The aggressive behaviour of skimmers was studied in a mixed species colony of black skimmers and common terns located on a sandy beach in New York. The levels and intensity of intraspecific aggression varied according to sex, season, and time of day. Males engaged in more intraspecific aggression, whereas females engaged in more interspecific defense. These differences suggest that interspecific defense is an appropriate response to the real threat of chick loss due to other birds (of both species). In general, females were defending space and their chicks, whereas males seemed to defend space, their chicks, and females. The aggression observed at any point in the reproductive cycle correlates with the magnitude of the threat.