Justification of Red List Category
This species is listed as Vulnerable because it is restricted to a very small range on just two small islands and, although it coexists with introduced rats on the island of Tristan da Cunha, and with mice on both Tristan da Cunha and Gough, the accidental introduction of rats, or another predator to its stronghold on Gough Island remains a risk.
The total population for G. comeri is estimated at 8,500 mature individuals based on 4,250 pairs on Gough during a 1983 survey. This is roughly equivalent to 13,000 individuals in total. There were an additional 170-255 pairs on Tristan in 1972-1974 (Richardson 1984), however it is unclear whether the population on Tristan should be countable under IUCN Red List guidelines since this is technically an introduction of G. comeri to Tristan rather than a reintroduction, and it should only be countable if the intent of the introduction was for conservation.
In 1984, the Tristan population was estimated at c.250 pairs and increasing. It is now distributed throughout the island (P. G. Ryan in litt. 2000). Based on this information the population is suspected to still be increasing.
Gallinula comeri was endemic to Gough Island (St Helena, Ascension and Tristan da Cunha, to UK) in the South Atlantic Ocean, but in 1956 a small number of birds from Gough were introduced to nearby Tristan da Cunha (Beintema 1997). There have been other possible releases onto Tristan but none have been documented. G. nesiotis, endemic to Tristan, had previously been driven extinct in the late-19th century (Nicoll 1906). In 1983, the Gough population was estimated at 2,000-3,000 pairs in 10-12 km2 of suitable habitat (Watkins and Furness 1986). This estimate has been recalculated as 4,250 pairs, based on the same pair density data but using new data on relative densities in different habitats and total habitat areas (Cuthbert and Sommer 2004). In 1972-1974, the Tristan population was estimated at 170-255 pairs and increasing (Richardson 1984). It is now distributed throughout the island, being scarce or absent only in the west (P. G. Ryan in litt. 2000).
On Gough, it is found near the coast, in boggy areas and close to streams, being most common in fern-bush, and somewhat less common in level areas of tussock grassland; it is very scarce or absent in wet heath (Watkins and Furness 1986, P. G. Ryan in litt. 1999, P. G. Ryan in litt. 2000, Cuthbert and Sommer 2004). On Tristan, where no tussock remains, it is found in fern-bush (Richardson 1984) and remaining stands of Phylica arborea on the mountainside near the Settlement. It feeds mostly on vegetable matter including seeds, but will also take invertebrates, carrion, and introduced mice Mus musculus and scavenge petrel carcasses. It also forages for invertebrates in abandoned and active albatross nests, petrel burrows, and will feed on garbage (Watkins and Furness 1986). It breeds from September to March, on Gough peaking between October and December, and laying between two and five eggs (Watkins and Furness 1986, P. G. Ryan in litt. 1999).
It is likely that G. nesiotis was extirpated from Tristan as a result of predation by black rat Rattus rattus (P. G. Ryan in litt. 1999), though this may have been in combination with feral cat and pig predation, habitat destruction and hunting by islanders (Groeneberg et al. 2008). The successful establishment of G. comeri on Tristan suggests that it is able to cope with current levels of predation by rats and mice, but the greatest risk to the species is still the accidental introduction of rats or other predators to its stronghold on Gough. Birds have been observed taking live house mice Mus musculus and scavenging mouse carcasses on Gough and Tristan (Wanless and Wilson 2007, A.L. Bond, unpublished data), making it vital that any attempted mouse eradication using poisoned baits takes adequate measures to reduce the potential impact of secondary poisoning on moorhens .
Conservation Actions Underway
On Tristan, a programme to eradicate cats was successful in the 1970s. Gough is a nature reserve and World Heritage Site, and is uninhabited apart from a meteorological station and a small team of scientists. Following a 2006 review of the impacts of introduced rodents on Tristan da Cunha and Gough, a feasibility study for house mouse eradication was published in 2008. This study outlined the research needed before an eradication attempt should be made. Activities have included investigating whether mice living in caves and lava tunnels would be exposed to poison bait dropped by helicopter (Cuthbert et al. 2011), how best to protect sufficient numbers of the two endemic landbirds including Gough moorhen from the risks of exposure to poison bait (Rexer-Huber and Parker 2011), and assessing the moorhen population for diseases and parasites.
Conservation Actions Proposed
Minimise the risk of the introduction of new species of plants and animals to Gough and Tristan. Repeat population monitoring at intervals of 5-10 years.
Text account compilers
Ekstrom, J., Khwaja, N., McClellan, R., Pilgrim, J., Shutes, S., Stattersfield, A., Taylor, J., Symes, A. & Stringer, C.
Hilton, G., Ryan, P.G. & Bond, A.
BirdLife International (2018) Species factsheet: Gallinula comeri. Downloaded from http://www.birdlife.org on 11/12/2018. Recommended citation for factsheets for more than one species: BirdLife International (2018) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 11/12/2018.