Millerbird (Acrocephalus familiaris)
Justification This species qualifies as Critical because it occurs on one extremely small island and undergoes marked fluctuations in population, owing to climatic events, reducing it to tiny numbers. The accidental introduction of mammalian predators, non-native pest plants or insect species could rapidly bring about its extinction.
Identification 13 cm. Small, nondescript, thin-billed warbler. Brown above, darkest on crown, white below. Voice Complex song of rapid, metallic notes.
Population estimate 31 - 731
Population trend stable
Range estimate 1 km2
Country endemic? Yes
Range & Population Acrocephalus familiaris is endemic to the steep, rocky island of Nihoa in the Northwestern Hawaiian Islands (USA). It previously occurred on Laysan also, where the nominate race was estimated to number 1,500 birds in 1915, but became extinct between 1916 and 1923. On Nihoa, the race kingi has been estimated to number from 31 to 731 birds since the late 1960s, with the most recent figure being 155, in July 1996. No long-term trend is evident, but fluctations between successive surveys appear to be real, and not solely a reflection of the difficulties in censusing or differences in season. The vegetated area of Nihoa measures just 0.43 km (68% of the island), of which 0.32-0.405 km is considered suitable habitat.
Ecology It prefers dense cover near the ground, particularly around the shrubs Chenopodium oahuense and Sida fallax. It feeds on insects and larvae, especially moths and caterpillars. Pairs show year-to-year fidelity in specific territories, with nesting occurring from at least January to September, but possibly throughout the year. Nests are located in dense shrubs (mainly C. oahuense) and two eggs are generally laid.
Threats Its extinction on Laysan was ultimately caused by the introduction of rabbits, which denuded the island of vegetation (causing severe insect food shortage). On Nihoa, the population size is probably regulated primarily by the weather, apparently being linked to precipitation levels, with droughts, storms and hurricanes inevitably having a negative impact. Variations in insect-supply and predation of eggs by Nihoa Finch Telespiza ultima also affect the population. Fire is a past and potential threat and introduction of detrimental non-native species is a permanent possibility.
Action Nihoa is part of the Hawaiian Islands National Wildlife Refuge, and legal access is controlled by a permit system that is restricted largely to biologists and other researchers. Strict protocols are followed to ensure that legal permittees do not accidentally introduce new species via seeds, eggs or insects travelling on clothes and equipment. Visiting scientists make efforts to control alien plants by hand weeding.
Targets Continue monitoring. Ensure strict protocols prevent further accidental introductions of alien species. *Translocate birds to Laysan, Kaho`olawe or Midway if discussions of this proposal conclude in its favour.
