Colonial seabirds that nest in stable, predictable environments tend to retain their mates and return to the same colony and nest site for consecutive seasons. Individually marked Caspian Terns (Sterna caspia) nesting on five islands in northeastern Lake Michigan were studied for four breeding seasons (1976-1979) to determine the effects of an unpredictable environment on their behavior and population structure. High lake levels submerged some colonies and exposed others to storm damage. This dissertation is divided into three parts: (1) breeding chronology and intraseasonal movement, (2) reproductive success and colony site tenacity, and (3) mate retention between consecutive breeding attempts. In the first part, I examine breeding chronology, and seasonal colony site use patterns to answer the following questions: 1) do terns associate with more than one colony during a single breeding season, and if so, 2) what are the factors that influence colony site movement in this population? I discovered that intraseasonal movement between colonies was common. Individuals searching for a mate or terns that experienced a reproductive failure were frequently observed at two or more colony sites. Intercolony movement by courting birds tended to be temporary as individuals eventually selected a nest territory at a specific colony site. Once egg laying was initiated, terns rarely were observed at colonies other than the one where they were breeding unless the reproductive effort was interrupted by disturbance or destruction of the nest. Following a reproductive failure, terns tended to desert the original colony and often renested at another colony or visited several colonies as non-breeders for the remainder of the summer. Because the most frequent cause of failure was storm damage to nests, intercolony movement probably is greatest during years of higher than average lake levels. In the second section, I examine the relationship between preference for colony site and reproductive success in the previous season. Colony site preference in first time breeders is also considered. Terns breeding for the first time showed no preference for the natal colony but experienced breeders tended to nest at the colony of previous breeding unless their previous reproductive effort was unsuccessful. Results indicated that one breeding adaptation of Caspian Terns is to use the same colony site if young are produced but to move to a new location if reproductive success has been terminated or threatened at the traditional site. In the final section I examine mate retention to determine (1) if individuals tend to keep the same mate for consecutive breeding attempts and (2) if mate retention is influenced by whether the pair raised offspring to fledging the previous year. Only 25% of the original pairs bred together for consecutive seasons, and mate retention was independent of previous reproductive success. In addition, all individuals that experienced reproducitve failures and renested within the same season kept the same mates. Therefore, terns that failed and renested were more likely to keep the same mates than successful individuals that did not nest again until the following year. These results indicate that mate retention following breeding success, and pair separation after reproductive failure may not always be the best breeding strategy. Factors other than previous reproductive success may influence mate retention in this population, and these include: stability of colony and nest sites, site of courtship and pair formation activities, size of breeding population and distribution of colony sites, and characteristics of the breeding season. This study on Caspian Terns suggests that many currently accepted generalizations about the breeding biology of colonial seabirds need to be modified if they are to apply to species breeding in highly unstable environments.