Population census and habitat use of Rodrigues Warbler Acrocephalus rodericanus

The Endangered Rodrigues Warbler Acrocephalus rodericanus is endemic to the Indian Ocean island of Rodrigues, one of the world's most devastated tropical islands, its native forest having been completely destroyed since human colonization three centuries ago. It is now found in thickets and woodland dominated entirely by non-native trees and shrubs. As a step to implementing conservation initiatives, a population census and habitat study was undertaken in April-June 1999. Using a combination of tape playback of song and point counts, at least 103 Rodrigues Warblers spread through nine wooded localities were observed, and a minimum population of 150 birds estimated. The majority (78%) and highest densities (2.3/ha) were found in habitat dominated by one introduced invasive tree, the rose-apple Syzygium jambos. Warblers were also found in plantations dominated by mahogany Swietenia mahagoni, tecoma Tabebuia pallida and Norfolk Island pine Araucaria cunninghamii, but at much lower densities (0.5/ha). Warbler densities were highest in habitat with a dense structure of small branches. There was a strong positive relationship between one index of human disturbance (number of cut branches) and warbler densities. This low-intensity cutting may promote the growth of new shoots thus perpetuating the dense vegetation structure that Rodrigues Warbler favours. At two localities, one supporting an existing population of warblers and the other prone to extirpation (shown from previous surveys), vegetation structure and composition were similar between sites, hinting that in some currently unoccupied areas, habitat is probably suitable and other factors are operating to preclude colonization and establishment. Further ecological studies would be desirable, especially to investigate the effect of nest-predation by introduced mammals and other factors suppressing warbler population growth.