Food and foraging activity of grey herons, Ardea cinerea, in a coastal area during the breeding season

Foraging strategies of grey herons Ardea cinerea in Arcachon Bay (southwest France) were studied in the main feeding areas during the breeding season (March-June 1993). In the study area, herons were mainly piscivorous both i n number and biomass. Eels were the most abundant prey in the herons' diet during the all study period. Small and medium sized fishes were caught more frequently by herons in May-June (when most of chicks were older) than in March-April, corresponding to the times when herons spent more time in the feeding areas. The average prey biomass taken by adults in each foraging se ssion was greater in May-June than in March-April. Reproductive timing (mai nly the hatching of chicks) and age-related requirements of food were proba bly the cause of these differences in foraging. The predigestion hypothesis was observed in 162 feeding sessions. Foraging activity of breeding herons was influenced by low tide periods and most herons (86.8%) fed during a fo ur hours period of low tide. The rate of intraspecific aggression was highe r in May-June (when most chicks were >4 weeks old and both parents were in the feeding areas) than in March-April.