HOMING SUCCESS OF MIGRANT VERSUS NONMIGRANT DARK-EYED JUNCOS (JUNCO HYEMALIS)

ABSTRACT

Within a population, the evolution of migratory behavior is accompanied by a suite of physiological, behavioral, and cognitive adaptations. Spatial memory is associated with the hippocampus in mammals and birds; in some cases, hippocampal neuroanatomy correlates with differences in behavior. In a recent study, a migratory subspecies of sparrow, the Dark-eyed Junco (Junco hyemalis hyemalis), performed better on room-scale spatial memory tests than did a nonmigratory conspecific. Migrants collected after migration also possessed greater hippocampal neuron density, which suggests a neurological basis for differences in spatial memory and a link between migratory behavior and enhanced spatial memory. It is likely that homing behavior, like migration, relies to some extent on spatial memory. In some instances, spatial memory performance has generalized across spatial scales, with pronounced differences at larger scales. We tested whether differences in spatial memory between migrants and nonmigrants, previously observed at a room scale, were detectable at a landscape scale; specifically, we investigated whether differences in homing ability could be detected after displacements of 1-40 km. We found no difference in number of returning individuals or in duration of return. Our results suggest that homing in this species may not rely on aspects of spatial memory that differed in aviary tests. Received 7 November 2003, accepted 15 November 2004.

Key words: Dark-eyed Junco, homing, Junco hyemalis, migration, spatial memory.

Éxito de Retorno al Sitio Natal de Individuos Migratorios y No Migratorios de Junco hyemalis

RESUMEN.-Dentro de una población, la evolución del comportamiento migratorio está acompañada por una serie de adaptaciones fisiológicas, comportamentales y cognitivas. La memoria espacial está asociada con el hipocampo en los mamíferos y las aves; en algunos casos, la anatomía nerviosa del hipocampo se correlaciona con diferencias en el comportamiento. En un estudio reciente, una subespecie migratoria, Junco hyemalis hyemalis, se desempeñó mejor en una prueba de memoria espacial a escala de habitation que un coespecifico no migratorio. Las aves migratorias colectadas luego de la migration también presentaron mayor densidad neuronal del hipocampo, lo que sugiere una base neurológica para las diferencias en la memoria espacial y una conexión entre el comportamiento migratorio y una capacidad aumentada de memoria espacial. Es probable que el comportamiento de retorno al sitio natal, como la migración, se base en cierta medida en la memoria espacial. En algunos casos, el desempeño de la memoria espacial se ha generalizado a través de las escalas espaciales, con diferencias pronunciadas a escalas mayores. En este estudio evaluamos si las diferencias en memoria espacial entre individuos migratorios y no migratorios, previamente observadas a escala de habitación, eran detectables a escala de paisaje. Específicamente, investigamos si las diferencias en la habilidad de retornar al sitio natal podían ser detectadas luego de desplazamientos de entre 1 y 40 km. No encontramos diferencias en el número de individuos que retornaron o en la duración del retorno. Nuestros resultados sugieren que la habilidad de retornar al sitio natal en esta especie podría no basarse en los aspectos de la memoria espacial que difirieron en las pruebas de aviario.

THE EVOLUTION OF migratory behavior-which usually requires the ability to navigate -has likely resulted in a suite of physiological, behavioral, and cognitive adaptations (Dingle 1996). Navigation requires an animal to assess its spatial position in relation to a goal. Navigating back to a familiar location requires memory and may involve memory of many intermediate spatial landmarks. Thus, for a migratory bird that returns to the same breeding and wintering territories (or both) each year, enhanced spatial processing abilities would be adaptive.

Although the link between spatial memory and migration -or the related behavior, homing-has not been well documented, returning to a familiar location after migration or displacement would be facilitated by accurate memory of spatial locations. Spatial memory is associated with the hippocampus in several mammals and birds. Many studies indicate that hippocampal neuroanatomy correlates with differences in behaviors that may involve spatial memory (Krebs et al. 1989, 1996; Sherry et al. 1989; Jacobs et al. 1990; Jacobs and Spencer 1994; but see Brodin and Lundborg 2003). Because the avian hippocampus is believed to be important in spatial processing, it is reasonable to assume that it also functions in navigation (Bingman et al. 1999, Macphail 2002), migration (Jacobs 1996), and homing.

The relationship between hippocampal size and enhanced spatial memory has been investigated in migratory birds. Migration-experienced (adult) Garden Warblers (Sylvia borin) possessed proportionately larger hippocampi than migration-naive (young) birds-a contrast not seen in the congeneric, nonmigratory Sardinian Warbler (S. mdanocephala; Healy et al. 1996). In a laboratory choice test with the same two warbler species, migrants remembered a location with more food for ≤12 months, whereas nonmigrants remembered the location for only two weeks (Mettke-Hofmann and Gwinner 2003). However, earlier homing studies with the two species suggest that the migratory Garden Warbler is not more likely to home successfully than the nonmigatory Sardinian Warbler (Ioale and Benvenuti 1983, Baillon et al. 1992).