Estimating the size of Little Grebe (Tachybaptus ruficollis) breeding populations

Full article

Published:

Volume 44(2), December 1997. Pages 157-161.

Language:

English

Keywords:

breeding population, census biases, census techniques, Europe, Little Grebe, NE Spain, population size

Abstract:

The estimation of Little Grebe Tachybaptus ruficollis breeding numbers has been found to be quite difficult, given the shy behavior of breeding birds. Different methods have been used, although their accuracy has never been tested. The breeding population was estimated in 1994 at 27 different water bodies in the Llobregat Delta (NE Spain). Breeding numbers were estimated by two methods: a) call census plus territory mapping, and b) direct nest searches. Breeding call surveys gave a 19% underestimate as compared with direct nest searches. Differences occurred in localities with more than one pair. The reliability of both methods was not affected by pond size or density of breeding pairs. However, distance between nests was lower in erroneously censused ponds and nest location also introduced biases. Localities with error in breeding call surveys had less vegetated edges than the rest. Call censuses were a good method for determining the presence of breeding birds, although the number of breeding pairs was usually underestimated. Estimates obtained by nest searches were more accurate but they required a greater investment of field work. The choice between these two methods will depend on research objectives and study area characteristics.