Geographic variation in White-throated Sparrow song may arise through cultural drift

Abstract

Geographic variation in song may arise to facilitate assortative mating among locally adapted individuals or to increase ability to communicate within local neighbourhoods. Alternatively, geographic song variation may be functionally neutral, emerging as a by-product of idiosyncrasies of founder effects and song learning in peripheral populations. We studied differences in the terminal notes of songs in White-throated Sparrows (Zonotrichia albicollis) in a peripheral population found west of the Rocky Mountains. Unlike the common triplet of notes found in terminal strophes of the main distribution of this species in eastern North America, birds in Prince George, British Columbia, Canada, had terminal notes composed of doublets. We mapped song variation on an east–west transect through central Alberta, Canada, but were unable to find a sharp division between areas where the terminal notes shift; instead, we found evidence of a cline of increasing frequency of doublet endings from east to west. We performed playback experiments of local versus foreign song types in populations where doublet terminal strophes (Prince George, BC) versus triplet terminal strophes (Algonquin Park, ON) predominate. We found a significant difference between the populations, with the western birds responding more strongly overall than the eastern birds, and males in both populations responding more strongly to the doublet-ending song type. Our results do not support the origination of regional variation to discriminate among local populations, but are consistent with neutral cultural shifts.