The ‘I‘iwi is one of the most spectacular of extant Hawaiian birds, with vermilion plumage, black wings and tail, and long, decurved bill. In pre-European Hawai‘i, beautiful feather capes, sometimes containing hundreds of thousands of ‘I‘iwi feathers, were a symbol of power and prestige among native Hawaiians. The ‘I‘iwi is a bird of the Hawaiian forests. Its decurved bill seems well adapted to exploit nectar from the similarly shaped flowers of lobelioid plants (Campanulaceae). ‘I‘iwi and ‘Apapane (Himatione sanguinea) are well known for their long flights over the forests in search of the flowers of the ‘öhi‘a (Metrosideros polymorpha) tree, their primary food source. Probably as a consequence of their high flights, ‘I‘iwi, ‘Apapane, and ‘Ö‘ü (Psittirostra psittacea) are the only 3 species of endemic Hawaiian honeycreepers in which the same subspecies occurs on more than one island. Contemporary interisland movements have not been documented.
‘I‘iwi are common in mesic and wet forests above 1,500 m elevation on the islands of Hawai‘i, Maui, and Kaua‘i, but their populations consist of less than 50 birds on the lower-elevation islands of O‘ahu and Moloka‘i, and they are now extinct on Läna‘i Island.
The species is highly susceptible to mortality from avian malaria, and viable populations persist only at higher elevations where disease-carrying mosquitoes are rare or absent. This species' range has contracted toward higher elevations during the past decade. One bite from a mosquito infected with Plasmodium relictum caused mortality in 90% of 10 juvenile ‘I‘iwi, and 100% of 10 other ‘I‘iwi bitten more than once died of malaria. One bird that survived malaria developed immunity and survived further challenges with multiple mosquito bites, but there is no evidence that ‘I‘iwi populations are developing disease resistance, as may be occurring with other Hawaiian species.
‘I‘iwi in breeding condition can be found in any month, but peak breeding occurs February to June, usually in association with the peak flowering of ‘ohi‘a plants. ‘I‘iwi pairs remain together during the breeding season and defend a small area around their nest, but they usually disperse from breeding areas after breeding. Negative correlations in densities between ‘I‘iwi and the introduced Japanese White-eye (Zosterops japonicus) may result from interspecific competition for limited nectar resources.
Despite its seasonal high densities and widespread distribution in higher-elevation forests, no aspect of the ‘I‘iwi's life history or biology has been well studied. Short descriptive accounts with notes on ‘I‘iwi distribution in the late 1800s were written by Rothschild (Rothschild 1893a), Wilson and Evans (Wilson and Evans 1890), and Perkins (Perkins 1903). Additional life history information based on collected birds was provided by Amadon (Amadon 1950) and Baldwin (Baldwin 1953). Most information on breeding ecology comes from a single study of unbanded birds on Kaua‘i Island by Eddinger (Eddinger 1970a). Systematic surveys of forest birds on all the main Hawaiian islands in 1976–1982 (Scott et al. 1986) provided the most current data on population size and distribution within much of the ‘I‘iwi's range. Banding studies (Ralph and Fancy Ralph and Fancy 1994c, Ralph and Fancy 1995, SGF) provide valuable information on timing of breeding and molting, local movements, site fidelity, and survival.
Habitat loss and modification because of development and agriculture, and introduction of disease vectors, avian diseases, mammalian predators, and alien plants all continue to threaten ‘I‘iwi populations, as well as those of other native Hawaiian birds.