Two censuses of the only nesting population of Rooks known for Spain were carried out during 1993 and 1994 in the southeast of León province, northwestern Spain. We found 15 colonies in 1993 with an estimated population of 3,147±147 birds, and 17 colonies with 1,540 nests in 1994, with an estimated population of 3,413 (171 birds). As compared with previous studies, a decline in the number of colonies and a closer aggregation of their spatial distribution was noted, the current mean distance among colonies being 1.82 km. In 1994, the area occupied by the Rook population was 700 square kilometres (710 x 10 km U.T.M squares). The most frequent nesting site has changed from Populus alba (as cited in previous studies) to Populus x canadensis and Platanus hispanica in the present study. The causes of the increase of the population of Corvus frugilegus (95/9.8%) in the last 15 years and the concentration in the spatial distribution of the colonies are discussed.
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