Diet composition, guild structure and trophic relationships of wintering birds of prey in an estuarine wetland (The Evros Delta National Park, Greece)

Seven species of birds of prey studied in the Evros Delta National Park belong to a common assemblage of a trophic guild. However, their diet study revealed different groups of feeding niches. A cluster analysis provided three groups with very similar diets: The eastern imperial eagle (Aquila heliaca) and greater spotted eagle (Clanga clanga) constituted a group feeding mainly on birds with their diets including 96% (13 bird species) and 98% (16 bird species), by number respectively, most bird prey being water birds. A second group, characterized by a considerable variety of prey types dominated by small mammals and birds, included the western marsh harrier (Circus aeruginosus) (67% mammals, 27% birds -at least 12 species ), black kite (Milvus migrans) (46% mammals, 39% birds -at least 11 species) and common  buzzard (Buteo buteo) (birds 36% -at least 7 species, mammals 22%). In all three species, birds dominated by biomass, whereas other prey such as arthropods (mainly insects), amphibians and reptiles had a low contribution by mass where they occurred. A third group included the common barn owl (Tyto alba), feeding mainly on small mammals (97%, at least 12 species) and little owl (Athene noctua). The little owl’s diet consisted of mammals (29%, -at least 6 species), birds (5% -at least 7 species) and, though insects preponderated by number (64%), mammals were dominant by biomass in both owls. Diversity indices estimated on class level were low, ranked as B. buteo > M. migrans > A. noctua > C. aeruginosus > A. heliaca = T. alba > A. clanga. Dietary similarities may be partly due to common habitat preferences and the role of other potential factors to niche partitioning is discussed.