Nest Site Selection by Kelp Gulls in Southern Africa

-We studied six colonies of Kelp Gulls (Larus dominicanus) in South Africa and Namibia, including mainland beach and cliff, and island sites. In the 20 distinct habitats examined, the gulls generally nested in somewhat horizontal areas with either intermediate vegetation cover (25 to 50%) or protruding rocks which provided similar cover. Characteristics favored for nesting were studied by comparing nest sites with matched points 2 m away. In all habitats the gulls selected flatter areas (mean slope less than 150) than the matched points (mean slopes up to 700). Selection of nest sites with more cover may serve to reduce chick loss by providing shelter (vegetation or rock crevices), and the gulls' choice of low rather than tall vegetation may ensure that adults can escape rapidly from the nest. Colonial birds select their habitat and nest sites according to environmental and social factors. For many species, the process of nest site selection involves: 1) habitat selection, 2) choosing a part of this habitat on the basis of particular physical factors (such as elevation, slope, rocks, vegetation), 3) defining territorial boundaries as a result of contacts with neighbors (not necessarily conspecifics), and 4) selecting the nest site. Habitat selection, the choice of a type of place in which to live, results in animals living in a restricted set of environmental conditions (Partridge 1978). Once a habitat is chosen, birds either select a nest site and then negotiate territorial boundaries with neighbors, or they may defend a territory possessing the general features required, and subsequently select the nest site in that territory. In a few species of gulls, territories and nest sites are acquired simultaneously. For example, Franklin's Gulls (Larus pipixcan) in the United States and Brown-hooded Gulls (L. maculipennis) in Argentina defend territories from a mat of floating vegetation which later becomes the nest site (Burger 1974a, b). In other gull species, territories are defended for weeks or months prior to nest building (Tinbergen 1956, Bongiorno 1970), and the territorial boundaries are fixed before the nest site is selected. These birds have elaborate behaviors for choosing the nest site, several locations being examined and many scrapes made before the final site is chosen (Tinbergen 1956, Bongiorno 1970). Gulls that select nest sites after acquiring territories must choose from the physical features available in those territories. Assuming, for example, that medium c ver (ca. 50%) is preferred and that vegetation cover varies throughout the colony, gulls who choose 30% cover in sparsely vegetated areas or 70% cover in densely vegetated areas might be selecting the best cover available in their particular territories. General nest site requirements of gulls have been examined in some detail, particularly in marsh-nesting species (Bongiorno 1970, Montevecchi 1978, Burger and Shisler 1978). The factors influencing colony and nest site selection in dry land colonies of gulls or terns have not been studied as ex ensively. Blokpoel et al. (1978) compa ed plots with and without nesting Common Terns (Sterna hirundo), and reported a preference for vegetated versus bare areas. Veen (1977) reported that Sandwich Terns (S. sandvicensis) exhibit rather uniform choices for nest sites with respect to vegetation and substrate, but he did not quantify these variables or compare nests with random points. Moreover, most investigators have worked in only one colony making generalizations for a species ten-