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Introduction
The island of São Tomé (Gulf of Guinea, Central Africa) is an important centre of endemism (Jones 1994). The number of endemic birds it holds is particularly remarkable for a small island (Stattersfield et al. 1998, Kier et al. 2009, Buchanan et al. 2011, Le Saout et al. 2013). It has 50 resident bird species, of which 17 are single-island endemic species, three are Gulf of Guinea endemic species and eight are endemic subspecies, ranging across eight orders and 19 families (Jones and Tye 2006, Melo 2007). The island is also unusual among oceanic islands with isolated and unique avifaunas in that there are no recorded anthropogenic extinctions of birds (Jones and Tye 2006).
The endemic avifauna of São Tomé is clearly associated with the persistence of the island's forest-dominated landscape (de Lima et al. 2013a). Preserving the remaining native forests and restoring degraded habitat are top conservation priorities, namely within and around the island's only protected area, the São Tomé Obô Natural Park (ONP), where most of the endemic species are found (de Lima 2012, Ndang'ang'a et al. 2014). These forests are under high level anthropogenic pressure (Salgueiro and Carvalho 2007). Threats such as land-use intensification, overexploitation and invasive species are likely to continue to have a strong impact on forest ecosystems and on the endemics in the nearby future (Jones et al. 1991).
Nine of São Tomé's endemic bird species are currently classified as threatened, including three which are 'Critically Endangered'; the Dwarf Olive Ibis Bostrychia bocagei, São Tomé Fiscal Lanius newtoni and São Tomé Grosbeak Neospiza concolor (IUCN 2013, Ndang'ang'a et al. 2014). The ibis is a lowland species found in old-growth or mature secondary rainforest in the south and centre of the island, and it breeds from September to February (Jones and Tye 2006, Maia et al. 2014, Azevedo 2015, Margarido 2015). The fiscal is known only from well-preserved forest, with low understorey density and in areas of high rainfall (Jones and Tye 2006), occurring from the lowlands up to 1,395 m above sea level (Maia and Alberto 2009). The grosbeak was thought to be a lowland old-growth forest specialist, but recent observations in secondary forest at 1,400 m suggest that it might be more widespread than previously...