Relative Abundance, Habitat Use, and Seasonal Variability of Raptor Assemblages in the Flooding Pampas of Argentina

Abstract

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-cover 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. The Chimango Caracara (Milvago chimango) was the dominant species in all land-cover types and seasons. The Southern Caracara (Caracara plancus), American Kestrel (Falco sparverius), and Roadside Hawk (Rupornis magnirostris) were all abundant and very frequently observed species, whereas Burrowing Owl (Athene cunicularia), White-tailed Kite (Elanus leucurus), and Long-winged Harrier (Circus buffoni) were less abundant but recorded during most surveys. The remaining raptors (Aplomado Falcon [Falco femoralis], Cinereous Harrier [Circus cinereus], Short-eared Owl [Asio flammeus], Sharp-shinned Hawk [Accipiter striatus], and Snail Kite [Rostrhamus sociabilis]) were much less abundant in the study area. We also recorded four other raptor species (Variable Hawk [Geranoaetus polyosoma], Black-chested Buzzard-Eagle [G. melanoleucus], Striped Owl [Asio clamator], and Barn Owl [Tyto alba]), but only outside of the standard transect surveys. Species composition differed among land-cover types, 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-cover types, but other less abundant species, such as C. plancus, A. cunicularia, and R. magnirostris, were more important to differentiate land-cover types 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-cover patterns in the study area, many other species may be threatened by the expansion of urban areas and agriculture in the Pampas region.