Distribution and habitat associations of the critically endangered bird species of São Tomé Island (Gulf of Guinea)

Abstract

Summary São Tomé holds 20 endemic bird species, including the little known and 'Critically Endangered' Dwarf Olive Ibis Bostrychia bocagei, São Tomé Fiscal Lanius newtoni and São Tomé Grosbeak Neospiza concolor. We conducted a systematic survey of the core forest area, performing 1,680 point counts and compiling occasional observations, which enabled the identification of new areas of occurrence for the target species. MaxEnt distribution modelling suggested that the ibis and fiscal have roughly half of the potential area of occurrence that had been assumed (127 and 117 km2, respectively), while it more than doubled that of the grosbeak (187 km2). The south-west central region of the island, most of which is included in the São Tomé Obô Natural Park, has the highest potential for the Critically Endangered birds. We confirmed the association of all target species with native forest. The ibis preferred high tree density, while the fiscal selected low tree density and intermediate altitudes. Despite very restricted ranges, population sizes seem to be larger than previously assumed. These results suggest that the fiscal and grosbeak might be better classified as 'Endangered', while the ibis should maintain its status under different criteria, due to a very restricted range during the breeding season. This work provides vital ecological knowledge to support conservation action focusing on these species and their habitats, highlighting the need to improve the effectiveness of the São Tomé Obô Natural Park in protecting its unique biodiversity.