Reproductive Success and Colony-Site Tenacity in Caspian Terns

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-Colony-site use by individually marked Caspian Terns (Sterna caspia) was studied at four breeding colonies in northeastern Lake Michigan during 1976-1979. I examined two related aspects of colony-site use: (1) colony-site preference in experienced breeders and (2) the relationship between reproductive success and colony-site tenacity. Terns showed a significant preference for the colony of previous breeding unless their preceding reproductive effort was unsuccessful. Caspian Terns tended to use the same colony site if young were produced but moved to a new location if reproductive success had been terminated or threatened at the traditional site. Received 11 Feburary 1987, accepted 15 January 1988. SELECTION of breeding habitat is a crucial determinant of avian reproductive success. Many species of gulls and terns are ideal for the study of habitat selection because they often nest in dense colonies and exhibit site tenacity, or the tendency to return to the colony site or nest site of the previous year providing the habitat remains suitable (Austin 1949, McNicholl 1975, Burger and Shisler 1980). Breeding birds appear to select nesting habitat where individuals will have a high probability of maximizing their reproductive success. Natural-selection theory predicts that individuals producing offspring at a colony site should continue to breed at that location as long as it remains relatively unchanged, whereas birds that have site-related reproductive failures (e.g. storm washouts, egg and chick predation, human disturbance) should move to another breeding colony, use another nest site in the same colony, or start a new colony at another location for subsequent nesting attempts. I examined the relationship between reproductive success and colony-site tenacity in Caspian Terns (Sterna caspia) breeding in northeastern Lake Michigan. Demonstrations of the adaptive value of habitat selection in colonial waterbirds are for the most part correlative, but reproductive success in Laughing Gulls (Larus atricilla) is related directly to nest location (Montevecchi 1978, Burger and Shisler 1980). More recently, Burger (1982) found that Black Skimmers (Rynchops ni1 Present address: Department of Fisheries and Wildlife, 200 Hodson Hall, University of Minnesota, St. Paul, Minnesota 55108 USA. ger) usually abandon unsuccessful colony sites but continue to nest at successful sites. Although several studies have demonstrated a relationship between nest-site tenacity (within the same colony) and previous success (Macdonald 1977, Brooke 1978, Ollason and Dunnet 1978), no study of larids or other colonial waterbirds has focused on the specific relationship between colony-site tenacity and the reproductive history of color-marked individuals. To determine if preference for colony site is influenced by whether chicks were raised to fledging at that location in the previous season, I studied colony-site preference in experienced breeders and the relationship between reproductive success and colony-site tenacity. STUDY AREA AND METHODS The study area included four islands in northeastern Lake Michigan. Colony sites were located on the northeastern point of High Island (45?45'N, 85?40'W); Hat Island (45?47'N, 85'18'W); Shoe Island, a gravel bar 1 km south of Hat Island; and Ile aux Galets (45?41'N, 851l1'W). The distance between colonies ranged from 1 to 39 km. The study area is described in greater detail by Cuthbert (1985b). Approximately 1,100 pairs of Caspian Terns nested at these sites from 1976 to 1979. These birds represented about 30% of the Great Lakes population during this period; the rest nested on Gravelly Island (45?31'N, 86?43'W) in northwestern Lake Michigan or in the Canadian colonies located on islands in northern Lake Huron (North Channel and Georgian Bay) and Lake Ontario (Blokpoel and Fetterolf 1978, Shugart et al. 1978,