Age-mediated spatial segregation of Swainson?s Hawks (Buteo swainsoni) in their wintering grounds in Argentina

SARASOLA, J.H.; SEOANE, J.; BECHARD, M.J.

Bariloche

Conferencia; I Worldwide Raptor Conference; 2013

Wintering habitat segregation by age is widespread among long distance migrant species. In some cases, older and more experimented individuals would select more suitable sites in wintering areas with those less experimented individuals occupying sub-optimal habitats. The Swainson?s Hawks is a long distance migrant raptor that breeds in U.S.A. and Canada and spend the boreal winter in central Argentina. It has been hypothesized that Swainson?s Hawks segregate in wintering areas according to their age but evidence supporting this assertion is scarce. In this work we examined the spatial segregation by age of Swainson?s Hawks wintering in agricultural landscapes of the Argentine pampas. Age class composition (adult vs. non- adults birds) was examined in 100 flocks (group size > 5 birds) of Swainson?s Hawks recorded during extensive surveys conducted in 2002-2004 austral summers. The proportion adult vs. non-adult was examined against land use and habitat variables at two different scales: at local site,recording habitat variables at a circular 500 m radius around the point where the flock was recorded, and at landscape scale in a 30x30 km plot that includes all the flocks recorded during 70 km surveys and eight point counts conducted at each plot. There was no evidence of complete segregation by age of wintering Swainson?s Hawks. Most of the flocks recorded were composed by a mix of adult and non-adult birds at different proportions. At local scale, however, proportion of adults in flocks was positively related with coverage of grasslands and negatively related to cultivated lands. When examined at landscape scale, however, the results were not conclusive. Our results indicate the lack of a complete segregation by age on wintering Swainson?s Hawks as it has been previously proposed. Some evidence, however, indicate a subjacent hierarchical mechanism of segregation where adult birds are found in higher proportion in the most favorable wintering habitat for the species.