Geographic variation in the songs of two closely related song-learning species, Anna's and Costa's hummingbird (Calypte anna, C. costae)

In species that learn their song, cultural transmission of song components can lead to the accumulation of variants that differ among populations, resulting in the formation of dialects. Three avian clades are thought to have independently evolved song learning - parrots, oscine passerines, and hummingbirds. Dialects have mainly been studied in passerines. We extend the study of dialects to the bee hummingbird clade, focusing on Annas and Costas hummingbirds (Calypte anna and C. costae). Both species are vocal learners. Annas produces complex, three phrase, multi-syllable songs and Costas produces simple, one phrase songs. We recorded 5-24 males per population (5 Costas and 6 Annas populations) across the species ranges in the Western United States and tested for evidence of geographic variation in song. We found minor population differences in frequency measures of Costas song, but song form was invariant across populations. Annas song was contrastingly variable with population differences in both syllable use and multiple spectral and temporal measures. The most strongly differentiated Annas population in our study, Seattle (Washington State), is the product of a recent northward range expansion facilitated by human activities that provide additional food sources for hummingbirds. The loss and modification of syllables in this population is suggestive of a founder effect on song. This study provides insight into song evolution in non-passerine vocal learners and contributes to understanding of how complex signals evolve.