LARGE-SCALE MOVEMENT PATTERNS BETWEEN SONG DIALECTS IN BROWN-HEADED COWBIRDS (MOLOTHRUS ATER)

Auk, The, Jul 2005 by Anderson, Kurt E, Rothstein, Stephen I, Fleischer, Robert C, O'Loghlen, Adrian L

ABSTRACT. -

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.

Key words: Brown-headed Cowbird, dispersal, geographic variation, local adaptation, mark-recapture, Molothrus ater, song dialects.

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

RESUMEN. - Investigaciones exhaustivas previas han intentado determinar si los dialectos de los cantos representan sistemas sociales aislados reproductivamente, de modo que los individuos tienden a tener un único dialecto durante toda su vida. Estudiamos este tema analizando datos de anillado de Molothrus ater provenientes de la ladera este de la Sierra Nevada de California. Durante 14 años, 1393 juveniles y 2568 individuos maduros fueron anillados a lo largo de una transccta de 40 km que incluyó tres dialectos. De estas aves juveniles y maduras, el 7.9% y el 12.1%, respectivamente, fueron recapturadas en un ano subsecuente. Todas las clases de aves maduras (machos del segundo ano, machos mas viejos y hembras) presentaron tasas de recaptura significativamente mayores que las aves anilladas como juveniles, pero no hubo diferencias entre las clases de aves maduras. En total, el 22.7% de los 110 juveniles recapturados en un año subsecuente fue atrapado en una región con un dialecto distinto al de la región en la que fue anillado, en comparacion con el 8.1% de los 310 individuos maduros. Ni las aves juveniles ni las adultas mostraron diferencias relacionadas con el sexo en la proporción de individuos recapturados en los años subsiguientes en regiones con diferentes dialectes. Las aves en todas las clases de sexo y edad presentaron mayores probabilidad de haberse desplazado a un nueva región cuando fueron recapturadas en un año subsecuente que cuando fueron recapturadas en el mismo año, lo que sugiere que los movimientos aparentes entre años fueron casos de dispersion, no viajes de forrajeo de corta duración. Aunque nuestros datos de anillado no pueden confirmar el flujo de genes, los altos niveles de movimiento que evidenciaron concuerdan con estudios genéticos y morfométricos que indican altos niveles de flujo génico entre estos dialectos de M. ater.

IN MANY BIRD species, song varies geographically as spatially discrete dialects (Mundinger 1982, Baker and Cunningham 1985, Catchpole and Slater 1995). Conforming to a local song dialect is likely to be adaptive, because female songbirds generally prefer males with local song types (Rothstein and Fleischer 1987a; Chilton et al. 1990; Lougheed and Handford 1992; O'Loghlen and Rothstein 1995, 2003; Bensch et al. 1998; Nelson 2000; MacDougall-Shackleton ec al. 2002). If dialects cause restricted dispersal or restrict the mating success of dispersers, they may add to reproductive isolation among dialect regions and, therefore, enhance genetic adaptation to local ecological conditions (Marier and Tamura 1962; Nottebohm 1969; Baker 1982, 1983; Catchpole and Slater 1995; Slabbekoorn and Smith 2002). Two studies indicated that gene flow is restricted between song dialects in the White-crowned Sparrow (Zonotrichia leucophrys; Baker and Mewaldt 1978, MacDougallShackleton and MacDougall-Shackleton 2001 ), but others have found no relationships between gene flow or genetic structure and dialect boundaries in other songbirds (Hafner and Petersen 1985, Payne and Westneat 1988, Fleischer et al. 1991, Lougheed and Handford 1992, Wright and Wilkinson 2001). Furthermore, songs are learned in many species and crystallized after dispersal from natal breeding areas (O'Loghlen 1995, Baptista and Gaunt 1997, Payne and Payne 1997), which would promote gene flow by allowing dispersers to function effectively in a non-natal dialect (Slabbekoorn and Smith 2002, Filers and Slabbekoorn 2003).