolution of novel vocal features.