The diet of nestlings of three Ardeidae species (Aves, Ciconiiformes) in the Axios Delta, Greece

The diets of the little egret (Egretta garzetta), the night heron (Nyctieorax nyctieorax) and the squacco heron (Ardeola ralloides) were studied by analyzing nestling regurgitations collected during five breeding seasons (1988-1990 and 1994-1995) at a heronry in the Axios Delta (Northern Greece). In total, 267 regurgitations from little egrets, 247 from night herons and 19 from squacco herons (only in 1995) were collected and analyzed. Each prey item was identified to the lowest possible taxon. The dry mass of each prey taxon was also estimated from oven-dried prey specimens collected in the field. At least 58 different prey taxa were identified among 5,108 items found in little egret regurgitations, at least 45 taxa among 2,373 items regurgitated by night herons and 12 taxa among 277 items from squacco herons regurgitations. Differences were detected between the three ardeid species in the proportion of each prey category. Little egret nestlings were mainly fed fish (39.6\% by number) and insects (32.0\%), but amphibians and fish were the most important groups by biomass (44.9\% and 32.9\% respectively). The proportions of prey categories varied significantly between years (X-12(2)= 922.91, p < 0.001). The night heron nestlings were mainly fed insects and the crustacean Triops cancriformis, although the dry mass of the latter contributed little to the consumed biomass (4.0\% compared to the 37.9\% of insects). Small mammals and reptiles were included in the diet of the young night herons, while the fish they ate were much bigger than those consumed by little egret nestlings. The proportions of the night herons' main prey categories varied significantly between years (X-24(2) = 598.67, p < 0.001). Squacco heron chicks were fed mainly insects (50.9\%) and amphibians (31.8\%), the latter being more important by biomass (73.6\%). In a cluster analysis, diet of young little egrets and night herons showed greater similarity from 1988 to 1990 than between 1994 and 1995, suggesting temporal changes in prey use. Study years tended to group separately for each species, but those of the little egret were more scattered in the cluster than the night heron's, thus reflecting greater prey use variability. The diet of the squacco heron was similar to that of the little egret when prey type frequencies were considered, but closer to the night heron's by dry mass. The dry mass differed significantly between the species (X-2 = 87.39, p < 0.001, Kruskal-Wallis test). This dietary segregation may be a mechanism that reduces competition among these ardeid species, especially when prey is limited.