Justification of Red List category
This species has a small population which is restricted to three isolated subpopulation, none of which hold more than 1,000 mature individuals. The preferred montane forest habitat is suffering ongoing degradation and there is evidence that a rapid reduction in abundance has taken place in the largest subpopulation over the past 25 years. It is therefore classified as Vulnerable.
Surveys in the late 1990s estimated a population of 7,123 (3,770-13,594) total individuals (Mulwa et al. 2007). This would roughly equate to 4,750 (2,500-9,100) mature individuals.
The species occurs in two of the three main blocks in the Taita Hills (the Dawida complex comprising eight occupied fragments and the Mbololo massif; no evidence of the species was found from the Sagala massif) and also on Mount Kasigau c. 50 km southeast (Mulwa et al. 2007). Mulwa et al. (2007) estimated numbers from within each block from fieldwork undertaken between November 1998 and September 1999; Dawida complex 878 individuals (95% confidence interval: 461-1,779), Mbololo 651 individuals (352-1,188) and Kasigau 5,594 individuals (2,957-10,627). These three areas are considered to represent three separate subpopulations, although there is the possibility that there may be further subdivision within the Dawida block.
Notably there is concern that the Kasigau population has since crashed (M. Adamjee in litt. 2022). The estimate was made at a time when the habitat was 'pristine' and the abundance of this species was exceptional, but the apparently insular nature of this population suggested fragility (Mulwa et al. 2007, R. Mulwa in litt. 2022). Subsequently the habitat has become at least partly degraded and in numerous recent visits abundance of Taita White-eye has been far lower, and a value of 1,000-1,500 individuals is considered a more accurate current range (M. Adamjee in litt. 2022). This revised value would suggest that no subpopulation is likely to hold more than 1,000 mature individuals, as well as indicating that at one site a rapid reduction is reported to have occurred. With degradation of the preferred montane forest habitat noted to be ongoing (Wakesa et al. 2020, R. Mulwa in litt. 2022) along with the noted reduction in abundance of the largest subpopulation, it is inferred that there is an ongoing decline in the number of mature individuals.
The population is inferred to be declining as a result of ongoing forest loss. Data from Global Forest Watch (2020) suggest that between 2009 and 2019, 1.2% of the forest in the species' range was lost. However, analysis by Wakesa et al. (2020) found that between 1973 and 2016, the larger forest blocks in the Taita Hills (Chawia, Fururu, Mbololo, Ngangao and Vuria) decreased by 23.2%. This roughly equates to a rate of 6% over 10 years. The direct rate of population decline is difficult to quantify as this species prefers edge habitats, however due to the restrictedness of the remaining habitat, the overall trend is declining.
Zosterops silvanus is found only in the remaining forest on Taita Hills and Mount Kasigau in Kenya (BirdLife International 2000). Surveys in the late 1990s found this species in the following fragments: Ndiwenyi, Mwachora, Macha, Fururu, Yale, Vuria, Chawia and Ngangao in the Dawida block, the Mbololo massif, and Mount Kasigau (Mulwa et al. 2007). Despite considerable search effort, the species has not been recorded in the Sagala block, which is dominated by the introduced/invasive Pinus patula (Mulwa et al. 2007).
The highest densities were recorded on Mount Kasigau, where the population is believed to be insular and dispersing individuals may be trapped within the small extent of remaining forest. There is evidence for some dispersal among the fragments within the Dawida block (Lens et al. 2002, Mulwa et al. 2007), but clear separation of the Mbololo block from the Dawida block (Callen et al. 2011).
This species is only found in the remaining forest on Taita Hills and Mount Kasigau (BirdLife International 2000). It inhabits edge habitat in highland forests and scrub, while it shuns non-native Pinus plantations (del Hoyo et al. 2020). It is recorded from 840-1,725 m elevation (del Hoyo et al. 2020).
Most indigenous forest has already been cleared in the Taita Hills for cultivation or reforestation with non-native timber. The remaining tiny area was thought to be under serious threat from both clearance and degradation (Brooks et al. 1998, Mulwa 1998, L. Bennun in litt. 1999), however three segments (Ngangao, Chawia and Mbolobo) have been gazetted (L. Lens in litt. 2019). Forest loss is still occurring within the species' range, but at a rate of <2% over the last ten years (Global Forest Watch 2020).
Conservation Actions Underway
The Mt Kasigau Forest is, so far, well protected by the local population, who rely on the mountain streams for their water supply (L. Bennun in litt. 1999). The Forest Department is now safeguarding the remaining forest fragments of the Taita Hills, which have been designated as an IBA. Much of the remaining forest habitat is formally protected and managed by either the Kenya Forest Service or the County Government (A. Ward-Francis in litt. 2016). A collaborative research project, currently underway, includes a large ornithological component which aims to provide the necessary ecological data to plan conservation policies for this species and other endemics in the area (L. Bennun in litt. 1999).
Text account compilers
Bennun, L., Butchart, S., Ekstrom, J., Pilgrim, J., Ward-Francis, A., Westrip, J.R.S., Mulwa, R., Adamjee, M., Matiku, P., Clark, J. & Rotton, H.
BirdLife International (2023) Species factsheet: Zosterops silvanus. Downloaded from http://datazone.birdlife.org/species/factsheet/taita-white-eye-zosterops-silvanus on 26/09/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://datazone.birdlife.org on 26/09/2023.