In 1992-95, 11-13 colonies of Rooks were inspected in northern Drenthe, totalling 6128 nests. A colony was defined as a group of nests at least 100 m away from other nests. Most Rooks nested in close proximity to one another, with >50% of the pairs in trees with >5 nests. The number of nests/tree was usually small (Fig. 1), but 85% of the Rooks nested in trees with at least two nests (Fig. 2). The median number of nests/colony varied only slightly between years (Table 1). Oak Quercus robur and beech Fagus sylvatica accounted for almost 85% of all nesting trees, but eleven other tree species were found to be occupied by Rooks. What tree species is used as nesting tree is mainly governed by their local availability, relative to feeding sites.