Sooty tern nesting success as a function of nest location, density and vegetationtype in a neotropical atoll

Nest density and reproductive success are used as indicators of habitat preference by animals. However, isolated treatment of these factors may be misleading. We studied habitat selection by Sooty Terns (Sterna fuscata) in an atoll in Brazil, composed of Farol and Cemiterio Islands. The islands were used separately to assess habitat choice, by analyzing nest density, nesting success, nesting locations and types of vegetation used for nesting. Differences in nest density were unrelated to nesting success. On Farol Island, nest density was similar in two vegetations (Cyperus ligularis and Portulaca oleracea), but higher in these than in a third vegetation (Iresine portulacoides), despite the latter’s high abundance. Nesting success, however, was signifi cantly higher on C. ligularis, indicating that this vegetation is advantageous for reproduction. On Cemiterio Island, covered largely with one vegetation type, nest density was higher at the periphery of the colony than at the center. Nesting success was similar at these two localities, andunrelated to vegetation coverage. We argue that the assessment of habitat quality may be inaccurate if based upon isolated indicators; inter-related effects of several reproductive parameters in association with other characteristics, such as vegetation type, are more useful. Other factors should also be considered. In this study, for instance, the differential distribution of predators in the islands may have affected nesting habitat choice by the Sooty Terns.

Maria Carolina Hazin and Regina H. Macedo, Revista Brasileira de Ornitologia 14 (3) 261-268