Data Zone
Justification of Red List category
This species is listed as Critically Endangered owing to a continuing decline in habitat quality caused by the presence of introduced mice. Mouse predation has forced this species out of coastal areas into sub-optimal upland habitat but recent surveys suggest that the population has been stable over the past decade. This stability may be temporary, as there are no significant barriers to future increases in the mouse population in upland areas in the absence of a successful eradication effort. There remains the need for urgent conservation intervention, which would also benefit the island's breeding seabirds, to secure its future.
Population justification
The total number of birds in 2020 was estimated at 1,917 individuals (95% CI: 1,550–2,500), derived from habitat specific density estimates from line transect distance sampling surveys around the island from 2018 to 2020 (Jones et al. 2020). Adults comprised 74.5 % of individuals observed in February and 81.2 % observed in September (Jones et al. 2020), hence the minimum population size is calculated at between 1,118 to 2,030 mature individuals. Jones et al. (2020) also estimated 720 breeding pairs, and a value of 1,440 mature individuals is taken as a best single estimate.
Ahead of the attempted mouse eradication in 2021, an insurance population of 100 Gough Island Finches was brought into captivity in case of adverse impacts on wild populations due to the deployment of poisoned bait (Chilvers and McClelland 2023). After the completion of the bait application 103 birds were released back into the wild and follow-up surveys showed there was still a significant wild population of birds present (Chilvers and McClelland 2023). Continuing surveys recorded a c. 60% reduction in density in both 2021 and 2022 versus pre-eradication but densities recovered to levels similar to those pre-eradication in 2023 (A. Steinfurth, H. Marshall and S. Oppel in litt. 2024). Accordingly, the current population size is considered to be similar to that estimated in 2020 by Jones et al. (2020).
An early estimate was of around 2,000 birds in 1956 (Holdgate 1957), while 1,500 pairs or 3,000 mature individuals was estimated based on density of pairs in surveyed areas in 1990-91 (Ryan and Cuthbert 2008). Densities were found to be considerably lower in 2007 at 20 pairs/km2 in the southern highlands and likely 5 pairs/km2 in the northern highlands versus 40 pairs/km2 assumed present across this whole area in 1990-91, which translated to a revised population estimate of 400-500 pairs in 2007 (Ryan and Cuthbert 2008). In re-analysing densities while accounting for habitat use more precisely Jones et al. (2020) indicate that densities in the highlands do not appear to have changed much in 30 years (1990-2020). However, this analysis does not account for differences in densities geographically within habitat types, which appears likely with fewer individuals recorded in the northern part of the highlands echoing the assumption made for the 2007 data (Ryan and Cuthbert 2008). This is unlikely to affect the finding of apparent stability of the highland population, only the precision of the population estimate (Jones et al. 2020). There has been a considerable reduction in the population in the lowlands that is evident in the earlier density data, and is strongly indicated to have been driven by the impact of mice (Cuthbert and Hilton 2004, Ryan and Cuthbert 2008, Jones et al. 2020), and this will have caused a significant historical population reduction. There are no barriers to mice accessing the finch populations in the highlands, but mouse densities have been lower to date which may be related to the abundance of alternative prey, largely seabirds. The eradication attempt will have temporarily reduced mouse populations, however their continued presence is a concern. It is hoped that in the short term the finch population may rebound in the lowlands but unless the mouse population is further controlled this would be a temporary situation.
Trend justification
Jones et al. (2020) found no discernible trends in the density of the species and that the population has remained stable in upland areas since 1990, however a rapid decline occurred prior to 2008 in lowland areas (Ryan and Cuthbert 2008), where densities remain very low likely due to mouse predation (Jones et al. 2020). It is hoped that following the mouse eradication attempt the reduction in the mouse population will allow some recovery of the finch population in the lowlands, where mouse predation is suspected to have been keeping finch densities exceptionally low after historical declines. However, such a recovery will like be reversed in the future with the expectation that mouse density will quickly return to levels found prior to the eradication attempt.
Rowettia goughensis is endemic to Gough Island (area 65.78 km2) Tristan da Cunha (Saint Helena, Ascension and Tristan da Cunha to UK) in the South Atlantic Ocean. It occurs from sea level to the highest point of the island (c. 910 m), and on Penguin Island, a vegetated offshore stack (Ryan and Cuthbert 2008, Ryan and Sharpe 2020), although this is not large enough to support a sustainable population. Initially reported to be most abundant at low elevations (Wilkins 1923) large declines in these habitats (Ryan Cuthbert 2008) have resulted in the highest densities now occurring in the high elevation moorland habitat in the centre of the island (Jones et al. 2020).
Scaling to a 2 km by 2 km grid, the area of occupancy (AOO) totals 96 km2 (24 cells). This exceeds the area encompassed by a minimum convex polygon around the occupied range (which totals 80 km2), hence the extent of occurrence is increased to match the AOO value at 96 km2. The principal threat to the species is from mortality of adults and nest predation by introduced house mice Mus musculus (Ryan and Cuthbert 2008, Jones et al. 2020), and with the failure of the attempted mouse eradication in 2021 this threat continues to apply to the whole population.
