We studied nest site selection and breeding success of Black-crowned Night-Herons (Nycticorax nycticorax) and Little Egrets (Egretta garzetta) in the Axios Delta (Greece) from 1988 to 1990. Both species nested above the middle of tamarisks and alders. Nest density varied each year from 333.3 nests/ha to 646.2 nests/ha for Blackcrowned Night-Herons and 291.7 nests/ha to 421.5 nests/ha for Little Egrets. Black-crowned Night-Herons started to breed earlier than Little Egrets, also placing their nests higher (mean nest height: 4.21 ? SD of 0.80 m and 3.54 ? SD of 0.71 m for Black-crowned Night-Herons and Little Egrets, respectively). The nearest neighbors were conspecifics for Black-crowned Night-Herons and heterospecifics (Black-crowned Night-Herons) for Little Egrets. The mean distance of the nearest neighbor was similar in both species (1.02 ? SD of 0.46 m and 1.11 ? SD of 0.37 m for Black-crowned Night-Herons and Little Egrets, respectively). In contrast to late-nesting Little Egrets that located their nests lower, Black-crowned Night-Herons maintained a constant nest distance from the ground. The mean clutch size (3.40 ? SD of 0.60 and 4.32 ? SD of 0.81 for Black-crowned Night-Herons and Little Egrets, respectively) and the mean chick survival per nest (2.48 ? SD of 0.95 and 2.53 ? SD of 1.28 for Black-crowned Night-Herons and Little Egrets, respectively) of both species varied among the study years. No variation was observed in the means of clutch size and chick survival per nest of Black-crowned Night-Herons among the 3 sub-periods of the breeding season. Little Egret clutch size and mean number of eggs hatched was smaller in late nesters, but no difference was observed in chick survival per nest between early and late nesters. Nest placement did not affect chick survival in Black-crowned Night-Herons and only marginally in Little Egrets.

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Kerkini Lake in Greece, a reservoir maintained principally for agricultural irrigation purposes on the Strymon River, has been classified as a ‘Wetland of International Importance’ according to the Ramsar Convention. Hydrological conditions favoured the formation of a shallow, very productive lake, rich in species diversity and biotic abundance. The importance of the wetland lay in its complex zonation of various habitats which mainly consisted of: shallow open water with sand-strips, beaches, and mud-flats; shallow-water areas with communities of emergent vegetation; a lowland hardwood riparian forest mainly of Salix and Tamarix species; a shrub-swamp; and extensive wet meadows. The diversity of habitats supported a rich waterbird fauna for breeding, stationing during migration, and wintering.Nevertheless, in 1982 a new high-dam was constructed, higher embankments along the banks of the Lake were established, and other works were carried out (to increase the water capacity of the reservoir), in response to irrigation demands. No consideration was given to the wetland resources. Operation of the new dam resulted in changes in the flooding etc. regime, with severe impact on the biotic resources of the wetland, especially by waterlevel increases of more than 5 m in less than 4 months during spring — the critical growing- and breedingseason. Wetland heterogeneity and the mosaic structure of water-land-vegetation were thereby lost.Now the riparian forest is diminishing in area and declining in productivity, the reed-swamp with emergent macrophytes has been lost, and wet meadows have also disappeared. Deep flooding and the loss of these habitats is reflected in marked decline in the densities of migratory waterbird populations and, most important, in drastic shrinkage in the populations of breeding species. Similar impact is observed in the dramatic decline of fish species diversity and abundance. In general, nowadays, Kerkini presents an alarming example of loss of biodiversity and accelerating impoverishment of its former biotic resources.In view of the projected plans to increase further the water-storage capacity of the Lake by raising its waterlevel, measures are urgently needed: (a) for preventing such actions; (b) for restoring and maintaining marsh conditions, and (c) for ecologically sound management and sustainable use of the wetland resources.

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