Henderson petrel  (Pterodroma atrata)

Classified as Endangered (EN) on the IUCN Red List (1).

Named after the only island on which it is known to breed, the Henderson petrel is a medium-sized, slightly built gadfly petrel (Pterodroma species) with fairly uniform grey-brown plumage. The underparts are slightly paler, and the feathers of the forehead and chin have greyish tips, giving a more mottled appearance around the beak. The underside of the wing has a pale leading edge between the wrist and the body, and the bases of the primary feathers are silvery-white (2) (3). As in other gadfly petrels, the Henderson petrel’s beak is relatively short and stout, and is equipped with a powerful hook and sharp cutting edge, used for gripping and cutting up its prey (4). The beak is black, while the legs are pink and the feet have dark tips (2) (3). The most common call of the Henderson petrel is a repeated kyek-kyek-kyek-kyek, given both in flight and from the ground (3).

Until quite recently, the Henderson petrel was considered to be a dark form of the Herald petrel, Pterodroma heraldica, which is distinguished by its pale belly. In addition to this plumage difference, the two species were split based on the fact that they do not appear to interbreed, tend to breed on different parts of the island and in slightly different seasons, and have different calls. Studies have also shown genetic differences between the two (5).

As its common name suggests, the Henderson petrel breeds on Henderson Island, one of the Pitcairn Islands in the South Pacific. The species is currently only known to breed on this island, although it may also have bred on Pitcairn Island in the past and appears to have stopped breeding on Ducie Island during the last century. Petrels resembling this species have also been recorded from other islands in the region, including the Gambier and Marquesas Islands in French Polynesia, and it is possible that the Henderson petrel may breed at these additional sites (2) (3).

On Henderson Island, this species nests on the island’s coral plateau, amongst dense forest (2), but nearer to the coast than the Herald petrel (5). No information is available on where it ranges at sea.

Relatively little is known about the biology of the Henderson petrel. However, like other, related species it is likely to feed mainly on squid, fish and some crustaceans (2) (3) (4), either caught by dipping down to the water’s surface during flight, or taken when the bird has alighted on the water (3). Gadfly petrels typically have a strong, swift flight that combines brief periods of flapping with glides on bent wings, the glide often taking the bird into a high arc in the air (3) (4).

The Henderson petrel may breed at any time of year, although peak breeding occurs between June and July (3) (5). Most petrel species return to the breeding colony a few weeks before nesting, and may use the same nest site from year to year (4). Courting pairs perform aerial displays (4), calling together in flight (5). Like other petrels, the Henderson petrel is likely to lay a single, large, white egg (4), which may hatch after around 50 days (5). This species lays its egg on the ground, and the male and female take turns at incubation (3), each undertaking ‘shifts’ which in related species may last up to 19 days at a time (3) (4) (6). These impressively long shifts allow the other member of the pair to forage over large distances out at sea (3). Young chicks are vulnerable to predation by crabs and by the introduced Polynesian rat (Rattus exulans) (2) (3), and breeding success in this species may be low, with fewer than a fifth of eggs going on to produce fledged chicks (6). Those chicks which do survive are thought to fledge at around 90 days (3).

The Henderson petrel is undergoing a long-term decline on Henderson Island as a result of chick predation by introduced Polynesian rats (R. exulans) (2) (6) (7) (8), which have been estimated to kill 25,000 petrel chicks on the island each year (9). If this species ever bred on Pitcairn Island, its disappearance there is also likely to have been caused by introduced rats and cats (2), and it has not been confirmed to breed on Ducie Island since 1922 (3), probably for the same reasons. However, other than for rats and a few introduced plants, the ecology of Henderson Island is still remarkably intact, with little human impact because of the island’s remote location and rough terrain. Although Pitcairn Islanders travel to the island to cut wood, this has little impact at current levels, and a potentially damaging attempt by a millionaire to build a house, airstrip and cattle ranch on the island in the 1980s was denied permission. Any increase in tourist visitors to this uninhabited island may be of concern as it could increase the risk of environmental damage and the accidental introduction of further mammalian predators (10) (11) (12).

The restriction of the Henderson petrel to one small known breeding site, or potentially only a handful of other sites at most, makes it particularly vulnerable to extinction. Climate change and sea level rise could also potentially affect this species (2).

Although rat and cat eradication attempts on Pitcairn Island have not yet been successful, rats have been successfully eliminated from Ducie and Oeno, raising the chances of the Henderson petrel becoming established on these islands in the future (2) (3). Although rat eradication on Henderson Island is made more difficult by its remote location (2), it will be vital if the decline of the Henderson petrel is to be halted. Fortunately, recent research has addressed potential obstacles to an eradication programme, and rat eradication is planned to start in 2011, subject to sufficient funding (9) (13).

Henderson Island, this species’ stronghold, is designated a World Heritage Site, and access to the island requires a licence (11). Although the remote location makes this difficult to enforce (12), it will be important to control visitors to ensure minimal impacts from tourism and to prevent the introduction of non-native species, including potentially devastating mammalian predators (8) (12). A Management Plan in place for the island aims to protect its wildlife and ecology, ensure wood-cutting by Pitcairn Islanders is sustainable, prevent long-term damage by tourists, and promote awareness of the value of the island and its wildlife (12). It will also be important to involve the local community and other key groups in saving this unique site (9). The Henderson petrel is listed on Appendix I of the Convention on Migratory Species (CMS), meaning that parties aim to strictly protect the species and conserve its habitat (14), and periodic surveys have been recommended to monitor its population trends (2).

ARKive is supported by OTEP, a joint programme of funding from the UK FCO and DFID which provides support to address priority environmental issues in the Overseas Territories, and Defra

To find out more about conservation on Henderson Island, see:

• BirdLife: EBA Factsheet - Henderson Island:
  http://www.birdlife.org/datazone/ebas/?action=EbaHTMDetails.asp&sid=217&m=0
• UNEP-WCMC: Henderson Island, United Kingdom:
  http://www.unep-wcmc.org/medialibrary/2011/06/30/b9c7b2f6/Henderson%20Island.pdf
• RSPB: Henderson Island Restoration Project:
  http://www.rspb.org.uk/ourwork/projects/details.aspx?id=tcm:9-241934

For more general information on conservation in the UK Overseas Territories, see:

• UK Overseas Territories Conservation Forum:
  http://www.ukotcf.org/
• Oldfield, S., Procter, D. and Fleming, L.V. (1999) Biodiversity: the UK Overseas Territories. Joint Nature Conservation Committee, Peterborough. Available at:
  http://www.jncc.gov.uk/page-3045