It is most common in tussock-grassland, wet heath and fjeldmark up to 800 m, and occurs at lower densities in fern-bush and peatbogs (Ryan and Cuthbert 2008). Breeding occurs from September to December, and chicks fledge in November and December. Clutch size is usually two eggs. The female constructs the nest, which is an open cup constructed on or close to the ground, sheltered by overhanging vegetation or a rock. Both sexes are involved in raising chicks (Ryan and Cuthbert 2008). It feeds primarily on invertebrates (80% of foraging time), but also eats fruit (Ryan and Cuthbert 2008), grass and sedge seeds, berries, and scavenges broken eggs and birds (Richardson 1984). It nests on the ground amongst or under vegetation, but mostly on steep slopes or cliffs. Different plumage types suggest that it takes at least three years to acquire full adult plumage (Ryan and Cuthbert 2008).
The presence of mice within the range of the species is inferred to be causing a continuing decline in the quality of habitat, as previously evidenced by the shift in peak abundance away from the lowlands where mice have been most abundant (Ryan and Cuthbert 2008, Jones et al. 2020). The eradication attempt in 2021 greatly reduced mouse density, however its failure will see the mouse population rapidly increasing over the next few years and resulting in an inferred continuing decline in habitat quality for Gough Island Finch.
The introduced House Mouse (Mus musculus) poses the greatest threat through competition (suspected) and predation (observed). Mice are known to have substantially altered invertebrate populations on other sub-Antarctic islands and R. goughensis is much less abundant on Gough than the equivalent species on nearby mouse-free islands (Ryan and Cuthbert 2008). Early reports suggest that the species was most abundant at low elevations (Wilkins 1923) but now occur at very low densities here having failed to recover from severe declines prior to 2008 (Ryan and Cuthbert 2008, Jones et al. 2020). Mice are currently most abundant in the lowlands and predation rates of dummy eggs are up to thirty times higher in these areas (Cuthbert and Hilton 2004). The proportion of juveniles in the population declined from 50% to 20% from 1991-2007, suggesting that recruitment is too low to sustain the population (Ryan and Cuthbert 2008).
Densities of mice are now at their lowest for many decades following the eradication effort in 2021. Unfortunately the effort did not successfully eradicate mice from the island, but it is hoped that the impact on the finch population will be reduced considerably for some years. Without further intervention the mouse population will rapidly return to a size where it is a severe threat across the range of the species once again.
The accidental introduction of the Black Rat Rattus rattus from visiting ships is a potential threat as there have been occasional reports of rats on the supply ship and island, the last in 1983 (Wace 1986).
Conservation and Research Actions Underway
Gough is a nature reserve and World Heritage Site and is uninhabited apart from staff who run a meteorological station and a small research team (Cooper and Ryan 1994, A. Bond in litt. 2016). Territory mapping to investigate pair density in different habitats was conducted in 2000-2001, in addition to an assessment of the role of mice as nest predators (Cuthbert and Sommer 2004, Cuthbert and Hilton 2004). Further investigation of diet overlap with mice, and predation by mice was undertaken during 2003-2006. Following a 2006 review of the impacts of introduced rodents on Tristan da Cunha and Gough a feasibility study for mouse eradication was published in 2008. The study recommended what research still needed to be undertaken before an eradication effort should be attempted, and since 2008 researchers on the island have been conducting this work. A series of projects have investigated questions arising from the study (e.g. Cuthbert et al. 2011, Rexer-Huber and Parker 2011), and have shown an eradication operation is feasible. Operational planning and fundraising for the eradication of mice from Gough began in 2015 (J. Kelly, pers. comm. 2016), and the operation took place in 2021. To minimise the risk of bait causing severe mortality in the native landbirds, this species and Gough Island Moorhen, wild birds were caught and held in captivity for the duration of the project. A hundred finches were held and, with the application of knowledge from prior trials eventually 103 were released back to the wild (Chilvers and McClelland 2023). The baiting operation covered the entire island but was not successful in eradicating mice, possibly due to extremely high levels of unexpected interference with the baiting stations by slugs unaffected by the poison, but potentially simply due to the exceptional difficulty in fully eradicating an established population of mice.
Conservation and Research Actions Proposed
Carry out regular surveys to monitor the population. The goal to eradicate mice from Gough Island (Ryan and Cuthbert 2008) remains the key conservation action for birds on the island. With mouse numbers now low it may be possible to instigate ongoing targeted baiting to try to keep the population down. Minimise the risk of other alien species becoming established on the island, particularly any rat Rattus species.
18 cm. Large, chunky, drab olive-coloured finch. Male uniform dull olive-green overall with yellowish forehead and eyebrow. Underparts slightly paler dull olive with prominent black bib. Thick-based, pointed, black bill. Female and juvenile buffy-olive, heavily streaked above and below with dark brown. Apparently two "streaky" immature plumages occur, including transitional phase between juvenile streaky and adult olive plumage. Voice Contact call is keet keet and song is high, keening whistle.
Text account compilers
Martin, R.
Contributors
Cooper, J., Ryan, P.G., Bond, A., Marshall, H., Steinfurth, A. & Oppel, S.
Recommended citation
BirdLife International (2024) Species factsheet: Gough Island Finch Rowettia goughensis. Downloaded from
https://datazone.birdlife.org/species/factsheet/gough-island-finch-rowettia-goughensis on 23/11/2024.
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 23/11/2024.