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Purple Finch ( Haemorhous purpureus ) populations in eastern and western North America differ in morphological traits and genetic characteristics, yet their vocal divergence has not been quantified. We analyzed flight-call structure in Eastern and Western birds using measurements of frequency and call duration. Multivariate analyses revealed strong acoustic differentiation between subspecies, driven primarily by differences in maximum frequency, minimum frequency, and frequency span. Duration was not significantly different. Variation among subspecies in flight call characteristics was much greater than variation within individuals or within subspecies. Discriminant function analyses correctly identified calls in 93% of samples that were originally identified by the recordist or by assignment based on known geographic ranges. The five misclassified birds were all recorded in the West, had been assigned subspecies status based on range, were detected only outside the typical breeding season and were statistically Easterns. After re-assignment, the chance of misclassifying a bird to the incorrect subspecies was near zero. Only one bird in our sample, recorded in the putative zone of overlap in the Pacific Northwest, was marginally intermediate in its frequency characteristics. The reliable identification of flight calls is useful because Eastern and Western Purple Finches are highly distinctive genetically and may warrant full species status. In addition, the growing popularity of public archives for bird sound recordings increases the possibilities to track movements of infraspecifically diverse taxa like Purple Finches that are highly vocal but whose plumage variation is subtle. Finally, the existence of structurally divergent calls suggests the possibility of broader ecological and evolutionary differentiation.