FLOCK FEEDING AND FOOD-INTAKE IN LITTLE EGRETS - THE EFFECTS OF PREY DISTRIBUTION AND BEHAVIOR

(1) Little egrets, Egretta garzetta, from a breeding colony in the Camargue, southern France, tended to feed in dense aggregations during early early morning. Later in the day the birds dispersed over the feeding area and foraged alone. (2) Of prey captured in these aggregations, 83% were mosquitofish, as revealed by regurgitates collected from chicks. (3) Capture rate of individuals within an aggregation decreased rapidly with time (-0.22 fish min-2) but the birds were able to improve their food intake by visiting a new site as maximum capture rate, measured directly after the initiation of an aggregation, decreased by only 0.06 fish min-2. So the egrets visited a series of aggregations and, as a result, the average capture rate decreased from 6.6 fish min-1 at 06.00 h to 2.0 fish min-1 at 08.30 h. (4) Sites used by these aggregations were typically small pools of open water within dense stands of submerged vegetation. Nocturnal respiration of the macrophytes almost depleted the water of oxygen which forced the fish to concentrate in the few areas of open water. (5) Soon after sunrise the dissolved oxygen level started to increase, reaching saturation at about 08.30 h. This enabled the fish to leave these pools and disperse into the vegetated parts of the marsh which caused the egrets to abandon the area. (6) Even in the open areas of the pools the oxygen levels were below 2 mg l-1 around sunrise, but here the fish had access to the water surface where they performed aquatic surface respiration. As the oxygen content of the water increased, the numbers of fish breaking the surface min-1 decreased drastically. (7) We attribute the high capture rate of fish by little egrets in the early morning to exposure of the fish which perform aquatic surface respiration. (8) This is the first field study where aquatic surface respiration is shown to increase predation risk of a fish.