Microgeographic song variation in island populations of the white-crowned sparrow (Zonotrichia leucophrys nutalli):: Innovation through recombination

Current geographic variation in bird song dialects may be used to infer historical processes involved in dialect formation. Discrete island populations, separated by water or unsuitable habitat, may be particularly useful as they allow for unequivocal subdivision of populations. We analyzed song recordings of nine populations of the white-crowned sparrow within the San Francisco Bay area, both on 'true' islands separated by water and in habitat islands along the Bay shore. We found one or two unique songtypes in each population with little variation within songtypes among individuals. Differences in songtypes concerned variation in syntax, syllable shapes, and spectral and temporal features. While each songtype exhibited unique features, there were often parts of the song that showed high similarity with songtypes of neighboring populations. We think that successfully dispersing males may learn from multiple tutors and produce songtype hybrids which could explain the variation in songtypes among populations. We found rare acoustic features on the 'true' islands, but also an apparent lack of impact by water barriers on the relationship between song similarity and geographic distance. A possible explanation may be that male dispersal, or just spread of song characteristics, is predominantly from island to mainland. Such a unidirectional pattern could result in rare song characteristics on islands, while general acoustic characteristics are still shared with the nearest mainland populations. Variation in similarity-based clustering of songtypes suggested that new songtypes emerge through recombination of components from existing songtypes.