| Sumario: | The chough Pyrrhocorax pyrrhocorax is at present a rare species in the Palearctic region, probably due to recent changes in traditional agro-pastoral systems. Naturally a cliff-nesting species, it has, however, recently occupied new areas with suitable feeding habitats using old buildings as nesting sites. Nest-site availability, breeding density, nest spacing and population numbers of choughs were compared in three widely spaced Spanish areas (totalling 1213 km2). In Madrid, nest availability is limited to cliffs, leading to higher breeding density and nest aggregation than in Los Monegros and Campo Azalvaro, where choughs use buildings. These artefacts maintain large populations (we estimated 1175 breeding pairs in 9716 km2) that are expanding, increasing the species' range and linking up with the original and isolated montane nuclei. Human disturbance, persecution and the near disappearance of old buildings are the main threats for these new populations.
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