Colony and nest site selection by roseate and common terms in the Azores archipelago - a multivariate analysis

Abstract

Multivariate techniques were used to examine overall characteristics of colonies, and detailed characteristics of nest sites within colonies, of Roseate and Common terns in the Azores archipelago. The characteristics of the nest sites were compared (1) with the available habitat and (2) between colonies, species and early and late nesting birds (Roseateterns only). Larger colonies occured far from the mainland, human setlements and other tern colonies, which reveals the importance of islets situated relatively far from sources of disturbance, predation and potencial competition for feeding resources. Both species showed markedly different nest site preferences: Roseate terns nested at areas with high relief and/or tall vegetation and Common terns selected open areas. Roseate terns nested within higher densities and their nestswere less visible from above than those of the Commons. Differences between the characteristics of early and late Roseate tern nests were attributed to seasonal changes in the vegetation structure of the habitat. Discriminant analysis indicated that the characteristics of Roseate tern nests differed more from the available habitat than those of Common terns, suggesting a more specialised nesting preference by Roseates. It is suggested that these nesting differences are primarily a result of preferences developed during allopatric speciation. Evidence of competition for nest sites between Roseate and Common terns was not found with this observational approach. It is speculated that Common terns might prevent younger Roseates from nesting in the open areas of mixed colonies. This idea is formulated in a model. The need to maintain optimal areas for nesting Roseate terns is stressed.