Hawaiian honeycreepers, famous as an example of adaptive radiation on islands, include species that converge in their appearance and way of life on more familiar birds from continents. While some honeycreepers retain an ancestral finchlike form and life history, others resemble and behave like sunbirds, warblers, or nuthatches. In contrast to this convergence, evolution has drawn a few honeycreepers out on novel pathways. What more bizarre evolutionary tangents could one imagine than the 'Akiapölä'au and Nukupu'u? In the absence of woodpeckers, these birds capture invertebrates living in bark or wood, but their tools and methods are entirely different. No other birds have, in a sense, evolved two bills in one. The long upper mandible curves downward like a black wiry hook, whereas the short, robust lower mandible juts forward as a sharp-pointed awl. The lower mandible is more or less straight in the 'Akiapölä'au, but curves with the upper mandible in the Nukupu'u.
Equipped with such tools, the tree-dwelling 'Akiapölä'au sets out foraging. Hitching its way along branches and twigs, the bird tests bark and epiphytes, pausing inquisitively to tap with the awl or probe with the hook. Once it detects a hiding caterpillar or spider, the bird goes to work excitedly. Noisily pounding and yanking, the 'Akiapölä'au excavates the substrate in pursuit of its retreating quarry. Ultimately the prey is cornered, hooked out into the open, and flogged against the bird's perch before being gulped down.
Perhaps their peculiar bills are unsuited to taking nectar, but whatever the reason, 'Akiapölä'au and Nukupu'u usually ignore flowers. 'Akiapölä'au do drink sap from the small, shallow wells they drill in live bark. A bird selects a few individual trees for drilling and visits each repeatedly, so that eventually a large surface of the tree's bark becomes peppered with holes. Drinking sap does not come easily for an 'Akiapölä'au, however, and requires the bird to tilt its head upward so that the fluid runs down inside the mandible guided by a short, fringed tongue. This recently discovered behavior resembles sap-sucking by woodpeckers.
The bill of a young 'Akiapölä'au seems to take many months to grow and harden, and during this time the juvenile remains with its parents. Protected on the family's large territory, the young bird slowly learns foraging skills. Generally only one young fledges, the sole offspring raised by its parents that year.
The 'Akiapölä'au survives as an endangered species on its home island of Hawai'i. Restricted to remnant forest above 1,500 m elevation, the total population numbers about 1,000 birds. Because the species prefers foraging on koa (Acacia koa), but nests almost exclusively in the crowns of 'öhi'a-lehua (Metrosideros polymorpha), the best habitat includes both tree species. Factors limiting its population and distribution include mosquito-borne diseases and the loss and fragmentation of habitat. Depredation by feral cats (Felis catus) and rats (Rattus spp.) and depletion of its arthropod prey base by imported predatory and parasitic insects are likely threats.
The Nukupu'u inhabited forests on three islands-Kaua'i, O'ahu, and Maui-and each island supported its own subspecies. The island forms are so distinct that the Nukupu'u may eventually be split into three species. A recent review of all Nukupu'u records from the twentieth century revealed that this bird is probably extinct. On a more optimistic note, concern for the 'Akiapölä'au has resulted in habitat protection and restoration in key parts of its range. With expanded management as a metapopulation, the 'Akiapölä'au can be spared the fate of its less fortunate sister species.
"Of all the native Drepanid birds none are more interesting in their habits than the species of Heterorhynchus " ( Perkins 1903 : 427). These admiring words, written a hundred years ago by the person most familiar with 'Akiapölä'au and Nukupu'u (then known under genus Heterorhynchus), still ring true today. Yet despite the charm and novelty of these birds, we know surprisingly little about them. Nineteenth-century naturalists described their distributions and habits at a time when both species were much more numerous. Since then the Nukupu'u has disappeared. Occasional reports from the twentieth century vary in reliability, some being misidentifications, and none has shed new light on this mysterious species ( Pratt and Pyle 2000 ). Consequently, for information on Nukupu'u we must depend on eyewitness reports prior to 1900 and on about 100 study skins distributed among museums worldwide. In this account, the three Nukupu'u taxa-Kaua'i Nukupu'u (Hemignathus l. hanapepe), O'ahu Nukupu'u (H. l. lucidus), and Maui Nukupu'u (H. l. affinis)-are treated separately to document comparisions among them (see Systematics: geographic variation; subspecies, below). Recent information on 'Akiapölä'au is cited from the little existing published research and gleaned from data contributed by numerous field biologists. It includes a three-year, weekend study of a now-extinct, color-banded population at Kanakaleonui on Mauna Kea (1989-1992; TKP and P. Chang). This fascinating species has yet to be the focus of intensive field research.