CONICET | Buscador de Institutos y Recursos Humanos

We evaluated the species composition, relative abundance, habitat use and seasonal variability of raptor assemblages in the Flooding Pampas of Argentina, which represents the southeastern part of the biome known as the Rio de la Plata Grasslands. We conducted seasonal roadside surveys to detect raptors in modified and natural habitats over a 3-yr period from spring 2006 through autumn 2009. We classified raptor species according to their relative abundances and occurrence frequencies, and compared the assemblage composition among land-use types (croplands, grazing fields, periurban areas, and grasslands) and seasons. The raptor assemblage in the Flooding Pampas comprised 16 species, representing approximately 43% of all raptor species in the biome. Milvago chimango was the dominant species in all land-use types and seasons. Caracara plancus, Falco sparverius, and Rupornis magnirostris were all very abundant and frequently observed species, whereas Athene cunicularia, Elanus leucurus, and Circus buffoni were less abundant but recorded during most surveys. The remaining raptors (Falco femoralis, Circus cinereus, Asio flammeus, Accipiter striatus, and Rostrhamus sociabilis) were much less abundant in the study area. We also recorded four other raptor species (Geranoaetus polyosoma, G. melanoleucus, Pseudoscops clamator and Tyto alba), but only outside of the standard transect surveys. Species composition differed among land-uses, but we detected no distinct overall seasonal patterns except that species diversity indices were lower in autumn and especially spring. Milvago chimango was important to determine similarity in assemblage composition within land-uses, but other less abundant species, such as C. plancus, A. cunicularia, and R. magnirostris, were more important to differentiate land-uses based on raptor composition. Species diversity was highest in grazing fields and grasslands, and lowest in periurban areas. Our results suggest that although some raptor species appear to benefit from land-use patterns in the study area, many other species may be threatened by the expansion of urban areas and agriculture in the Pampas region.