NEST-SITE SELECTION BY ROSEATE TERNS AND COMMON TERNS IN THE AZORES

Abstract

Nest-site characteristics of Roseate Terns (Sterna dougallii) and Common Terns (S. hirundo) in mixed and unmixed colonies were examined at a number of sites in the Azores archipelago using discriminant analysis. Colonies were stratified and points located in a systematic way. Physical parameters, nearest-neighbor distance, vegetation cover, and density of nests were measured. Measured nest-site parameters were compared between species and with habitat points selected in the colony. Significant interspecific differences in nesting sites were found. Roseate Terns nested in areas with high relief and/or tall vegetation and with higher nesting densities. Common Terns selected more-open areas. There was considerable variability in nest-site selection of both tern species among colonies. Roseate Terns were more specialized as their nest sites differed more significantly than those of Common Terns from the available habitats. In contrast to findings of studies in North America, nest con- cealment by Roseate Terns did not influence hatching success. Competition may be important in nest resource partitioning in individual mixed colonies depending on colony size and habitat structure. The need to maintain/create optimal nesting areas for Roseate Terns is stressed. As habitat descriminators varied greatly between colonies, management should be planned on a colony-by-colony basis. Received 15 March 1994, accepted 15 May 1994.