The present study is focused on the pathogenicity of a parasitic digenea, Clinostomum complanatum among dead piscivorous birds, collected after mass mortalities in northern Iran. A total of 126 birds (15 species) were examined for parasitic infections. Birds of four species belong to the family Ardeidae were found to be infected with immature and mature worms. C. complanatum was more prevalent in Ardea purpurea followed by Nycticorax nycticorax, Egretta alba and Egretta garzetta. Pathological study revealed acute inflammation of the submucosa of the oral cavity and oesophagus which can lead to impairment of deglutition and malnutrition which in turn can weaken the immune system. Immature parasites were found to penetrate the tissues sometimes to muscular layer but adults were seen attached to the oral cavity and the lumen side of the oesophagus suggesting parasite goes through a tissue migration after infecting the definitive host and prior to adult stage.