Many species of birds fly in flocks numbering in the hundreds or even thousands of individuals. Such flocks make highly coordinated maneuvers when flying to or from feeding grounds and roosts or even during panic flights to escape aerial or terrestrial predators. How movements within a flock are coordinated, preventing collisions and possible injury to flock members, remains largely unknown. Although collisions of migrating birds with buildings and television towers result in mortality of thousands of birds annually (Stoddard 1962, Kemper 1964, Banks 1979, Gill 1990:587), fatal collisions among conspecifics are reported much less commonly. We report here a case of a fatal collision between two Caspian Terns (Sterna caspia) in southern California.