Nest site selection and temporal patterns in habitat use of roseate and common terns

-Roseate Terns (Sterna dougallii) nest in a wide variety of habitats. We examined nest-site selection in a mixed-species colony of Roseate and Common (S. hirundo) terns on the interdune area of a barrier beach to determine species differences, to identify which characteristics at nest sites differed from the available habitat, and to compare nest-site preferences of earlyand late-nesting Roseate Terns. Both species nested in the same area, but Roseate Terns nested under dense vegetation and Common Terns nested in more open sites. For Roseate Terns, cover within 0.5 m of the nest was greater than that within 1 m; the opposite was true for Common Terns. Cover within 5 m was similar for both species. Roseate and Common terns select different habitat features. Site characteristics of earlyand latenesting Roseate Tern nests differed. Late-nesting terns used sites with greater cover within 0.5 m, less cover within 5 m, taller vegetation, and with less visibility compared with earlynesting terns. Late-nesting Roseate Terns were still able to find sites in dense cover. At this colony, competition between the two species may not be limiting, and abundant sites remain available. Received 23 October 1987, accepted 30 January 1988. HABITAT selection affects reproductive fitness and survival during all phases of the life cycle of animals. For birds, habitat selection during the breeding season is important because species are confined to the nest site during incubation and the chick period, except for species with precocial young (reviewed by Burger 1980, 1985). Thus, nest-site selection is a decision a pair must live with for several weeks or months if their breeding attempt is to be successful. Variations in available habitat make selection possible (Partridge 1978) and allow partitioning among species (Cody 1985). Competition for nest sites might be intense in mixed-species colonies where species nest in close proximity, leading to a clear separation of site preferences. Often the species in such colonies vary markedly in size, and in direct competition for space the larger species usually wins (Schoener 1974). Temporal differences affect nest-site choices; early-nesting species may have clearly established sites before the second species arrives (Burger 1983). We previously described the nest-site characteristics of Common and Roseate terns (Sterna hirundo and S. dougallii) in a New York sand beach colony, but did not compare nest sites with available habitat or examine temporal differences in nest-site choices (Gochfeld and Burger 1988). In this study we compared nests of Roseate and Common terns intermixed in the same parts of a large colony, compared nest sites with available habitat, and examined differences in nest-site choices of earlyand latenesting Roseate Terns. The colony provided an opportunity to study nest-site selection and resource partitioning because the two tern species are similar in size, arrive at the colony at about the same time, have similar breeding phenologies, and have similar-size eggs, nests, and young. The Roseate Tern breeds on all continents except Antarctica, but populations are disjunct. Its center of abundance is in tropical latitudes (Nisbet 1980, Gochfeld 1983). It has declined dramatically in North America (Buckley and Buckley 1981) and Europe, and was listed in 1987 as an endangered species in the United States. Understanding its nest-site requirements is thus important, not only for information about nest-site selection, resource allocation, and spatial competition, but for an endangered species, which in the northeastern United States is restricted to very few colonies (Nisbet in Kress et al. 1983).