Microgeographical variation in birdsong: Savannah sparrows exhibit microdialects in an island population

Abstract

Social communication often involves vocal learning, whereby young animals learn their vocalizations early in life by imitating the sounds of adults. In animals that learn their vocalizations, it is common to find patterns of geographical variation known as ‘vocal dialects’, acoustic features shared within a cluster of animals that differ from the vocalizations of animals in nearby clusters. Dialects may form when animals learn their vocalizations early in life and then disperse short distances, or when they modify their vocalizations to match local vocal patterns after dispersal. Dialects are typically studied at a regional or continental scale, but they may also persist at smaller scales in so-called ‘microgeographical song dialects’ or ‘microdialects’. Microdialects have received little research attention. In this study, we investigate microdialects and dispersal distance in song-learning Savannah sparrows, Passerculus sandwichensis, through a 4-year study of birds living on three islands in the Bay of Fundy, Canada. Our analyses yield evidence of microgeographical variation: birds showed higher acoustic similarity to their neighbours than to faraway individuals in the same population. When we classified songs on the basis of their highly variable middle sections, we found that particular song types formed spatial clusters of similar-sounding individuals. Therefore, acoustic variation in Savannah sparrow song appears to show dialects across small geographical distances. In an analysis of dispersal from natal sites to breeding sites, we found a median distance of 189 m, consistent with previous findings. Our results suggest that limited dispersal distances, combined with the social processes of vocal learning (overproduction and selective attrition), likely contribute to the presence of microdialects.