Aggressive behavior of Grey Heron (Ardea cinerea) nestlings was observed in approximately 25% of the nests at two of three heronries (Kiersity and Kąty Rybackie; north Poland) studied in 2000-2002. Pecking (mostly at the youngest chick) was the commonest type of aggressive behavior (over 60% of cases). The chick mortality rate was higher in nests with sibling aggression than in other nests, however significant differences were found only at Kąty Rybackie. Foraging of parent birds nearby, in an area with abundant high quality and easily caught prey, could have caused the lack of sibling aggression in the colony at Mosty. At Kąty Rybackie, many chicks died in nests where aggressive behavior had been noted. It appears that siblicide, not predation, is the most important factor affecting Grey Heron breeding success in these colonies.