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
Lesser koa finch was smaller than the other scientifically described koa finches, measuring on average only five inches in length. Its life cycle and feeding habits apparently centered around the koa plant from which it got nectar and fruit (and for which it was named). The koa also provided refuge for small flocks of the finch as it avoided people and the noon high sun. It was seen congregating with its larger relative, the greater koa-finch. Several specimens were collected and sent to London, Cambridge, New York, Philadelphia, and Berlin. The bird appeared to have always had a small population. When people brought