Until 1995, the Hawai?i, Kaua?i, and O?ahu ?amakihi were considered subspecies under the superspecies Common ?Amakihi (Hemignathus virens). Consequently, most research on H. virens dealt with all subspecies as one species. For this reason, and because of the similarities among the species, the Hawai?i, Kaua?i, O?ahu, and extinct Greater ?amakihi have been considered in a single account (however, recent genetic studies have led to the recognition of the Kaua?i and O?ahu ?amakihi as distinct species). Future revisions may provide separate accounts for each of these species.

The ?amakihi are a group of closely related, small, endemic olive green birds with black lores and short, decurved bills. Found from sea level to the subalpine zone primarily in native forests on all major Hawaiian islands except Ni?ihau and Kaho?olawe during the nineteenth century, they are presently common on the islands of Hawai?i, Maui, and Kaua?i, locally common on O?ahu, uncommon on Moloka?i, and extirpated on L?na?i. The ?amakihi are characteristic birds of the ??hi?a (Metrosideros polymorpha) forests on all islands; they thrive in both xeric and mesic forests and now range from elevations of 100 m in some areas to the uppermost forest regions (2,900 m). Highest densities are found above 1,500 m in drier woodland and forest on Hawai?i Island. The Greater ?Amakihi, which had an extremely limited historical distribution on Hawai?i Island, became extinct early in the twentieth century.

Surviving ?amakihi species are among the least specialized and therefore the most adaptable of Hawaiian native forest birds. All are nonmigratory and omnivorous, taking insects and other arthropods, nectar, juices and pulp of fruit, and sap from fluxes in trees. Foraging alone, in pairs, or in loose, mixed- species flocks, they have fully tubular tongues with fringed tips and, except for the Greater ?Amakihi, decurved bills adapted for nectar-feeding.

Although endemic to the islands of Hawai?i, Maui, Moloka?i, and formerly L?na?i, most research on the Hawai?i ?Amakihi was conducted on Hawai?i Island; unless otherwise noted, data presented apply to the Hawai?i ?Amakihi on this island. Key studies include data on its natural history (Amadon 1950, Baldwin 1953), breeding biology (Berger 1969, van Riper 1975, 1987), foraging behavior (Kamil 1978, Kamil and van Riper 1982), feeding adaptations (Richards and Bock 1973), bioenergetics (MacMillen 1974), parasite levels (van Riper 1975, 1981), and population history and status (Banko 1984, Scott et al. 1986). Few studies are available from Maui and Moloka?i Islands; parallel studies on these islands would add greatly to our understanding of these species.

The Kaua?i ?Amakihi, the largest surviving species in the ?amakihi complex, is similar to ?amakihi of the other Hawaiian Islands, but has a larger, heavier, more decurved bill and a shorter tail. Limited to Kaua?i Island, this species is characterized as a bark specialist, prying with its bill and picking insects from crevices. Like other ?amakihi, it has a fully tubular tongue adapted for taking nectar. Data on the biology of this species are incomplete, and include limited information on its natural history (Rothschild 1893?1900, Perkins 1903, Munro 1960, Wilson and Evans 1890?1899, Pratt 1979, Berger 1981), bioenergetics (MacMillen 1974), breeding biology (Eddinger 1970), and population history and status (Banko 1984, Scott et al. 1986).

The O?ahu ?Amakihi has been little studied and is poorly known, even though it is widespread and locally common in forests on the most populous of the Hawaiian Islands. It is endemic to O?ahu Island, and received full species status rather than merely subspecies status after the recent split of the Common ?Amakihi (Am. Ornithol. Union 1995). The O?ahu ?Amakihi is adaptable in habitat use and generalized in foraging behavior, and it inhabits a variety of forests with both native and introduced plants. It maintains populations at lower elevations than other native Hawaiian passerines, and in some areas it is common in suburban yards and parks. This species may even be expanding at low elevations and beginning to repopulate portions of its former range. Recent evidence indicates that this species may have developed resistance to avian malaria, allowing it to survive at low elevations, where introduced diseases transmitted by mosquitoes have decimated most other native birds.

The Greater ?Amakihi, originally named Green Solitaire by early collectors, was the largest and also the least known of all ?amakihi species. It was discovered in 1892 by H. Palmer (Rothschild 1893?1900), was last recorded in 1901, and is presumed extinct. The primary cause for its extinction was likely habitat loss; predation and disease may have been contributing factors. It had a very limited range in dense forest above Hilo on Hawai?i Island, and was never common, at least in historic times. The only firsthand information on the behavior of this species is from Perkins 1903.