The 'Apapane is the most abundant species of Hawaiian honeycreeper and is perhaps best known for its wide-ranging flights in search of localized blooms of ō'hi'a (Metrosideros polymorpha) flowers, its primary food source. 'Apapane are common in mesic and wet forests above 1,000 m elevation on the islands of Hawai'i, Maui, and Kaua'i; locally common at higher elevations on O'ahu; and rare or absent on Lāna'i and Moloka'i.
The 'Apapane and the 'I'iwi (Vestiaria coccinea) are the only two species of Hawaiian honeycreeper in which the same subspecies occurs on more than one island, although historically this is also true of the now very rare 'O'ū (Psittirostra psittacea). The highest densities of 'Apapane are found in forests dominated by 'ōhi'a and above the distribution of mosquitoes, which transmit avian malaria and avian pox to native birds. The widespread movements of the 'Apapane in response to the seasonal and patchy distribution of 'ōhi'a flowering have important implications for disease transmission, since the 'Apapane is a primary carrier of avian malaria and avian pox in Hawai'i.
The 'Apapane has an incredibly diverse array of songs and calls that vary between and even within islands. Their bright crimson plum-age, black wings and tail, prominent white undertail-coverts and abdomen, and long, decurved bill are characteristic as they move quickly among 'ōhi'a flower clusters, probing for nectar and gleaning insects. 'Apapane density may exceed 3,000 birds/km2at times of 'ōhi'a flowering, among the highest for a noncolonial species. Birds in breeding condition may be found in any month of the year, but peak breeding occurs February through June. Pairs remain together during the breeding season and defend a small area around the nest, but most 'Apapane disperse from breeding areas after nesting, and fidelity to local breeding areas seems low for most individuals.
Despite their seasonal high densities and widespread distribution in higher-elevation forests, no aspect of 'Apapane life history or biology has been well studied. Loss and modification of native forests for agriculture and development have greatly reduced the numbers and distribution of 'Apapane in the Hawaiian Islands, and introduced mammalian predators and avian diseases continue to limit populations of 'Apapane and other native Hawaiian birds.
The Laysan Honeycreeper (Himatione sanguinea freethii), a subspecies of 'Apapane from Laysan I. in the Northwestern Hawaiian Is., was first discovered in 1891 but became extinct in 1923 as a result of the almost complete destruction of the island's vegetation by introduced rabbits. This bird was described by Rothschild ( Rothschild 1893a ) as "by far the rarest of the Laysan Island birds, though I have observed a fair number, generally in pairs. They are very active in their movements, flitting about in the scrub. It feeds on very small insects as a rule, but I have also observed it sucking honey from the flowers."