Bristle-thighed curlew (Numenius tahitiensis)
|Size||Length: 40 – 44 cm (2)|
Wingspan: 82 – 90 cm (2)
Male weight: 254 – 533 g (2)
Female weight: 372 – 796 g (2)
Classified as Vulnerable (VU) on the IUCN Red List 2007 (1) and listed on Appendix II of the Convention on the Conservation of Migratory Species (CMS) (3).
This medium-sized shorebird possesses a long, down curved, touch-sensitive bill; a unifying characteristic of all curlews. The curlew’s genus name Numenius means ‘of the new moon’ and refers to the long, crescent-shaped bill. It has plumage that is mostly flesh-coloured to pale brown, with darker brown broad stripes on the head, and the upperparts are spotted cinnamon and dark brown. The underparts are streaked buff, the belly and feathers under the tail are whitish, and the legs are a pale blue-grey (2) (4). Their name comes from the bristle-like extensions at the base of their legs, although these are generally inconspicuous and can only be seen when close up. Females are heavier than males and have longer wings and a shorter bill. Juveniles are similar to adults except for the presence of larger cinnamon-buff spots on the upperparts, and virtually unstreaked underparts (2).
The bristle-thighed curlew breeds on the lower Yukon River and central Seward Peninsula in western Alaska, United States. It winters on many small oceanic islands in the South Pacific, from the Marshall and Hawaiian Islands, south to Santa Cruz, Fiji, Tonga, Samoa, Marquesas, Tuamotu Island and Pitcairn Island (2).
During the breeding season it can be found on exposed hilly tundra. Over winter, it occurs on oceanic islands, generally in the interior amongst dense vegetation, but sometimes on sandy beaches, rocky shores, tidal mudflats and exposed reefs (2) (4).
Breeding in the bristle-thighed curlew probably starts around May, when the bird can be found on the Alaskan tundra (2). They are monogamous birds, forming long-term bonds, and are not only faithful to a partner, but also to breeding and wintering sites, returning to the same place year after year (5). Generally, four eggs are laid into a simple, bare depression in mossy vegetation, and both sexes will incubate the eggs for about 25 days (2) (6). The chicks leave the nest shortly after hatching, and continue to receive parental care, initially from both parents, and then just the male as the female deserts the brood before the chicks fledge (2) (6). Whilst breeding, the adults aggressively defend the eggs and chicks, and often attack potential predators or perform displays to distract them (6).
After the breeding season juveniles and adults congregate on the Yukon River delta. Here they feed on berries, insects and other foods in preparation for the migration to their wintering grounds; an impressive journey in which they fly non-stop for over 4000 kilometres (2) (6). They arrive at the oceanic islands in late August to early September, where they will undergo a moult. During this moult, over 50 percent of adults become completely flightless, for a period of two weeks (2).
Whilst on their wintering grounds, the curlews are opportunistic feeders, taking crustaceans, insects, spiders, snails, small fish, scorpions and the eggs of seabirds (2) (6). They show remarkable ingenuity by using rocks to crack the thick egg shells, a rare example of tool use by birds (6). Occasionally they also feed on seabird carrion and fish regurgitated by seabirds and lizards (2).
The fairly small population of bristle-thighed curlews is believed to be declining, primarily due to the impacts of introduced predators on their wintering grounds (1). The flightless period during the moult would have evolved during a time when there were no mammalian predators on the South Pacific islands where it spends the winter. Today, with the establishment of humans on these islands, and the subsequent introduction of mammals, moulting leaves the curlew in an extremely vulnerable position. Introduced cats, dogs and possibly pigs, prey heavily on the flightless curlews, causing a significant decline in numbers (4) (6).
Hunting was a threat to this species in the past; on the Tuamotu islands it was traditionally caught for food. However, the possession and use of firearms is now restricted on these islands and therefore hunting no longer poses a significant threat (2). On their breeding grounds the curlew suffers predation by Arctic skuas, common ravens and foxes, and the expanding development of gold mines and mining roads on the Seward Peninsula is an increasing threat that could have a significant impact on this bird, due to its reliance on this small area for breeding (2) (6).
The bristle-thighed curlew occurs in a number of protected areas. Both the Yukon Delta and some of the remote Hawaiian Islands, are designated National Wildlife Refuges, which protects breeding grounds and several stop-over and wintering sites (4) (6). Wildlife and habitat is monitored in the Yukon Delta, but further specific monitoring of the curlew is recommended (4). Hawaii’s Comprehensive Wildlife Conservation Strategy proposes a number of conservation actions for the bristle-thighed curlew including protecting the current habitat of the curlew in Hawaii, and protecting and restoring additional wetland habitat, especially where it can be reclaimed from abandoned urban or agricultural uses (7). However, to protect this species from further declines, it is important to tackle the primary threat of introduced mammals across its wintering range.
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- Carrion: the flesh of a dead animal.
- Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
- Monogamous: having only one mate during a breeding season, or throughout the breeding life of a pair.
IUCN Red List (November, 2011)