The diet of the black-crowned night heron (Nycticorax nycticorax) feeding in June and July along the southern shore of the St. Lawrence Estuary was composed primarily of the threespine stickleback Gasterosteus aculeatus. Three other sticklebacks, G. wheatlandi, Pungitius pungitius, and Apeltes quadracus occur in areas where the birds feed but were not eaten. It is suggested that differential predation on G. aculeatus may diminish interspecific competition for nest sites where the four stickleback species co-occur on the breeding grounds.