The elepaios are a group of three closely related species endemic to the Hawaiian Islands, and the only members of the monarch flycatcher family (Monarchidae) in the United States. Elepaios occur on the islands of Hawaii, Oahu, and Kauai, but are absent from the other islands, even in the fossil record (1, 2). The form on each island was originally described as a separate species, but they were later lumped (3), and this taxonomy was followed by the American Ornithologists' Union for many years (4). In 2010, based on a combination of genetic, morphological, and behavioral evidence (5, 6, 7), the American Ornithologists' Union approved a petition to split the Elepaio into three species once again (8).
The three elepaio species differ in plumage coloration and vocalizations, and in some aspects of their ecology, including habitat use (9, 10). Elepaios on all islands exhibit a two-year delay in plumage maturation in both sexes, but the subadult plumage differs among islands in degree of similarity to adult plumage (11, 9). Sexual dimorphism in throat color is more pronounced on younger islands (Hawaii) than on older islands (Kauai).
The Oahu Elepaio is by far the rarest of the three elepaio species. It has declined seriously in the last few decades and now numbers only about 1,200 individuals (12), and is listed as Endangered by the U.S. Fish and Wildlife Service (13). Nest depredation by the non-native Black Rat (Rattus rattus) is the greatest threat to the Oahu Elepaio. Efforts to control rats have resulted in some local population increases (14, 15, 16). Remarkably, the Oahu Elepaio is evolving in response to depredation by building its nests higher off the ground where they are less accessible to rats (10). The elepaios are affected by introduced mosquito-borne diseases, but have greater immunity than most of the Hawaiian honeycreepers, and in some areas they are found at low elevations (17).
Unusual among native Hawaiian birds, elepaios are adaptable in their habitat use and the Oahu Elepaio in particular often inhabits disturbed secondary forest dominated by non-native plant species. Elepaios are versatile foragers and use a remarkable variety of behaviors to search for and capture prey at all heights and on all substrates in the forest, including the ground, tree trunks, branches, leaves, and in the air.
Elepaios are long-lived, sedentary, and occupy the same small territory throughout the year, and individuals can be found in the same location year after year. Banded Oahu Elepaios have lived to be at least 22 years old. Younger birds in subadult plumage are sexually mature, but are subordinate to older birds, often acting as floaters until they acquire their own territory.
Bold and inquisitive, elepaios often investigate and even follow hikers, holding their tail cocked up in a curious posture, and can be attracted easily by squeaking. The elepaios are important in the mythology of the Hawaiian people, and are considered the guardian spirit of Hawaiian canoe makers.