Abundance, movement and habitat use of Swainson's hawks in their wintering grounds, Argentina

Abstract

Knowledge of habitat use by a particular species is necessary in developing species conservation plans. Large-scale mortality of Swainson’s hawks (Buteo swainsoni) in Argentina during austral summer 1995-96 identified the need to address several questions about relationships between hawks and their wintering ground environment. In 1996, an international and interdisciplinary research project involving numerous institutions and agencies was initiated to understand causes of Swainson’s hawk mortality and to prevent future mortality events. A research component addressed questions about the patterns of abundance; movement and habitat use of Swainson’s hawks in particular wintering areas. In this study, population abundances, individual movements and habitat use, both at population and individual levels of resolution, were analyzed at a landscape scale in two study areas. These areas were located in La Pampa and Santa Fe provinces of Argentina and the study was conducted in 1996-97 and 1997-98 austral summers. Field methods included systematic surveys, conventional radio tagging of individual hawks and focal observations. Densities of hawks varied between 4.40 and 5.02 hawks/km2 in La Pampa and 3.51 hawk/km2 in Santa Fe, with high variations in these values through the season. Hawks widely dispersed at regional level (far away from the limits of the study area) and used a defined study area with a random pattern, without constituting tight groups or flocks. Finally, both at population and individual levels, hawks selected permanent pastures (for foraging), plowed fields (for sunbathing/resting and sometimes for foraging) and woodlands (for roosting), but their clustered spatial distribution was not associated with the abundance of these selected habitat types. These results reflected similarities in the pattern of habitat use with the breeding areas, and they could be expected from adaptations of a migratory raptor to the dynamic changes of heterogeneous agricultural landscapes, especially in relation to food availability. Patterns of habitat use showed a combination of risk factors that made hawks susceptible to pesticide applications in their wintering ground. In addition, values of population abundance found in this study as well as patterns of movement gave a baseline for the development of conservation measures directed to explore and prevent the incidence of agricultural practices, such as pesticide applications and changes in land use, on populations of Swainson’s hawks wintering in the Argentinean pampas.