Acrocephalus familiaris 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Aves Passeriformes Acrocephalidae

Scientific Name: Acrocephalus familiaris (Rothschild, 1892)
Common Name(s):
English Millerbird, Nihoa Millerbird, Nihoa Reed-warbler
Taxonomic Source(s): del Hoyo, J., Collar, N.J., Christie, D.A., Elliott, A., Fishpool, L.D.C., Boesman, P. and Kirwan, G.M. 2016. HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 2: Passerines. Lynx Edicions and BirdLife International, Barcelona, Spain and Cambridge, UK.
Identification information: 13 cm. Small, nondescript, thin-billed warbler. Brown above, darkest on crown, white below. Voice Simple song of rapid, metallic notes.

Assessment Information [top]

Red List Category & Criteria: Critically Endangered B1ac(iv) ver 3.1
Year Published: 2016
Date Assessed: 2016-10-01
Assessor(s): BirdLife International
Reviewer(s): Butchart, S. & Symes, A.
Contributor(s): Baker, H.C., Baker, P.E., Camp, R., Conant, S., Freifeld, H., Fretz, S., MacDonald, M., Morin, M., VanderWerf, E., Plentovich, S., Wallace, G. & Farmer, C.
Facilitator/Compiler(s): Benstead, P., Bird, J., Calvert, R., Capper, D., Khwaja, N., Stattersfield, A., Stuart, T., Symes, A., Derhé, M., Wright, L, North, A.
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.

Previously published Red List assessments:

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, monitoring methods used from 1967 to 2014, and data recorded intermittently since the late 1960s, did not yield precise population estimates or trends for this species (Morin et al. 1997, Gorresen et al. 2012, Gorresen et al. 2016). 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 (Marshall et al. 2012, Gorresen et al. 2016). The last estimates using these methods were 468 ± 166 (SD) individuals in September 2013 and 893 ± 303 (SD) in September 2014 (Gorresen et al. 2016). Wide 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 61.9 ha (86% of the island; Gorresen et al. 2012), and Millerbirds are distributed relatively evenly across the island, with some areas of slightly higher densities (MacDonald 2012, Gorresen et al. 2016). Conservation translocations were undertaken in September 2011 and August 2012, when a total of 50 Millerbirds were moved from Nihoa to Laysan, a distance of 1,037 km (Freifeld et al. 2016). The birds began breeding successfully within six months of the first release. Based on post-release monitoring through 2014, the nascent population on Laysan was estimated at 164 birds (Dalton et al. 2014, Freifeld et al. 2016).

Countries occurrence:
United States (Hawaiian Is.)
Additional data:
Continuing decline in area of occupancy (AOO):No
Extreme fluctuations in area of occupancy (AOO):NoEstimated extent of occurrence (EOO) - km2:2
Continuing decline in extent of occurrence (EOO):NoExtreme fluctuations in extent of occurrence (EOO):No
Number of Locations:1Continuing decline in number of locations:Unknown
Extreme fluctuations in the number of locations:No
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:The naturally occurring population on Nihoa was estimated at 489  ± 70 (SD) in 2010, 699 ± 78 in 2011, 610 ± 210 in 2012, 468 ± 166 in 2013, and 893 ± 303 in 2014 (Gorresen et al. 2016). The most recent estimate of the nascent population on Laysan was 164 in September 2014 (Dalton et al. 2014, Freifeld et al. 2016). Considering that the Nihoa subpopulation fluctuates widely, 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.

Trend Justification:  Although there is a lack of precision over the monitoring methods, the existing data does suggest that Millerbird numbers on Nihoa have experienced pronounced fluctuations and have likely ranged between fewer than 50 and more than 800 individuals (VanderWerf et al. 2011).

