Influence of woodland cover on habitat selection and reproductive parameters of tropical roseate terns: implications for colony management

Abstract

We examined the effect of vegetation structure, in particular canopy closure, on colony site occupancy, nesting densities, and reproductive parameters of roseate terns Sterna dougallii breeding in a Pisonia grandis dominated woodland on Aride Island, Seychelles, western Indian Ocean. Long-term observations (1995Long-term observations ( to 2006 revealed that areas with high vegetation density and canopy cover (> 50%) were abandoned, in favour of nearby more open forest areas, such as clearings. The attractiveness of a forest clearing (0 to 25% canopy cover) to breeding birds was also largely supported by experimental manipulation of vegetation density in 2004. Most birds moved from areas under canopy cover to experimentally cleared plots, where they nested at higher densities and had a higher probability of successfully fledging a chick. However, some individuals remained in their original areas, despite their greater canopy cover, and had a lower fledging success. This site tenacity is presumably explained by an imprinting process leading some birds to breed in successive years in the same, familiar locations, despite their nest-sites having become sub-optimal for fledging success. Roseate terns choosing a nest site in woodland on Aride must trade off the need for some cover, offering protection from the sun, against the need for easy access through gaps in the canopy to fly to and from their nests. A suitable nest-site should also minimize chick/parent infestation by ticks and mortality caused by contamination of feathers with the sticky fruits of Pisonia grandis. We suggest that, when they are not formed naturally, small artificial forest clearings within the usual breeding area are likely to be attractive for roseate terns and may result in enhanced colony productivity. These findings may be applicable to other seabird colonies (e.g. sooty terns) found under forest cover on oceanic islands throughout the Indo-Pacific region.

Demography and conservation of the White-tailed Tropicbird Phaethon lepturus on Aride Island, Western Indian Ocean

“…In our study, however, adult tropicbird mortality due to Pisonia fruiting events averages 4.4% per year, but might account for as much as 8.5%, being presumably an important factor in explaining adult mortality (averaging 23.2% of total adult mortality). On Aride Island, other seabird species, mainly those that spend considerable time on the ground, such as Sooty Terns Sterna fuscata, Roseate Terns Sterna dougallii and Lesser Noddies Anous tenuirostris, appeared to be affected by Pisonia (Monticelli et al 2008b; personal observations).…”

“…It seems that the habitat return on Aride Island to a predominantly native climax forest, following its designation as a nature reserve, has contributed to a decline of some seabird species (Feare et al 1997;Monticelli et al 2008b;this study). Habitat management, such as the maintenance of canopy gaps, with the consequent decline of fern density, has been already shown to be an important conservation tool for other breeding seabirds of Aride Island, such as the Roseate and Sooty Terns (Feare et al 1997;Monticelli et al 2008b), and it may be crucial for the White-tailed Tropicbird. Although endemic landbirds depend on the presence of a native forest to survive, habitat management for seabirds might, presently, have positive impacts on such birds.…”

“…Habitat changes, especially in relation to vegetation structure, seem to be important in explaining distribution and abundance of seabirds on Aride Island. Increasing canopy cover was an important factor in understanding the spatial distribution of breeding Roseate Terns on Aride Island and explained why forest areas with high canopy closure were readily abandoned (Monticelli et al 2008b). Sooty Terns, although breeding in high numbers under an enclosed tree canopy on Aride Island, are present at significantly lower densities compared to other islands of the archipelago with an open habitat, and Aride's population size might be constrained by canopy and understorey density (Feare et al 1997).…”

Juvenile survival in a tropical population of roseate terns: interannual variation and effect of tick parasitism

“…Conversely, years of high roseate tern productivity are likely to be also favorable for ticks and result in enhanced levels of infestation in the following season. We also showed (Monticelli et al 2008a) that characteristics of the roseate tern nesting habitat are a proximate factor influencing the infestation levels of nestlings by ticks. On Aride, roseate terns nest on the forest floor under a tree (Pisonia grandis) canopy, where the more enclosed and vegetated areas present higher tick densities, presumably due to damper conditions that favor tick survival.…”

“…On Aride, roseate terns nest on the forest floor under a tree (Pisonia grandis) canopy, where the more enclosed and vegetated areas present higher tick densities, presumably due to damper conditions that favor tick survival. A large clearing created within the usual breeding area in 2004 decreased the level of chick infestation by ticks (Monticelli et al 2008a), hence none of the fledglings involved in the present study were infested in that year (Table 3). During the other years, 13% of the 525 roseate tern fledglings included in the analyses were infested during chick growth or at the time of ringing.…”