COLONY SITE SELECTION BY 8 SPECIES OF GULLS AND TERNS BREEDING IN THE VALLI-DI-COMACCHIO (ITALY)

Former studies of nesting habitat selection by seabirds have been limited to single species, or two-species assemblages, and have not assessed the influence of interspecific interactions. We studied the nest distribution of eight species of seabirds, breeding in mixed colonies, in order to investigate the influence of any specific preferences for foraging and nesting habitats, of habitat availability, and of social interactions. The distribution of the foraging zones did not influence colony location. The topography of the islets (mainly their size) influenced the distribution of the colonies in only two species (yellow-legged gulls and little terns). Each species significantly preferred certain types of nesting habitats for its nest site, differing in cover and type of vegetation. Yellow-legged gulls and Little terns acted as <<monospecific colonialists>>, slender-billed gulls and black-headed gulls tend to be <<plurispecific colonialists>>, and Mediterranean gulls, sandwich terns, gull-billed terns, and common terns as <<indifferent colonialists>>. If the nest distribution within the habitat types was determined only by the specific habitat preferences, we would have expected the association of two species within the same patches to depend on their habitat overlap; the relationship between habitat overlap and species association were complex, and we suggest that the nest distribution was determined by an interplay of habitat preferences, together with the tendency to prefer or to avoid certain other species of seabirds.