Acrocephalus familiaris


Taxonomy [top]

Kingdom Phylum Class Order Family

Scientific Name: Acrocephalus familiaris
Species Authority: (Rothschild, 1892)
Common Name(s):
English Millerbird, Nihoa Millerbird, Nihoa Reed-warbler
Taxonomic Notes:

Assessment Information [top]

Red List Category & Criteria: Critically Endangered B1ac(iv) ver 3.1
Year Published: 2012
Date Assessed: 2012-05-01
Assessor(s): BirdLife International
Reviewer(s): Butchart, S. & Symes, A.
Contributor(s): Baker, H., Baker, P., Camp, R., Conant, S., Freifeld, H., Fretz, S., MacDonald, M., Morin, M., VanderWerf, E. & Plentovich, S.
Facilitator/Compiler(s): Benstead, P., Bird, J., Calvert, R., Capper, D., Khwaja, N., Stattersfield, A., Stuart, T., Symes, A. & Derhé, M.
This species qualifies as Critically Endangered because it occurs on one extremely small island and undergoes marked population fluctuations, owing to climatic events, reducing it to tiny numbers. Such fluctuations have led to extensive and strikingly low levels of genetic diversity, making it extremely vulnerable to extinction through exposure to stochastic factors including the accidental introduction of mammalian predators, non-native pest plants or insect species, as well as severe climatic events and disease.

Geographic Range [top]

Range Description: Acrocephalus familiaris is endemic to the steep, rocky island of Nihoa in the North-western Hawaiian Islands, U.S.A. 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, existing monitoring methods, and data recorded intermittently since the late 1960s, do not yield precise population estimates or trends for this species (Morin et al. 1997, H. Freifeld in litt. 2010). However, the existing data do suggest that Millerbird numbers on Nihoa have experienced pronounced fluctuations and have likely ranged between fewer than 50 and more than 800 individuals (H. Freifeld in litt. 2010), with the most recent estimates of 507 ± 295 individuals in September 2010 and 775 ± 298 individuals in September 2011 (Kohley et al. 2010 VanderWerf et al. 2011). These fluctuations have had a significant impact on the genetic diversity of the remaining population, with the effective number of breeders being estimated as between 5 and 13 individuals (using samples collected in 2007 and 2009 [Addison et al. 2011]). The vegetated area of Nihoa is just 0.43 km2 (68% of the island), and Millerbirds are distributed patchily within this area (H. Freifeld in litt. 2010). A first translocation was undertaken in September  2011 when 24 Millerbirds were moved from Nihoa to Laysan, a distance of 1,037 km (Farmer et al. 2011a). Upon arrival at Laysan the 24 birds were released and each was resighted at least once during the initial three-week period. During the first breeding season on Laysan, nesting birds on Laysan produced at least five fledglings (Kohley and Rutt 2012, J. Vetter, unpublished data).

United States
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Population [top]

Population: The population was estimated at 507 ± 295  individuals in September 2010 and 775 ± 298 individuals in September 2011 (Kohley et al. 2010 VanderWerf et al. 2011). Considering the population appears to fluctuate, the band 250-999 mature individuals seems appropriate. This estimate equates to 375-1,499 individuals in total, rounded here to 350-1,500 individuals.
Population Trend: Stable

Habitat and Ecology [top]

Habitat and Ecology: It is found in dense cover near the ground, particularly around the shrubs Sida fallax , Solanum nelsonii, and Chenopodium oahuense on Nihoa (VanderWerf et al. 2011), and Scaevola taccada and Eragrostis variabilis on Laysan (Kohley and Rutt 2012). On Nihoa the main foods include small beetles, spiders, roaches and larvae (M. A. MacDonald in litt. 2008). The extinct Laysan population was thought to have fed primarily on moths (Morin et al. 1997), however the species is thought to be a catholic feeder, feeding on the most readily available prey items (H. Freifeld in litt. 2010). Forthcoming results of research will include a comparison of potential Millerbird prey on Nihoa and Laysan islands (M.A. MacDonald in prep.), Pairs show year-to-year fidelity in specific territories, with nesting apparently correlated with precipitation and most breeding taking place in the winter months (peaking January-March), although the breeding period may be extended in years of high summer rainfall (M. A. MacDonald in prep). Nests are located in dense shrubs and two eggs are generally laid (Morin et al. 1997).

Systems: Terrestrial

Threats [top]

Major Threat(s): The species is suffering extensive and strikingly low levels of genetic diversity as a result of recent severe bottlenecks caused by climatic events, anthropogenic influences and the introduction of exotic mammals, plants and insects. Its extinction on Laysan in the early 20th century was ultimately caused by the introduction of rabbits and livestock, which denuded the island of vegetation (severe declines in the species's invertebrate prey and suitable habitat). On Nihoa, the population size is probably regulated primarily by precipitation levels, which affect the abundance of arthropod prey (extended droughts for example, are likely to have a negative impact). Severe weather events such as hurricanes may cause direct mortality of Millerbirds; a single severe storm could extinguish the entire population (H. Freifeld in litt. 2010). Since the species has an extremely small range and has severely low levels of genetic diversity, it is particularly vulnerable to extinction through exposure to disease. In the past, population outbreaks of grasshoppers (e.g. the gray bird grasshopper Schistocerca nitens) denuded 90% of the island's vegetation, especially the shrubs in which Millerbirds nest. Outbreaks such as these may have been responsible for reductions in the bird population to well below 200 individuals in 1994, 1996, and 2005 (Latchininsky 2008). Fire is a past and potential threat (Morin et al. 1997) and introduction of detrimental non-native species is a permanent possibility. Nihoa Finches Telespiza ultima may prey upon the eggs of Millerbirds, but since the two species evolved together, it is unlikely to present a significant threat (M. A. MacDonald in litt. 2008).

Conservation Actions [top]

Conservation Actions: Conservation Actions Underway
Nihoa is part of the Hawaiian Islands National Wildlife Refuge and Papahānaumokuākea Marine National Monument (H. Freifeld in litt. 2010). Legal access is controlled by a permit system that restricts access to biologists, other researchers, and native Hawaiian cultural practitioners. Strict biosecurity protocols are followed to ensure that legal visitors do not accidentally introduce new species via seeds, eggs or arthropods travelling on clothes and equipment. Visiting scientists make efforts to control alien plants by hand weeding (H. Baker and P. Baker in litt. 1999). A scoping document for translocations was produced in 2007, identifying Laysan, Kure and Lisianski as the most suitable islands on the basis of island size, elevation and a lack of predators. Development of detailed translocation, release, and monitoring methods (Farmer et al. 2011b) has resulted in a successful in 2011 (Farmer et al. 2011a) and a second translocation is scheduled for 2012.

Conservation Actions Proposed
Continue monitoring. Improve monitoring methods (H. Freifeld in litt. 2010). Remove newly discovered population of the highly invasive plant Cenchrous echinatus that was discovered in September 2011 (Vanderwerf et al. 2011). Ensure strict protocols prevent further accidental introductions of alien species. Carry out additional translocations to establish populations on other islands (e.g. Kure and Lisianski) as appropriate (S. Conant in litt. 2008).

Citation: BirdLife International 2012. Acrocephalus familiaris. The IUCN Red List of Threatened Species. Version 2015.2. <>. Downloaded on 01 September 2015.
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