PLATTEEUW M, BOTOND KISS J, YE ZMUD M & SADOUL N (2004) Colonial birds in large-scale wetlands: the Danube delta as an example for the delta of Rhine and Meuse?. LIMOSA 77 (2): 79-92.

During the breeding seasons of 2001 and 2002 two comprehensive surveys of colonially breeding waterbirds (Pelicaniformes and Ciconiiformes) were carried out in both the Romanian and Ukrainian parts of the Danube Delta. Apart from being the first time in history that this internationally important area for these groups of birds was comprehensively surveyed, the aim of this study was to obtain insight in the crucial factors affecting site selection and population size of colonial waterbirds. In 2002, the year with the most complete survey, a total of 209 colonies was found, holding almost 40,000 breeding pairs of 13 species. The most abundant species were Great Cormorant (22,787 pairs), Pygmy Cormorant (9,341 pairs) and Great White Pelican (4,150 pairs). Among the wading birds, remarkable figures included 3,340 pairs of Glossy Ibis, 2,964 pairs of Black-crowned Night-heron, 1,725 pairs of Little Egret and 1,279 pairs of Squacco Heron. For most of the species involved the survey was rather complete, except for Purple Heron. This is a notoriously skulky breeding bird of inundated reed beds, of which colonies are hard to find, both from the ground and during aerial surveys. Of the total of 236 different colony sites, located during both years, the vast majority was situated in trees (148). A further 60 were found in inundated reedbeds, 16 in reedbeds with sparse trees, 10 on bare ground on islets and two on floating aquatic vegetation. Therefore, all colony sites were well protected against access by terrestrial predators (e.g. Red Fox) and generally well out of the way of human influence. For Pygmy Cormorant and Squacco Heron, two examples of not very far ranging species with well-defined and different feeding grounds, the proximity of suitable feeding habitats (as mapped for the area by Hanganu et al. 2002) to the colony sites was of some influence on colony size. This and, even more, the fact that most colonies were situated close to the numerous smaller lakes and ponds in the central part of the delta, indicate that, besides safety of the actual breeding site, the availability of feeding areas also played an important role. Probably, the decisions of potential breeders take place on different scale levels. Firstly, a global selection of a suitable area is made on the basis of availability of good feeding grounds within a species-dependent flight range (e.g. 50-60 km for pelicans and up to 10 km for smaller herons). The farthest ranging species also generally have the highest food requirements and therefore need larger feeding areas. For these species, colonies may establish all over the area, but for smaller herons and ibises the area is largely confined to the central part. Within this 'flyable' range, the safest and most isolated sites will be chosen as colony sites. Then, the actual colony sizes will depend on both the availability of safe nest sites within each colony and the carrying capacity in terms of food availability within the daily flight range from the colony site. It is argued that these considerations on site selection and spatial configuration of feeding and breeding habitats are fundamental in restoration attempts for man-made river systems such as the Dutch Rhine and Meuse delta area. Spatial planning of these attempts should not only focus on local conditions but also take into account the actual and potential configuration of wetland habitats on a larger scale. Great White Pelican Pelecanus onocrotalus Dalmatian Pelican Pelecanus crispus Great Cormorant Phalacrocorax carbo Pygmy Cormorant Microcarbo pygmeus Western Great Egret Ardea alba = Egretta alba Grey Heron Ardea cinerea Purple Heron Ardea purpurea Little Egret Egretta garzetta Squacco Heron Ardeola ralloides Cattle Egret Bubulcus ibis = Ardeola ibis Black-crowned Night Heron Nycticorax nycticorax Eurasian Spoonbill Platalea leucorodia Glossy Ibis Plegadis falcinellus

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