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Tristan Moorhen Gallinula nesiotis Scientific name definitions

Barry Taylor
Version: 1.0 — Published March 4, 2020
Text last updated January 1, 1996

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Identification

25 cm; 3 adult comeri 505–530 (513) g. Sexes alike. Superficially resembles G. chloropus but smaller, much more strongly built, short-winged, and almost completely flightless but able to climb well and partially fly over obstacles; also differs in plumage, having black, not grey, neck and underparts, no white on flanks, and red legs heavily blotched with greenish yellow. Immature largely greyish brown, with dark sooty grey head, chestnut brown upperparts and sooty grey belly with cream feather tips; shield shiny red; bill dull, dark red with yellow-green tip. Juvenile predominantly brown, belly paler with cream feather tips; two dirty cream undertail bars; shield and bill dull green, bill with whitish tip; tibiae yellow; tarsi and feet green. Races differ only in skeletal measurements, nesiotis being slightly smaller and lighter.

Systematics History

In the past, sometimes placed in genus Porphyriornis. Previously considered to include Gough I form comeri as a race, as supported by DNA studies (1); however, if comeri is accepted as distinct from G. chloropus, it should also be separated from present species. Treated as monotypic.

Subspecies

Reports from the Challenger expedition of 1873 suggest that this species was extant at that time (2), but villagers contacted on the Valhalla expedition in 1906 stated that it was no longer present (3). Probably extirpated by black rats Rattus rattus, although hunting and habitat loss probably also played a part.

Distribution

Tristan da Cunha I (S Atlantic Ocean).

Habitat

Race comeri occurs throughout Gough I in very dense vegetation of tussock grass and bushes in the shrub and tree-fern zones, generally below 500 m and mainly close to coast in boggy areas and along streams. On Tristan da Cunha, where introduced, inhabits rugged, luxuriant, inaccessible fern-bush (Blechnum), with dense Phylica arborea trees, at 300–900 m.

Movement

None. One at Cape Town in 1893 apparently an escape.

Diet and Foraging

Gough I birds appear to feed as much on vegetable matter, seeds and carrion as on invertebrates. Generally remains in cover but feeds in open if undisturbed. Eats grass, and takes grass-heads with scythe-like motion of bill. Scavenges from carcasses of Soft-plumaged Petrels (Pterodroma mollis) and Broad-billed Prions (Pachyptila vittata) partly eaten by Brown Skuas (Catharacta antarctica); also feeds on garbage at meteorological station. Enters petrel burrows, apparently in search of food, and seen foraging in abandoned albatross nests, presumably for invertebrates; also scavenges around active albatross nests.

Sounds and Vocal Behavior

Voice metallic and loud. Common call a harsh, staccato, high-pitched, far-carrying “koo-ik”, often repeated, and taken up by surrounding birds; also loud, screaming series of “koo-ik” notes; and low-pitched, monotonous, whispered, repeated “ik” or “ook” given by pair members and audible at close range. Female gives harsh repeated call near nest.

Breeding

On Gough lays Sept–Mar; on Tristan probably breeds Dec–Mar. Monogamous and territorial; pair-bond apparently permanent. Nest circular and cup-shaped; made of sticks; bowl diameter 14–20 cm; rim 10–15 cm above ground (in captivity 75 cm above ground); placed in grass (Poa flabellata) tussock, with access tunnel from edge of tussock to nest; both sexes build. Eggs 2–5 (up to 6 in captivity); incubation 21 days, by both sexes; downy chick black, with fringe of long silky hairs on throat, bill crimson at base, blue-grey towards tip, with white spot on top of upper mandible, legs and feet black; young fed by both parents; seen to be given fresh flesh from bird carcasses. Immature plumage retained into second year. 2 broods per season recorded, with c. 14 weeks between clutches; young of first brood help to feed second brood.
VULNERABLE. Numbers difficult to assess because of secretive nature. Race comeri common on Gough I, where total population probably 2000–3000 pairs; territory size c. 0·5 ha; no avian competitors, but egg predation by skuas recorded. Introduced to Tristan da Cunha in 1956, whence nominate race was extirpated by around 1900; current population c. 250 pairs. Both populations appear stable, and habitats unlikely to change significantly, but there is permanent risk of mammalian predators becoming established on the islands; however, chances of this happening have been minimized. Re-establishment on Tristan da Cunha successful despite large population of black rats (Rattus rattus); earlier extinction there was attributed to rat predation but possibly more due to combination of habitat destruction, predation by cats and pigs, and hunting with dogs. Captive stock exists in a few European zoos, and species breeds successfully in captivity, but hybridization with G. chloropus occurs and must be avoided.
Distribution of the Tristan Moorhen
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Distribution of the Tristan Moorhen

Recommended Citation

Taylor, B. (2020). Tristan Moorhen (Gallinula nesiotis), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.trimoo2.01