Inaccessible Island Finch Nesospiza acunhae Scientific name definitions

17–21 cm; 24–49 g. An exceptionally variable species. Male nominate race is olive-green above and paler olive-grey below, with yellowish forecrown and throat; lores and feathers around eye pale grey; flight-feathers dark olive-brown with yellow-olive outer margins; iris dark brown; bill and legs dark grey. Female is on average slightly smaller than male, also duller and more streaked above. Juvenile and immature are even more heavily streaked than female. Race fraseri is brighter green above and more intensely yellow below than nominate, is on average larger in head, wing and tarsus length, but smaller bill; dunnei is about two-thirds larger than nominate and previous, and has much larger, deeper bill.

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Nominate race largely confined to coastal lowlands and cliffs dominated by Spartina arundinacea tussock grassland; replaced on the plateau by fraseri, although some immatures of this race feed along coast in winter and spring. Race dunnei largely confined to areas of tall Phylica arborea woodland, where there is abundant fruit. All three races interbreed in ecotone between Blechnum heath and Phylica woodland on the plateau.

Mostly resident. Some immatures of race fraseri apparently forage along coast in winter, returning to the plateau in Sept–Oct.

Diet mainly seeds, fruits and invertebrates. Small-billed nominate race eats seeds of Spartina and other grasses, sedges and forbs, as well as invertebrates; often forages on flies (Diptera) in intertidal zone and among stranded seaweeds. Race fraseri eats mainly sedge seeds and Nertera fruit, as well as invertebrates; its brighter plumage apparently results from greater intake of carotenoids, linked to dietary differences (between food provided by fernbush vegetation on the plateau and that of Spartina tussock grassland on coastal scarps). Race dunnei and large-billed hybrids feed mainly on Phylica fruit, although they also take invertebrates. Hybrids often glean insects from epiphytes on Phylica trees on E plateau. Chicks fed with caterpillars and other invertebrates; chicks of dunnei fed with Phylica fruit from an early age. Forages on various substrates, from bark, foliage and lichens to flowers, boulders and ground.

Male song a repeated phrase of 3–4 notes, “whit it teeu”, more varied than that of N. questi; song of race dunnei deeper and slower than those of other races, and hybrids have distinct song. Females have subdued, querulous whistle. Both sexes give a chipping contact call, also alarm call when skua (Catharacta) appears.

Season Nov–Feb; single-brooded. Usually monogamous, and partners remain together for successive breeding attempts; one male of nominate race had two females on adjacent territories. Possible helper at one nest of race dunnei. Territorial, both sexes repelling intruders. Nest built by female, a cup made from grass and sedge leaves, placed in dense vegetation, usually close to ground, sometimes up to 1·2 m up in dense Spartina tussock. Clutch 1–2 eggs (average 1·8), light blue, finely speckled dark brown; may lay several replacements following failures; incubation by female, fed near nest by male, period 17–18 days; chicks brooded by female for first 5–7 days, after which both parents feed chicks, both also remove faecal pellets when chicks small, but allow these to accumulate on nest rim when chicks large, nestling period 18–21 days; fledglings remain in dense cover for 10 days after leaving nest, then accompany adults for up to 6 weeks. Main causes of failure are predation by Tristan Thrush (Turdus eremita), also inclement weather; starvation possibly a problem for dunnei, which have much lower breeding success (only 13% of eggs result in fledglings) than small-billed birds and hybrids (52%).

VULNERABLE. Restricted-range species: present in Tristan Islands EBA. Abundant and widespread on Inaccessible I (14 km²), with estimated 10,000 pairs. Population of nominate race on main island of Tristan disappeared within 50 years of settlement, probably as a result of predation by introduced house mice (Mus domesticus) and feral cats (Felis catus). Density varies with habitat, up to 18 pairs/ha in coastal tussock, but only 2–6 pairs/ha on the plateau. Although this species is very numerous within its extremely small range, and is thought not currently to be declining, the possible arrival of invasive species could rapidly result in steep decline in its numbers, potentially leading to extinction within a short time period (as has happened on Tristan da Cunha). Paradoxically, Inaccessible I has become far more accessible with arrival of several motorized boats on Tristan, increasing risk of accidental introduction of mammalian predators and other invasive species. A further threat is invasion of introduced New Zealand flax (Phormium tenax) into areas of Phylica woodland, leading to degradation of habitat.