Colony-site use patterns were studied in Caspian Terns (Sterna caspia) nesting on five islands in northeastern Lake Michigan to address the following questions: (1) do individual terns frequent more than one colony site during a single breeding season; and (2) what are the factors that influence intraseasonal colony site movement in this population? Intraseasonal movements of terns between colonies in northeastern Lake Michigan were common. Birds that were not nesting were observed significantly more often at two or more colonies than nesting terns, and intercolony movement was precipitated by lack of a mate or nest site and by reproductive failure (e.g., nest destruction by storms or predators and investigator disturbance). Intercolony movement during courtship and following reproductive failures probably is intensified during periods of flooding or high water in the Great Lakes.