Current Population Trend:Stable
Additional data:
Number of mature individuals:250-999Continuing decline of mature individuals:No
Extreme fluctuations:YesPopulation severely fragmented:No
No. of subpopulations:1Continuing decline in subpopulations:Unknown
Extreme fluctuations in subpopulations:NoAll individuals in one subpopulation:Yes
No. of individuals in largest subpopulation:100

Habitat and Ecology [top]

Habitat and Ecology:It is found in dense cover near the ground, particularly around the shrubs Sida fallax and Solanum nelsonii on Nihoa (VanderWerf et al. 2011, MacDonald 2012), and Scaevola taccada and Eragrostis variabilis on Laysan (Kohley and Rutt 2012, Wilcox et al. 2013, Dalton et al. 2014). On Nihoa the main foods include small beetles, spiders, roaches and larvae (Morin et al. 1997, MacDonald 2012). The original, naturally occurring Laysan population was thought to have fed primarily on moths (Henshaw 1902), however, the species is thought to be a catholic feeder, feeding on the most readily available arthropod prey items (MacDonald 2012). 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 (MacDonald 2012). The translocated population on Laysan breeds from at least February through September, with a peak in spring-summer months (Wilcox et al. 2013). Nests are located in dense shrubs or bunchgrass, and two eggs are generally laid (Morin et al. 1997, Wilcox et al. 2013).

Continuing decline in area, extent and/or quality of habitat:Unknown
Generation Length (years):4.4
Movement patterns:Not a Migrant

Threats [top]

Major Threat(s): The species exhibits extensive and strikingly low levels of genetic diversity as a result of recent severe bottlenecks (Addison and Diamond 2011). These may have been caused by climatic events, anthropogenic influences (impacts of Nihoa's prehistoric human population, and perhaps of modern introduction of nonnative plants and insects) or other stochastic fluctuations in habitat and consequently in population size. The Millerbird's 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 (USFWS 1984). 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. Population outbreaks of the nonnative grasshoppers (e.g. the Gray Bird Grasshopper Schistocerca nitens) have periodically denuded 90% of the island's vegetated area, 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, especially predators such as rats (Rattus spp.) is a permanent possibility. Nihoa and Laysan Finches (Telespiza ultima and T. cantans, respectively) may prey upon the eggs of Millerbirds, but since these species evolved together, it is unlikely to present a significant threat (M. A. MacDonald in litt. 2008).

Conservation Actions [top]

Conservation Actions: Conservation and Research 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 pulling and other methods (eg. Plentovich et al. 2014). 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 (Morin and Conant 2007). Development of detailed translocation, release, and monitoring methods were finalized in 2011 (Farmer et al. 2011) and two successful translocations to Laysan in 2011 and 2012 resulted in a nascent population there, last estimated at 164 birds (Freifeld et al. 2016). The population will continue to be monitored and the data used to determine population persistence and growth. New, more rigorous population monitoring methods were introduced on Nihoa in recent years (Gorresen et al. 2012, Gorresen et al. 2016); these are expected to yield estimates of total numbers with improved confidence and, if monitoring is done annually, early detection of significant trends. 

Conservation and Research Actions Proposed

Continue monitoring millerbirds on Nihoa and Laysan. Monitoring on Laysan is particularly important to document vital rates, dispersal, and habitat use, and the role of these in population persistence. Continued study of the reintroduced population on Laysan is essential to inform conservation translocations of this and other Hawaiian passerines in the future. However, monitoring and study on Laysan has become far more challenging since the closure of the long-term, year-round USFWS field camp on the island in 2013. Continue to monitor Nihoa for the highly invasive plant Cenchrous echinatus that was re-discovered in September 2011, and continue to monitor and manage invasive alien plants on Laysan (e.g., Morin and Conant 1998). Ensure strict adherence to quarantine protocols to prevent further accidental introductions of alien species. Carry out additional translocations to establish populations on other islands as appropriate (S. Conant in litt. 2008, Freifeld et al. 2016).

Citation: BirdLife International. 2016. Acrocephalus familiaris. The IUCN Red List of Threatened Species 2016: e.T22714797A94427463. . Downloaded on 26 May 2018.
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