The extinct ‘akialoa species were among the most morphologically specialized of all Hawaiian birds in the subfamily Drepanidinae. With inconspicuous yellow-green plumage, their most distinctive features were their spectacular decurved bills, up to half the body length. These large Hawaiian honeycreepers foraged principally by creeping along tree trunks and large branches, probing for insects under moss and bark, and taking nectar from tubular flowers. In behavior and bill morphology, they were broadly convergent with Southeast Asian spider-hunters (Nectariniidae: Arachnothera spp.).
Four taxa of ‘akialoa existed during historical times, differing from one another primarily in body size and bill length, with the largest, longest-billed form on Kaua‘i Island in the northwest and the smallest, shortest-billed form on Hawai‘i Island in the southeast. Birds of intermediate size existed on the islands of O‘ahu and Läna‘i. Because of this size gradient, and the extreme scarcity of specimens from O‘ahu and Läna‘i Islands, taxonomists differ on species limits applied to these 4 taxa, recognizing as few as 1 polytypic species or as many as 4 separate species.
The most recent treatment by the American Ornithologists' Union (American Ornithologists' Union 1998a) recognizes 2 species of ‘akialoa: (1) Greater ‘Akialoa, with 3 sub-species (referred to as groups in American Ornithologists' Union 1998a), 1 each on Kaua‘i, O‘ahu, and Läna‘i Islands; (2) Lesser ‘Akialoa, on Hawai‘i Island. Because the forms described from each of the 4 islands are distinctive and their taxonomic status uncertain, we treat each separately in this account, to the extent possible given what little is known of these extinct birds. Hereafter, each island form is referred to by name as follows: The 3 subspecies of Greater ‘Akialoa are Kaua‘i ‘Akialoa (H. e. stej-negeri), O‘ahu ‘Akialoa (H. e. ellisianus, Figure 1, top), and Läna‘i ‘Akialoa (H. e. lanaiensis); and the Lesser ‘Akialoa is called Hawai‘i ‘Akialoa (H. obscurus, Figure 1, bottom).
Prehistoric forms of ‘akialoa have been described from fossil bones, including the Hoopoe-billed ‘Akialoa (H. upupirostris), which was similar in size to the Kaua‘i ‘Akialoa, and found on Kaua‘i and O‘ahu Islands (James and Olson 1991). Fossil bones of an ‘akialoa similar to and probably conspecific with the Läna‘i ‘Akialoa have also been found on Moloka‘i and Maui Islands, along with a second, smaller species on Maui Island and a very large species on Hawai‘i Island (H. F. James pers. comm.). All these prehistoric species likely fell victim to human-induced changes (e.g., habitat destruction, predation, and disease), and should be considered part of the modern fauna of Hawaiian Islands. Collectively, these fossil forms and the 4 historically known forms are referred to as the ‘akialoa complex.
Common when first encountered in 1779 by the Cook Expedition (which first recorded the name “Akaiearooa”), the historically known ‘akialoa species suffered rapid population declines concurrent with the spread of avian diseases and extensive habitat modification and destruction during the nineteenth century. Already uncommon or scarce when first encountered by Europeans, just 2 or 3 specimens of the O‘ahu ‘Akialoa (from mid-1830s) and 3 of the Läna‘i ‘Akialoa (from 1892) are known to exist in museums worldwide, and next to nothing is known of these 2 taxa. The Hawai‘i and Kaua‘i ‘akialoa remained common until the 1890s, but the former became extinct in the early to mid-1900s, while the latter was last sighted in the remote Alaka‘i region of Kaua‘i Island in the late 1960s. All species are now believed extinct.