Is driven transect sampling suitable for estimating red-legged partridge Alectoris rufa densities
Borralho, R., Rego, F. & Vaz Pinto, P. 1996: Is driven transect sampling suitable for estimating red-legged partridge Alectoris rufa densities? - Wildl. Biol. 2: 259-268.
An omnibus procedure for Alectoris partridges density estimation is still lacking. The suitability of driven line and strip transect sampling for estimating red-legged partridge Alectoris rufa densities on mixed farmland, in southern Portugal, in March, May and July 1993 and 1994 was evaluated. Driven transect counts along a grid of dirt tracks were performed and compared to early spring territory mapping counts. Distance data sets exhibited prominent peaks near the origin, apparently caused by attraction of the birds to the tracks; nevertheless, line transect estimates computed using Fourier series models were similar to mapping counts in March of both years, as well as adult density estimates in July, but density was underestimated in May 1994. Line transect estimates were robust to the shift from individuals to clusters as sighting units, and to different grouping options, but not to data truncation. Driven strip transect estimates were computed using 10 m, 20 m, 40 m, 80 m, and 160 m strip widths. In March, estimates computed using the first three widths overestimated density and underestimated it in 1994 when the 160-m strip was used. May and July adult density estimates were not significantly different from those in March. In this study, driven line transect sampling was an efficient and accurate method of estimating red-legged partridge density on a farmland area in early spring and summer; however, further research is needed to evaluate its suitability in habitats of lower visibility, as detection models seem to require long-distance sightings to compensate for the attractiveness of tracks. Strip transects are not recommended because of the difficulty of choosing an adequate strip width for density estimation. Both methods revealed similar utility as indices of relative population abundance.
Key words: Alectoris rufa, driven transect, line transect, mixed farmland, Portugal, red-legged partridge, strip transect
Rui Borralho*, Departamento de Engenharia Florestal, Instituto Superior de Agronomia, Tapada da Ajuda, 1399 Lisboa Codex, Portugal
Francisco Rego, Departamento de Matemática, Instituto Superior de Agronomia, Tapada da Ajuda, 1399 Lisboa Codex, Portugal
Pedro Vaz Pinto, ERENA, Av. Visconde Valmor, 11-4º, 1000 Lisboa, Portugal
*Present address: ERENA, Av. Visconde Valmor, 11-4º, 1000 Lisboa, Portugal
Received 20 September 1996, accepted 23 December 1996
Associate Editor: Peter J. Hudson