The elepaios are a group of three closely related species endemic to the Hawaiian Islands, and are the only members of the monarch flycatcher family (Monarchidae) in the United States. Elepaios occur on the islands of Hawai'i, O'ahu, and Kaua'i, but are absent from the other islands, even in the fossil record (James and Olson 1991, Burney et al. 2001). The form on each island was originally described as a separate species, but they were later lumped (Bryan and Greenway 1944), and this taxonomy was followed for many years (American Ornithologists' Union 1998). In 2010, the AOU approved a petition by the author to split the Elepaio into three species again (Chesser et al. 2010), based on a combination of genetic, morphological, and behavioral evidence (Conant et al. 1998, VanderWerf 2007, VanderWerf et al. 2010). The three species are covered in a single Birds of North America account because they are similar in many respects; subheadings were added to highlight species differences where sufficient information was available.
Despite similarities among islands, the three elepaio species differ in plumage coloration and vocalizations, and in some aspects of their ecology, such as habitat use (VanderWerf 2012a,b). Elepaios on all islands exhibit a 2-year delay in plumage maturation in both sexes, but the subadult plumages differ among islands in degree of similarity to adult plumage (VanderWerf 2001, 2012a). Sexual dimorphism in throat color is more pronounced on younger (Hawai'i) than on older (Kaua'i) islands.
The elepaios are adaptable species that occupy a variety of forested habitats, from dense rain forest to dry, open woodland. They exhibit substantial variation in plumage color and body size that is caused by climatic variation, particularly on Hawai'i (VanderWerf 2012a). They 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 Hawai'i (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 (VanderWerf 2012a), and they are not genetically differentiated (VanderWerf et al. 2009).
The Hawaii Elepaio and Kauai Elepaio are fairly common and widespread at higher elevations on their respective islands, and the Kauai Elepaio is the only native forest bird species on Kaua'i that is not declining (Gorresen et al. 2009). In contrast, the Oahu Elepaio has declined seriously in the last few decades and now numbers only 1,200 birds (VanderWerf et al. 2013), and is listed as Endangered (USFWS 2006). Nest predation by non-native black rats (Rattus rattus) is the greatest threat to the Oahu Elepaio. Efforts to control rats have resulted in some local population increases (VanderWerf and Smith 2002, VanderWerf 2009, VanderWerf et al. 2011). Remarkably, the Oahu Elepaio is evolving in response to predation by building its nests higher off the ground where they are less accessible to rats (VanderWerf 2012c).
Unusual among native Hawaiian birds, elepaios are also adaptable in their habitat use and sometimes inhabit disturbed secondary forest composed of non-native plants. 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, trunks, branches, leaves, and in the air. They have greater immunity than most of the Hawaiian honeycreepers to introduced mosquito-borne diseases, and in some areas they are found at low elevations (VanderWerf et al. 2006).
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 individuals have lived to be at least 23 years old on Hawai'i, 22 years old on O'ahu, and 13 years old on Kaua'i. 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.