Editor's Note: initial writing of this account combined treatment of the ‘Ö‘ü and the Läna‘i Hookbill, owing to their close relationship and their scarcity. Future revisions of the account will attempt separate treatments of the two species.
The ‘Ö‘ü and the Läna‘i Hookbill are plump, predominantly olive green, thick-billed Hawaiian honeycreepers. The ‘Ö‘ü is now very rare and restricted to remote, high-elevation native forest, and the Läna‘i Hookbill is extinct. They are closely related species, belonging to a specialized tribe, Psittirostrini, consisting of nine historically known Hawaiian species with heavy, finchlike to parrotlike bills. Because of their taxonomic affinity and the paucity of information on the Läna‘i Hookbill, we are treating the two species in a single account.
The ‘Ö‘ü was one of the most common Hawaiian honeycreepers during the early period of ornithological exploration from 1789 to 1900, known from the islands of Kaua‘i, O‘ahu, Moloka‘i, Maui, Läna‘i, and Hawai‘i. The ‘Ö‘ü is most closely associated with mid-elevation ‘öhi‘a lehua (Metrosideros polymorpha) forest with ‘ie‘ie vines (Freycinetia arborea), but historically ranged from high-elevation mämane (Sophora chrysophylla) forests to near sea level, depending on the availability of fruits and insects. This species is renowned for its extended flights between foraging areas. The naturalist aboard Captain James Cook's ship collected ‘Ö‘ü specimens when Europeans first visited the Hawaiian Islands in 1779, thus making the ‘Ö‘ü one of the first Hawaiian forest birds to be described by taxonomists. From before European contact through the early 1900s, Hawaiians collected ‘Ö‘ü for their olive-green plumage for use in feather work worn by royalty.
The ‘Ö‘ü is well represented in museums and private collections, with 323 known specimens (Banko 1986). Ironically, what was once one of the most common and widespread of Hawaiian birds is now (1997) so scarce that its continued existence is in question. It may still exist in critically low numbers on only two of the Hawaiian Islands, Kaua‘i and Hawai‘i. Although ‘Ö‘ü were seen regularly until the 1980s, the accounts of these sightings provide fragments of life history information and document the decline of this unique component of the Hawaiian avifauna.
The historical record is meager for the Läna‘i Hookbill. This species is known from a single specimen collected in 1913 on Läna‘i Island by George C. Munro. It was observed only twice in the wild, once in 1916 and a second time in 1918 (Munro 1944a). On the basis of Munro's specimen, R. C. L. Perkins (Perkins 1919) originally described the species, but subsequent reviewers challenged the description, suggesting that the specimen is an aberrant ‘Ö‘ü (Greenway 1939, Amadon 1950). In 1989, the controversial specimen was reprepared, its skull removed and replaced with an epoxy resin cast (James et al. 1989a). Examination of the cranial osteology, myology, plumage, and bill morphology revealed the validity of Perkins's original description. The unique shape of the mandibles and rarity of the species, compared with the ‘Ö‘ü on Läna‘i Island, suggests that the Läna‘i Hookbill was an extreme specialist with limited range. It may have been restricted exclusively to the ‘akoko (Chamaesyce spp.)– and öpuhe (Urera glabra)–dominated montane dry forest where it was collected. We can only speculate about most aspects of this species' biology on the basis of collection and sighting locations, field notes from Munro, and morphology of the holotype specimen. Its discovery and extinction occurred in less than ten years; it was last seen in 1918.
Members of the Psittirostrini tribe have been especially vulnerable to extinction since first human contact. Fossil remains of nine historically undescribed species tentatively belong to this tribe (Olson and James 1982b). These species were probably extirpated as a result of lowland-habitat modification carried out by early Polynesians (Olson and James Olson and James 1982b, Olson and James 1982a, Cuddihy and Stone 1990, Steadman 1995b). Historically known members of this tribe have not fared much better, as habitat degradation expanded into higher elevations with European arrival (Cuddihy and Stone 1990). These species include the endangered Laysan (Telespiza cantans) and Nihoa (Telespiza ultima) finches, Palila (Loxioides bailleui), and Maui Parrotbill (Pseudonestor xanthophrys); and the extinct Lesser Koa-Finch (Rhodacanthis flaviceps), Greater Koa-Finch (Rhodacanthis palmeri), and Kona Grosbeak (Chloridops kona; Pyle 1997e, U.S. Fish and Wildlife Service [USFWS] U.S. Fish and Wildlife Service 1992b). The ‘Ö‘ü was classified as Endangered by the USFWS in 1967 and by the Hawaii Division of Fish and Game in 1969, and is now considered Critically Endangered, with a 50% probability of extinction within five years or two generations (Mace and Lande 1991, Ellis et al. 1992c, Collar et al. 1994).