Oʻahu nukupuʻu bill to pluck insects found underneath tree bark, the Oahu nukupuu fed on insects on flowering trees such as the koa ("Acacia koa") and ohia ("Metrosideros polymorpha"). These trees attract a large amount of insects due to their nectar-filled flowers. The Oahu nukupuu fed on koa in high elevation forests and fed on ohia in low elevations. The species was believed to have vanished as the spread of disease occurred, killing off nukupuu populations across the islands. In order to control the rat population in the sugar cane fields, mongooses were introduced to Hawaii and were suspected to be predators
Acacia koa confirmed this close relationship; the estimated time of divergence is about 1.4 million years ago. "A. heterophylla" sequences nest within those of the more diverse "A. koa", making the latter paraphyletic. Both species are thought to be descended from an ancestral species in Australia, presumably their sister species, "Acacia melanoxylon". Dispersals most likely occurred via seed-carrying by birds such as petrels. Both species have very similar ecological niches, which differ from that of "A. melanoxylon". A closely related species, koaia or koaie ("A. koaia"), is found in dry areas. It is most easily distinguished by having smaller seeds that are