Within-population song evolution in white-throated sparrows (Zonotrichia albicollis)

Cultural evolution of birdsong occurs when songs change over generations and such changes can spread directionally or via drift within populations. We describe the emergence and directional spread of a novel song variant within a population of White-throated Sparrows in central British Columbia, Canada. Birds in this population have been singing a doublet-ending song since the early 2000s. In 2015, we detected a novel variant, consisting of a doublet-ending song with a distinctive amplitude modulation in the first note (Modulated-Doublet). We banded and recorded birds from 2015 to 2020 and classified songs using both audio and spectrographic analysis. The proportion of birds singing the Modulated-Doublet increased, replacing the Unmodulated-Doublet over time. Additionally, the modulation became more pronounced, both across the population and within individual birds, over successive years. The rapid spread suggests there may be several transmission biases driving the adoption of this novel song over the older established variant.