Nesting success of White Terns and White-tailed Tropicbirds on Cousine Island, Seychelles

Introduction Seabird life history strategies are often characterised by high adult survival, delayed maturity at breeding, low clutch sizes and variable reproductive output (Schaffner 1990, Bowler et al. 2002, Hockey and Wilson 2003, Ramos et al. 2005). At low latitudes seabirds have small clutches, large eggs and long incubation and fledging periods (Hockey and Wilson 2003), and clutch size has been related to food availability and predation risk (Shea and Ricklefs 1996). Primary production in tropical marine ecosystems is generally considered to be low and seasonal, and food may be scarce, especially at oceanic islands (Hockey and Wilson 2003). Avian predators on small islands are generally few, but predators may be attracted to seabird eggs and chicks when large numbers of seabirds are breeding at high densities (Schaffner 1991, Hockey and Wilson 2003). Although most tropical islands are free of large indigenous mammalian predators, predation by smaller terrestrial predators like crabs and skinks occurs (Schaffner 1991, Ramos et al. 2005). Intra- and interspecific competition between breeding birds may also lower breeding success (Bowler et al. 2002, Ramos et al. 2005). Cousine Island is one of the smallest islands in the Seychelles archipelago. Seven tropical seabirds breed at the island; two of the least common are the White Tern Gygis alba and the White-tailed Tropicbird Phaethon lepturus. An estimated 1 000–1 500 pairs of White Terns and 450–850 pairs of White-tailed Tropicbirds breed on this island (Skerret et al. 2001). White Terns are tree-nesting birds that lay a single egg on an exposed fork or in an artificial structure, whereas White-tailed Tropicbirds are ground nesters that breed in a shady and sheltered crevice (Schaffner 1991, del Hoyo et al. 1996, Bowler and Pillay 2000). The incubation period of White Terns is 34–36 d (five weeks) and the nestling period 68 d (seven weeks), whereas the incubation period of the White-tailed Tropicbird is 40–42 d (six weeks) and the nestling period 77–85 d or 11–12 weeks (del Hoyo et al. 1996, Higgins and Davies 1996). Both species exhibit life-history traits specific to tropical island breeding. Each species has a clutch size of one and nesting success is generally low, from 30–50% for White-tailed Tropicbirds (Schaffner 1991, Ramos and Pacheco 2003) and 29–40% for White Terns (Vanderwerf 2003). The aim of this study was to compare the reproductive parameters of White Terns and White-tailed Tropicbirds on Cousine Island and investigate if egg or chick failures were associated with different stages of the incubation and nestling periods. The study also examined the associations between reproductive parameters and nest types (White Terns), the two monsoon seasons and nest reuse. We postulate reasons for observed differences in the measured parameters between the two specie