Data Zone
Justification of Red List category
This species is restricted to the twin islands of Biak-Supiori but despite the very small distribution the population size is not suspected to approach thresholds for listing as threatened under Criterion C. The population is suspected to be declining due to an ongoing slow rate of forest cover loss. The extent of occurrence, only 3,600 km2, indicates that the species may be at risk, although the population is not severely fragmented and there are parts of the range where the main threat is not believed to apply. But forest loss is ongoing, hence a continuing decline in the area, extent or quality of habitat is estimated. In conjunction with the very small extent of occurrence, the species is considered to approach the thresholds for listing as threatened, meeting Criterion B1b(iii) but not a second subcriterion. Accordingly, Biak Gerygone is assessed as Near Threatened.
Population justification
The population size of this species has not previously been directly quantified. Other species of Gerygone occur at quite high densities (20-400 birds/km2; e.g. Fogden 1976, Bell 1982), although qualitatively G. hypoxantha is considered rare (Pratt and Beehler 2015, Beehler and Pratt 2016), while the limited eBird (2022) data available support that it may occur at lower densities than congeners. Using the global landcover 2 estimate of forest cover for the island (closed to open broadleaved evergreen or semi-deciduous forest) and excluding fragments smaller than 1 km2 results in an area classified as forest habitat of 1,572 km2. Consequently, assuming (somewhat precautionarily) a combined occupancy/density of 10-30 mature individuals/km2, the population is suspected of numbering 15,000-45,000 mature individuals, although there remains high uncertainty with this.
Trend justification
No direct assessment of rate of population change has been made for the species. It is considered to be forest dependent and there has been a slow rate of forest cover loss, up to 4% in the ten years to 2022 (data from Global Forest Watch 2022, using Hansen et al. [2013] data and methods disclosed therein) and this is suspected to be causing broadly equivalent reductions in population size of this species.
G. hypoxantha is endemic to the twin-islands of Biak-Supiori in Geelvink Bay, West Papua, Indonesia.
The species inhabits old-growth and secondary forest habitats, mangroves and other riparian or littoral habitats in the lowlands and is not as rare or specialised in its occurrence as previously thought (G. Dutson in litt. 2016).
Forest on Biak has come under heavy pressure from logging and subsistence farming, though large-scale logging has not been economically feasible for some time (Wikramanayake et al. 2002). The rate of forest cover loss recorded through satellite monitoring has been slow in the past two decades, and in the three generations to 2022, the maximum rate is estimated at 4.0% (Global Forest Watch 2022, using Hansen et al. [2013] data and methods disclosed therein). Much of Supiori comprises virtually impenetrable, forested limestone mountains, which are likely to be safe from habitat degradation.
Conservation Actions Underway
The species is assumed to be present in the 110 km2 Biak-Utara protected area and throughout the 420 km2 Pulau Supiori Nature Reserve (UNEP-WCMC 2021a,b), both of which comprise virtually impenetrable limestone areas (Wikramanayake et al. 2002).
Conservation Actions Proposed
Conduct surveys on both islands to establish its current distribution, population status and assess its habitat requirements. Afford formal protection to further key sites where appropriate.
10 cm. A small, discrete species, pale yellow underneath and mousy grey-brown above with white lores and broken eye ring. Bill is relatively long, but not as bulky as that of G. magnirostris. Tends to be generally quiet, making it difficult to locate.
Text account compilers
Berryman, A.
Contributors
Butchart, S., Dutson, G. & Martin, R.
Recommended citation
BirdLife International (2024) Species factsheet: Biak Gerygone Gerygone hypoxantha. Downloaded from
https://datazone.birdlife.org/species/factsheet/biak-gerygone-gerygone-hypoxantha on 24/11/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 24/11/2024.