Justification of Red List Category
This species is listed as Critically Endangered because it is estimated to have an extremely small population which is declining rapidly owing to predation by introduced mammals. In addition, it has a very small range, and its habitat is declining in quality and extent.
The population estimate of 191-327 mature individuals is derived from 160-296 mature individuals on the mainland (from Nichols et al.  and Ormsby et al. ) and a population of 46 individuals (roughly equivalent to 31 mature individuals) on Iles aux Aigrettes (Ferrière et al. 2015).
Surveys have revealed that the population size declined from an estimated 346 pairs in 1975 to c.200 pairs in 1993 and 93-148 pairs in 2001 (Nichols et al. 2004), and just 7-17% of nesting attempts successfully fledged one chick between 1998 and 2001, hence current rapid declines are likely to continue into the future. Maggs (2016) conducted a population viability analysis of this species and concluded that the population would go extinct within 50 years; this may even be the case within 3 generations as Maggs et al. (2015) extrapolated from current declines to project 14% declines per annum.
This species is endemic to Mauritius. It declined rapidly from 350 pairs in the mid-1970s, to c.275 pairs by the mid-1980s. Intensive fieldwork indicated a further reduction to an estimated 200 pairs between 1990 and 1993 (Safford 1997), and 93-148 pairs between 1998 and 2001 (R. Switzer in litt. 2003, Nichols et al. 2004, Anon 2006) within an area of less than 25 km2 located in the southwest of the Black River Gorges National Park. Finally, a survey in 2012 concluded that the population could have declined to as few as 80 pairs within the same region (Ormsby et al. 2012). It is widespread in upland native forest, but largely absent from the whole Macchabé-Brise Fer area and Fouge Range (Safford 1997). The species's core distribution has contracted since 1975 - it has disappeared from some outlying sites (Tamarin Falls, Jouanis and Monvert) and the core area has decreased by 50% (Nichols et al. 2005). Its status in central Mauritius (Montagne Lagrave and the central plateau relics) remains uncertain (R. Switzer in litt. 2003, Nichols et al. 2004), although one was seen on Mt Lagrave in the breeding season in 2011/2012 (Ormsby et al. 2012, V. Tatayah in litt. 2012). Highest densities are between Montagne Cocotte and the Combo area with up to 10 pairs/km2 (C. Jones in litt. 2000). Birds exploit isolated habitats over a wide area of the central plateau, including many relict patches of native vegetation (Safford 1997). Following twenty years of conservation work on Ile aux Aigrettes, 16 white-eyes were released on the predator-free island between December 2006 and March 2007 (Cole et al. 2006, Anon 2006). A further release of c.20 birds between 2007 and 2010 enabled the establishment of a small sub-population on Ile aux Aigrettes, and birds have since successfully fledged young there every breeding season since 2008/09 (Maggs et al. 2011). In March 2015, the population on Ile aux Aigrettes stood at 46 individuals (Ferrière et al. 2015).
It is restricted to the wettest native upland forests. It feeds on nectar, fruit and insects, and travels considerable distances to productive flowers (Cheke 1987). Some introduced plant species have become extremely important nectar sources (Safford 1991). High densities may be associated with mosaics of small plantations of exotic trees, where nest predation may be low, interspersed with native vegetation for foraging (Safford 1997). The species defends territories of c.0.5ha, and aggressively defends a favoured flower or nest-sites against conspecifics and Mauritius Grey White-eye Z. mauritianus (Nichols et al. 2005, Maggs et al. 2015). The clutch-size is 2-3 eggs (Safford 2013). In recent decades, pairs on the mainland have not generally fledged more than one offspring per nesting attempt and productivity is predicted at 0.2 fledglings per female per season (Maggs et al. 2015). Productivity has been higher on Ile aux Aigrettes, where pairs are known to fledge between 1 and 3 nestlings per nesting attempt.
Zosterops chloronothus has suffered chronically from continuing habitat destruction and degradation as a result of invasion by exotic plants. Nest predation by introduced rat species has been identified as a major threat (Maggs et al. 2015) and other bird species are also considered a threat (C. Jones in litt. 2000, Nichols et al. 2005): a study of Mauritius Grey White-eye found only 8% of nests resulted in fledglings, with predation by native Mauritius Black Bulbul Hypsipetes olivaceus and probably the invasive Red-whiskered Bulbul Pycnonotus jocosus the most frequent cause of nest failure (Sørensen 2005).
Conservation Actions Underway
The species has long been protected by law. The Black River National Park partly covers the species's distribution. Habitat around Bassin Blanc may be bought by compulsory purchase in the future (Jones and Hartley 1995, R. Safford in litt. 1999). A species recovery programme was initiated by the Mauritian Wildlife Foundation in September 2005, in which territories identified in the most recent survey were revisited and the breeding behaviour of these birds was closely monitored (Cole et al. 2006, R. Cole in litt. 2007). Between 2005 and 2008 an intensive management plan was applied, involving wild population monitoring, predator control at nest sites, rescue/harvest of wild nests, artificial incubation and hand-rearing of offspring, and a trial release of birds to the predator-free, restored offshore islet Ile aux Aigrettes in December 2006 (Cole et al. 2006, R. Cole in litt. 2007). All individuals from rescued nests were hand-reared by a team from Chester Zoo, the Durrell Wildlife Conservation Trust or the Mauritian Wildlife Foundation (Sørensen 2005).
A total of 38 individuals, all originating from wild nests, were released onto Ile aux Aigrettes between 2006 and 2010 (Cole et al. 2006, Maggs et al. 2011, R. Cole in litt. 2007, G. Maggs in litt. 2010). The Ile aux Aigrettes sub-population is closely monitored and provided with supplementary food, and the first fledglings were produced in 2008 (Maggs et al. 2011, G. Maggs in litt. 2010). In 2010, 14 fledglings were produced from five breeding pairs, and the total population currently stands at 46 individuals (Ferrière et al. 2015, G. Maggs in litt. 2010). Releases and nest rescues have now ceased, and intensive research has been conducted to increase knowledge of the species and its current threats, and to investigate potential management techniques for long-term management of the species (G. Maggs in litt. 2016).
10 cm. Small, drab, warbler-like bird of forest. Dull olive-green above with paler underparts, tending towards cream on belly and yellow on vent. Similar spp. Confused only with Mauritius Grey White-eye Z. borbonicus mauritianus from which it differs by having dark, not grey rump, noticeable white eye-ring, and being overall olive, not grey. Also has longer, fine, decurved bill. Voice Metallic plik plik contact note and warbled song. Hints Not easy to find in the Black River Gorge area in south-west Mauritius.
Text account compilers
Benstead, P., Bird, J., Calvert, R., Ekstrom, J., McClellan, R., Shutes, S., Symes, A., Warren, B. & Westrip, J.
Cole, R., Jones, C., Nichols, R., Safford, R., Switzer, R., Tatayah, V. & Maggs, G.
BirdLife International (2021) Species factsheet: Zosterops chloronothos. Downloaded from http://www.birdlife.org on 12/04/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 12/04/2021.