Extensive past research has attempted to determine whether song dialects represent reproductively isolated social systems, with individuals tending to spend their entire lives in a single dialect. We addressed that issue by analyzing banding data for Brown-headed Cowbirds (Molothrus ater) on the eastern slope of the Sierra Nevada of California. For 14 years, 1,393 juveniles and 2,568 mature individuals were banded along a 40-km span encompassing three dialects. Of those juvenile and mature birds, 7.9% and 12.1%, respectively, were recaptured in a later year. All classes of mature birds (second-year males, older males, and females) had significantly higher recapture rates than birds banded as juveniles, but there were no differences among the mature classes. Overall, 22.7% of 110 juveniles recaptured in a subsequent year were trapped in a dialect region other than the one in which they were banded, as compared with 8.1% of 310 mature birds. Neither juvenile nor mature birds showed sex-related differences in proportions recaptured in subsequent years in different dialect regions. Birds in all sex-age classes were more likely to have moved to a new dialect region when recaptured in a subsequent year than when recaptured within the year, which suggests that apparent movements between years were cases of dispersal, rather than short-term foraging trips. Although our banding data cannot confirm gene flow, the high levels of movement they show agree with genetic and morphometric studies indicating high levels of gene flow among these cowbird dialects.

Patrones de Movimiento de Gran Escala entre Dialectos del Canto de Molothrus ater