Henderson, a raised island in the south-central Pacific, is one of a group of isolated islands which includes volcanic Pitcairn (the only currently inhabited island, well known as the final refuge for the Bounty mutineers, and treated as Secondary Area s137; see 'Restricted-range species', below) and Oeno and Ducie atolls, constituting the political unit of the Pitcairn Islands, a dependent territory of the United Kingdom.

Henderson is covered in dense scrub forest and, with the exception of a few introduced plant species including coconut palms, is one of the few relatively undisturbed elevated limestone islands in the world (see also EBA 099).

All the restricted-range species occur in the interior forest and a couple are also common in forest edge and surrounding scrub.

Acrocephalus taiti, a Henderson endemic, was formerly treated as a race of A. vaughani by Sibley and Monroe (1990), with the two other races vaughani on Pitcairn and rimatarae on Rimatara in French Polynesia. Consequently this EBA was originally defined as including Pitcairn Island (ICBP 1992) until the three were recognized as separate species (Graves 1992, Sibley and Monroe 1993), resulting in Pitcairn being treated as a Secondary Area (s137) and Rimatara, formerly treated as a Secondary Area but now with two endemic species, as an EBA (211). A. taiti differs from its two close relatives not only in plumage and vocalization but also in forming breeding groups of unrelated birds, though it has been suggested that this is a direct consequence of Henderson's stable habitat which is at carrying capacity for the species (Brooke and Hartley 1995).

The vulnerability of Henderson was exposed in 1982-1983 when a millionaire sought (unsuccessfully) to make it his home (Serpell et al. 1983). Current threats come from the Pacific rat Rattus exulans, introduced by Polynesian settlers, and wood-cutting (of miro and tao for carving into curios) by Pitcairners, although present very low levels of clearance apparently make little impact on the avifauna.

Numerically healthy populations of the restricted-range species recorded in 1987 and 1992 indicate that Henderson's surviving landbirds have apparently adapted to the presence of Pacific rat or are at least able to coexist with it. Using different techniques, population estimates are c.3,240 and 6,200 individuals for Porzana atra, c.3,420 and 3,400 individuals for Ptilinopus insularis, 720-1,820 individuals and 1,200 pairs for Vini stepheni, and c.10,800 and 9,500 breeding individuals for Acrocephalus taiti (Graves 1992, Brooke and Hartley 1995, Brooke and Jones 1995, Jones et al. 1995d, Trevelyan 1995).

Nevertheless all the endemic species are threatened on account of their tiny ranges and their consequent permanent inherent risk of extinction. Acrocephalus taiti was not evaluated as a separate species by Collar et al. (1994), although it clearly would also qualify for Vulnerable status. The greatest potential danger is the possible accidental introduction of other mammalian predators, especially black rat R. rattus, and diseases (e.g. avian malaria and pox). The introduction of some of the exotic Pitcairn plant species could have very serious consequences for the native Henderson vegetation (Waldren et al. 1995) and ultimately for other wildlife.

Henderson is a wintering site of international significance for Bristle-thighed Curlew Numenius tahitiensis (40-50 birds in 1991), a threatened (Vulnerable) restricted-range species breeding in Western Alaska (Secondary Area s002) (Brooke 1995a).

Henderson is also important for seabirds (11 breeding species) and, in particular, for three currently recognized gadfly petrels-Kermadec Petrel Pterodroma neglecta, Herald Petrel P. heraldica and Murphy's Petrel P. ultima-which all suffer severely from rat predation with less than 20% of eggs laid yielding fledglings (Brooke 1995b). The two morphs of P. heraldica, which both occur on Henderson, are likely to be separate species because they show reproductive isolation, both nesting and courting assortatively in different parts of the island, and having different calls (Brooke and Rowe 1996); Henderson could be the world stronghold for the dark form, the proposed 'Henderson Petrel'. As rat predation severely affects its breeding success and perhaps even its survival on this island, the newly described species will qualify as threatened. Although rat eradication may not be practical on Henderson, it is being considered on Oeno with a view to providing a secure refuge for this bird.

Between January 1991 and March 1992, a detailed survey of the Pitcairn Islands in general and Henderson in particular was carried out by the Sir Peter Scott Commemorative Expedition (see Brooke 1992, Benton and Spencer 1995). This expedition aimed to provide a comprehensive study of the terrestrial and marine biota, the geology, geomorphology and archaeology, and to provide a management plan for Henderson, which has been designated a World Heritage Site.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Henderson Island. Downloaded from https://datazone.birdlife.org/eba/factsheet/217 on 23/11/2024.