Distribution, density and productivity of great bustards Otis tarda in northwestern Spain:: a regional approach

This paper describes at a regional scale the distribution pattern, density, productivity and sex ratio of great bustards in northwestern Spain and explores the role played by habitat type, terrain characteristics and human disturbance on variation in its demographic parameters. Data from 136 plots covering an area of 7314 km(2) were obtained in two censuses carried out in the spring and summer of 1998. The density of the great bustards was 1.39 individuals/km(2) in the pre-breeding period, decreasing by 22% in the post-breeding period. Density was significantly higher in central plots than in peripheral plots. Mean productivity was 0.24 chicks per female and showed a high variability among plots, being significantly lower in the densest plots. The overall sex ratio was 1.35 females per male during the pre-breeding period. Productivity related positively to areas holding small fields and a high interspersion of land uses. Density and productivity were negatively affected by river density and altitude, and seasonal density variation was positively correlated with human population density.