Raptor assemblages in grasslands of Southern Brazil : species richness and abundance and the influence of the survey method

Abstract

Background: Raptor distributions and abundances are poorly known in southern Brazil. Given that raptors have low population densities, methods typically applied to census birds are usually inappropriate for surveying raptors. Roadside surveys allow large areas to be searched at a relatively low cost and are often applied to count raptors. We conducted eight paired roadside surveys (roadside point transects and roadside strip transects of 54.5 ~ 58.6 km long) to assess the species richness and abundance of raptors in two grassland landscapes of southern Brazil and to compare results of the two methods. Results: In total, 1,890 observations of 18 raptor species (118.13 individuals (ind.)/transect) were recorded. Raptor abundances were higher in the altitudinal grasslands (299.25 ± 48.91 ind./transect) than in the coastal plain (86.63 ± 44.04 ind./transect). Abundances were higher when raptors were surveyed with point transects (9.38 ind. · [10 km2]−1 · h−1) than with strip transects (3.55 ind. · [10 km2] −1 · h−1). No significant differences were found in relation to species richness, although five species were exclusively recorded with point transects versus one with strip transects. Conclusions: Roadside point transects may reduce bias due to differences in species detection caused by habitat features and in abundance estimates resulting from survey habitat being disproportional to their availability. Point counts may enhance the quality of the search for raptors in a surveyed area, increasing the detection probability. The choice of survey method depends on the objectives of the study, but our results favor the use of point transects rather than strip transects to survey raptors in open landscapes.