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 (9). Sexual dimorphism in throat color is more pronounced on younger islands (Hawaii) than on older islands (Kauai).
Unusual among native Hawaiian birds, elepaios use a range of habitats and sometimes inhabit disturbed secondary forest dominated by non-native plant species (11). They 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.
All the elepaio species are adaptable, and the Hawaii Elepaio in particular occupies forested habitats ranging from dense rainforest to dry, open woodland. Individuals in these different habitats exhibit substantial variation in plumage color and body size that is caused by climatic variation (10). Birds are larger and have shorter bills in colder, high elevation environments, in accordance with Bergmann's and Allen's rules, and they are more heavily pigmented in warm humid environments, following Gloger's rule. The variation in plumage color lead to descriptions of three different subspecies on Hawaii (C. s. sandwichensis, C. s. ridgwayi, and C. s. bryani), but this variation is continuous and clinal and birds from different parts of the island do not exhibit sufficiently consistent morphological differences to warrant subspecific definition (10), nor are they genetically differentiated (7).
The Hawaii Elepaio is fairly common and widespread at higher elevations (12). The elepaios are affected by introduced mosquito-borne diseases, but have greater immunity than most of the Hawaiian honeycreepers, and in some areas elepaios are found at low elevations (13, 14).
All three elepaio species 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 Hawaii Elepaios have lived to be at least 23 years old. Younger birds in subadult plumage are subordinate and seldom breed, 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.