'Consorci del Museu de Ciencies Naturals de Barcelona'
Abstract
We studied bias in density estimations derived from strip transects in dry open–country in the Canary Islands. We also present some critical remarks on García–del–Rey’s (2005) paper regarding strip transects and the validity of comparisons based on population densities of birds in scrublands on Tenerife island using two different methods: territory mapping and strip transect sampling. Although strip transects with census belts of 25 m do not account for detectability, this method only slightly undervalues true density estimates, and allowed to detect more than 85% of birds present in poorly vegetated environments in the Canary Islands. Previously published works on distribution and abundance of terrestrial birds in the Canary Islands using the strip transect sampling with belts of 25 m on both sides of the observer, thus provide reliable information that only slightly underestimates true densities.
Key words: Birds, Canary Islands, Census methods, Strip transects, Open–country environments, Population
density, Territory mapping.We studied bias in density estimations derived from strip transects in dry open–country in the Canary Islands. We also present some critical remarks on García–del–Rey’s (2005) paper regarding strip transects and the validity of comparisons based on population densities of birds in scrublands on Tenerife island using two different methods: territory mapping and strip transect sampling. Although strip transects with census belts of 25 m do not account for detectability, this method only slightly undervalues true density estimates, and allowed to detect more than 85% of birds present in poorly vegetated environments in the Canary Islands. Previously published works on distribution and abundance of terrestrial birds in the Canary Islands using the strip transect sampling with belts of 25 m on both sides of the observer, thus provide reliable information that only slightly underestimates true densities.
Key words: Birds, Canary Islands, Census methods, Strip transects, Open–country environments, Population
density, Territory mapping.Se estudian los sesgos derivados del empleo del método del taxiado (transectos lineales con bandas de 25 m a cada lado del observador) para obtener densidades en ambientes estepáricos de las Islas Canarias. También se presentan algunos comentarios críticos al trabajo de García–del–Rey (2005) que compara estimas de densidad obtenidas en Tenerife utilizando dos métodos diferentes: mapeo de territorios y transecto lineal. Aunque el método del taxiado estima densidades relativas no corregidas por la detectabilidad de las especies, este método proporciona valores de densidad muy parecidos a los reales, ya que permite detectar en ambientes con poca cobertura vegetal a más del 85% de los individuos dentro de bandas de 25 m a cada lado del observador. Por tanto, los trabajos previamente publicados sobre densidades de aves en Canarias proporcionan estimas fiables sólo ligeramente infravaloradas.
Palabras clave: Aves, Islas Canarias, Métodos de censo, Transecto lineal, Ambientes estepáricos, Densidad
de población, Mapeo de territorios
