GROUP FORAGING IN LITTLE EGRETS (EGRETTA-GARZETTA) - FROM FIELD EVIDENCE TO EXPERIMENTAL INVESTIGATION

Results from both field and experimental foraging studies of little egrets (Egretta garzetta) are used to discuss the relationship between flocking, prey density and foraging efficiency. In the field, the latter was affected by group size. The two parameters studied, striking efficiency (percentage of successful pecks) and the number of captures per minute improved in large groups. On the other hand, only in one of two field sites did the results reveal an improvement under the form of a reduced variance in striking efficiency, of birds in a small group as compared to solitary birds. During the experiments, the number of captures per minute was higher in a pool with high prey density than in situation with low prey density, while differences were not significant when comparing medium with low or high prey density. Group size had no effect on capture rate. Striking efficiency, searching effort and prey selectivity were not affected by flocking or prey density. The possible benefits of flock feeding are discussed. © 1990.