The Nihoa Millerbird and Laysan Millerbird are the only known Old World warblers (subfamily Sylviinae) that colonized the Hawaiian Archipelago, the most remote group of islands in the world. The Laysan form, discovered first, was named “millerbird” (Henshaw 1902a) because of its fondness for feeding on large miller moths (Family Noctuidae: probably Agrotis spp.). The Laysan and the Nihoa millerbirds are generally regarded as (at least) separate subspecies.
The extant Nihoa form is endemic to the tiny (63 ha; 40 ha vegetated) island of Nihoa, and the now extinct Laysan Millerbird was endemic to Laysan Island (407 ha; 187 ha vegetated), approximately 1,060 km to the northwest of Nihoa (U.S. Fish and Wildlife Service 1984d, Conant 1985a, Morin and Conant 1994). The Nihoa Millerbird was discovered in 1923, in the same year that the Laysan subspecies was concluded to be extinct (Wetmore 1923 in Olson 1996a). There is no evidence that this genus ever occurred elsewhere in the Hawaiian Archipelago (Olson and Ziegler 1995).
Both forms are generally characterized as small and drab. Observers frequently describe them as highly active, which can be explained by their insect foraging habits.
Although first described only in 1892 (Rothschild 1892b), the Laysan Millerbird became extinct between 1916 and 1923 (Ely and Clapp 1973, Sincock and Kridler 1977, Sincock 1979). Minimal information exists on the Laysan Millerbird, since few naturalists observed this species in life (Fisher Fisher 1903b, Fisher 1903c, Bailey 1956a).
The Laysan Millerbird's extinction, along with a host of other endemic species' extinctions, was ultimately caused by the introduction of Old World rabbits (Oryctolagus cuniculus) on Laysan in approximately 1903. With no terrestrial mammalian predators present, the rabbit population exploded and denuded Laysan of vegetation (Bailey 1956a, Ely and Clapp 1973).
Access to the Nihoa Millerbird is limited not only by the remoteness of Nihoa Island and required access permits, but also by the difficulty of landing on this steep, rocky island, often surrounded by rough seas. Although this endangered species has been somewhat protected by limited and difficult access, these restrictions also have limited research on its natural history. Aside from almost yearly population estimates made by U.S. Fish and Wildlife Service refuge biologists during brief visits, most information about millerbird natural history is anecdotal and based on short visits made in the 1960s and 1970s (Sincock and Kridler 1977) and early 1980s (SC). Research is also curtailed by justifiable concerns about the negative impact of even minimal human activities on the seabird colonies of Nihoa I., as well as its numerous endemic plants and invertebrates.