The great bustard Otis tarda in Spain: Present status, recent trends and an evaluation of earlier censuses Original Research Article

We estimate the size of the Spanish population of great bustards Otis tarda at around 17,000–19,000 birds, using the results of the most recent counts available. To evaluate the reliability of earlier censuses and estimations, we compared the results of the first census carried out in 1981–1982 with more reliable counts made at specific intensive study areas during 1987–1988 and 1993–1994. For these comparisons we selected five test areas where experienced observers had been studying great bustard populations during several years. All counts and sex-ratio samples recorded during 1987–1988 and 1993–1994 were similar between these two years and much higher than those obtained during 1981–1982 at the same areas. This supported our conclusion that many individuals, particularly females, had been missed during the 1981–1982 census. Extrapolating the results from test areas to the whole of Spain, we again obtained 17,000 birds as an estimate of the Spanish population. Since 1980, when the species was legally protected, numbers have probably remained stable in most areas, while the smallest marginal groups have tended to disappear.