VOLUME 19 - N. 2

Abstract:

The phenology and structure ofthe nest-sites of a Connorant colony in dead trees, during 1992- 1993, are described. Sites were c1assified using three criteria: (I) topography ofthe colony; (2) density of breeding trees; (3) structure ofbreeding trees. Photographs were used to mark the nests during the breeding periodo The number ofnests increased to a peak in early May and then declined; many nest sites were used successfully by more than one pair. The structure of nest sites was more important than their position. The percentage of sites reaching the nest stage decreased during the breeding season. Adult birds showing much white on the head are more frequent in the centre ofthe colony. Sites taken by breeding immature-plumaged birds are like those taken by adults, but immatures seem to succeed in building nests and breeding only on sites of lower quality. Beginning from 1992 some breeding attempts occurred on sites outside the present colony. Current level of competition between Cormorants and Grey Herons which breed in the same area is low. Differences between this and a similar German colony are discussed.