The 'Anianiau is the smallest Hawaiian honeycreeper in existence today, weighing only 9-10 g. The male is a nearly uniform bright yellow to yellow green, whereas the female is a slightly duller yellow green. Attributes such as pale lores, nearly uniform coloration, and small size distinguish this species from other similar Hawaiian honeycreepers.
'Anianiau primarily feed on nectar from native trees including 'ōhi'a, ōhelo, and other native and introduced plants. They also glean insects from the outer canopy of trees, and are rarely seen on the trunk or larger branches. The 'Anianiau is a socially monogamous species, which breeds from February to June, and usually lays three eggs. Both sexes care for the young, and the male helps construct the nest and feeds both the incubating female and hatchlings.
The 'Anianiau is strictly confined to Kaua'i. Originally distributed throughout the entire island, the 'Anianiau is now restricted to mountain ranges mostly above 600m in elevation in forests of the Koke'e, Waimea, and Alaka'i regions. Although its range and population has declined in the past 100 years, the 'Anianiau's population is thought to be stable in its remaining habitat.