A husky Hawaiian honeycreeper drably colored in shades of brown, the Po'ouli (pronounced poh-oh-U-lee and translated "black-faced") was named for its most striking feature by M. Kawena Pukui, noted authority on Hawaiian language and culture. This naming followed the bird's remarkable discovery in 1973 by T. Casey (TC), J. Jacobi, and other college students on a University of Hawaii expedition to the rain forest wilderness of Haleakala Volcano on Maui Island. Since its unexpected debut, the Po'ouli has become a star representative of the unique and dangerously rare species among the Hawaiian avifauna. Today, fewer than ten individual Po'ouli are known to survive.
The Po'ouli has revealed few details of its biology. In the 1970s, the single population of a few hundred individuals inhabited only 600 hectares of rain-drenched, tangled elfin forest on the windward slopes of Haleakala. The Po'ouli seldom calls or sings and typically is seen long before it is heard. It is encountered singly, in pairs, or in family groups, often in association with other foraging insectivorous honeycreepers. A Po'ouli typically hops into view along a tree limb, head down, methodically searching bark and epiphytes for food. It pauses only to rip apart loose bark, lichen clumps, or moss cushions for hidden invertebrate prey. Its diet is yet another strange example of specialization among the Hawaiian honeycreepers, for besides consuming arthropods and fruit, the Po'ouli eats extraordinary numbers of land snails. Despite this innovation, the monogamous breeding system and nesting biology of the Po'ouli show no divergence from those of most other honeycreepers.
The Po'ouli is listed as an Endangered Species by both federal and state governments. The state of Hawaii manages the Hanawi Natural Area Reserve to protect the bird's habitat, and a new recovery program conducted by the National Biological Service is attempting to improve the future of the fewer than 10 surviving Po'ouli.
Knowledge of the Po'ouli is based on few studies. The genus and species were described from 2 immature specimens (Casey and Jacobi 1974). These skins and pickled carcasses were reexamined for plumage characters (Engilis et al. 1996) and dissected for studies of anatomy (Bock 1978, James and Olson 1991) and diet (Baldwin and Casey 1983). Paleontological investigations brought to light the species' former geographic distribution (James and Olson 1991). Comparisons of foraging behavior of 4 Maui honeycreepers included the Po'ouli (Casey 1978, Mountainspring et al. 1990). Other miscellaneous observations made in the mid-1970s by TC are incorporated into our account. The Po'ouli evaded an attempt to estimate its population by the Hawaii Forest Bird Survey, but incidental sightings have yielded enough data to sketch its range, habitat use, and foraging ecology (Scott et al. 1986, Mountainspring et al. 1990). A study of 2 nests revealed much about the nesting biology of this species (Engilis et al. 1996, Kepler et al. 1996b). Recent searches (Engilis 1990, P. Baker, M. Reynolds, and T. Snetsinger pers. comm.) document the species' continued decline, while research begun by P. Baker promises new information.