We investigated the emergence over time of a novel song variant (doublet-ending song) in a western Canadian sub-population of white-throated sparrows; this variant differs from the species-typical, triplet-ending song. By analysing recent (1999–2014) and historic (1950/1960s) recordings, we show that populations west (British Columbia) and immediately east (Alberta) of the Rockies, and from central Canada (Ontario) initially all had triplet-ending songs. The shift to doublet-ending songs first arose west of the Rockies, and has increased immediately east of the Rockies in the last decade. The Ontario population retained predominantly triplet-ending songs. Note lengths have increased over time in all populations, while inter-strophe interval has decreased, allowing doublet-ending birds the ability to have greater strophe repeats for a given song length. We explore whether the emergence and apparent spread of the doublet-ending songs can be explained by cultural drift, or may be under selection by conveying an advantage during counter-singing.
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