The Maui nukupuu is a medium-sized, approximately 23 gram, Hawaiian honeycreeper (family Fringillidae, subfamily Drepanidinae) with an extraordinarily thin, curved bill, slightly longer than the bird’s head. The lower mandible is half the length of the upper mandible and follows its curvature rather than being straight as in the related akiapolaau (Hemignathus munroi) of Hawaii island. Plumage of adult males is olive green with a yellow head, throat, and breast, whereas adult females and immatures have an olive-green head and yellow or yellowish gray under-parts (USFWS 2006, p. 2-92).