I'iwi are very conspicuous in the upper elevation native forests. They're active and vocal, and make a variety of unusual sounds. One characteristic call is described as sounding like a "rusty hinge". Others include clucks, whistles, barks, hoots, mimicry of other birds, etc. They have a distinctive vermillion body and curved bill, and black wings. (They're more orange than the 'apapane's scarlet.) The juvenile has a more interesting and often more attractive combination of gold-green head, light gray body and black wings. Like the 'apapane, their wings whir when they fly.

The long-lost mamo were similar in overall shape to the i'iwi, but larger and black. The gold highlight feathers of the Hawai'i mamo were the most prized feathers of the Hawaiian birds, and were used to create Kamehameha's golden cape. Moloka'i was the home of the black mamo ("hoa").