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Vol. 52, 2013
Raptor assemblages in grasslands of Southern Brazil: species
richness and abundance and the influence of the survey method
Felipe
Zilio1,5*, Alan Bolzan2, André de Mendonça-Lima3,
Cristiane Oliveira da Silva3,
Laura Verrastro4,5 and Márcio Borges-Martins4,5
1Programa
de pós-graduação em Biologia Animal, Departamento de Zoologia,
Universidade Federal do Rio Grande do Sul, Av Bento Gonçalves, 9500,
Porto Alegre CEP 91501–970 Rio Grande do Sul, Brazil
2Curso de Ciência Biológicas, Departamento de
Ecologia, Universidade Federal do Rio Grande do Sul, Av Bento
Gonçalves, 9500, Porto Alegre CEP 91501–970, Rio Grande do Sul, Brazil
3Programa de pós-graduação em Ecologia, Departamento
de Ecologia, Universidade Federal do Rio Grande do Sul, Av Bento
Gonçalves, 9500, Porto Alegre CEP 91501–970, Rio Grande do Sul, Brazil
4Curso de Ciência Biológicas, Departamento de
Zoologia, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS,
Brazil
5Laboratório de Herpetologia, Departamento de
Zoologia, Universidade Federal do Rio Grande do Sul, Av Bento
Gonçalves, 9500, Porto Alegre CEP 91501–970 Rio Grande do Sul, Brazil
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.
Key words: Diurnal raptors; Open landscapes;
Point transect; Roadside survey; Strip transect.
*Correspondence: E-mail: fzilio@msn.com
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