email a friend
printable version
Millerbird Acrocephalus familiaris
BirdLife is updating this factsheet for the 2016 Red List
Please email us with any relevant information
BirdLife Species Champion Become a BirdLife Preventing Extinctions Programme Supporter
For information about BirdLife Species Champions and Species Guardians visit the BirdLife Preventing Extinctions Programme.

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.

Taxonomic source(s)
AOU. 1998. Check-list of North American birds. American Ornithologists' Union, Washington, D.C.
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.

13 cm. Small, nondescript, thin-billed warbler. Brown above, darkest on crown, white below. Voice Simple song of rapid, metallic notes.

Distribution and population
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).

Population justification
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.

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).

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).

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 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 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) which in September 2011 resulted in a successful translocation 2011 (Farmer et al. 2011a) of 24 birds. A further 26 birds were moved in August 2012 and at the end of October 2012 the population was estimated to be 67. The population will continue to be monitored and the data used to determine if a further translocation is required (G.E. Wallace in litt. 2013).

Conservation and Research 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).

Addison, J. A.; Diamond, A. W. 2011. Population genetics and population size of the Critically Endangered Nihoa Millerbird (Acrocephalus familiaris kingi). The Auk 128(2): 265-272.

Farmer, C, H. Freifeld, R. Kohley, P. Luscomb, R. Rounds, C. Rutt, G. Wallace, T. Work, W. Aldeguer, F. Amidon, D. Tsukayama, and E. VanderWerf. 2011. Millerbird Translocation, 2-16 September 2011. Nihoa and Laysan Islands, Northwest Hawaiian Islands, Papahānaumokuākea Marine National Monument. U.S. Fish and Wildlife Service, Honolulu, Hawai'i.

Farmer, C., R. Kohley, H. Freifeld, and S. Plentovich. 2011. Nihoa Millerbird (Acrocephalus familaris kingi) translocation protocols (final). U.S. Fish and Wildlife Service, , Honolulu, Hawai'i.

IUCN. 2015. The IUCN Red List of Threatened Species. Version 2015-4. Available at: (Accessed: 19 November 2015).

Kohley R., C. Farmer, D. Tsukayama, R. Hammond, and W. Aldeguer. 2010. Nihoa Millerbird captive-feeding trials and Nihoa biological monitoring expedition, September 19-October 5, 2010. Nihoa Island, Northwest Hawaiian Islands, Papahānaumokuākea Marine National Monument. USFWS, Honolulu, Hawai'i.

Kohley, R. and Rutt, C. 2012. Monitoring Translocated Millerbirds on Laysan, 10 September 2011 to 19 March 2012. Laysan Island, Northwest Hawaiian Islands, Papahānaumokuākea Marine National Monument. U.S. Fish and Wildlife Service, Honolulu, Hawai'i.

Latchininsky, A. V. 2008. Grasshopper outbreak challenges conservation status of a small Hawaiian Island. Journal of Insect Conservation 12: 343-357.

MacDonald, M. A. 2008. Nihoa Millerbird Pre-translocation & Nihoa Biological Monitoring Expedition, July 17 - September 22, 2007.

Morin, M. S.; Conant, S.; Conant, P. 1997. Laysan and Nihoa Millerbird (Acrocephalus familiaris). In: Poole, A.; Gill, F. (ed.), The birds of North America, No. 302, pp. 1-19. The Academy of Naural Sciences and The American Ornithologists' Union, Philadelphia and Washington, DC.

VanderWerf, E.A., D. H. Tsukayama, F. A. Amidon, and W. Aldeguer. 2011. Nihoa Island biological monitoring and management, 2-16 September 2011. Unpublished report. U.S. Fish and Wildlife Service.

Further web sources of information
Alliance for Zero Extinction (AZE) species/site profile. This species has been identified as an AZE trigger due to its IUCN Red List status and limited range.

Audubon WatchList

Click here for more information about the Alliance for Zero Extinction (AZE)

Explore HBW Alive for further information on this species

View photos and videos and hear sounds of this species from the Internet Bird Collection

Text account compilers
Benstead, P., Bird, J., Calvert, R., Capper, D., Khwaja, N., Stattersfield, A., Stuart, T., Symes, A., Derhé, M. & Wright, L

Baker, H.C., Baker, P.E., Camp, R., Conant, S., Freifeld, H., Fretz, S., MacDonald, M., Morin, M., VanderWerf, E., Plentovich, S. & Wallace, G.

IUCN Red List evaluators
Symes, A.

Recommended citation
BirdLife International (2016) Species factsheet: Acrocephalus familiaris. Downloaded from on 26/10/2016. Recommended citation for factsheets for more than one species: BirdLife International (2016) IUCN Red List for birds. Downloaded from on 26/10/2016.

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

To provide new information to update this factsheet or to correct any errors, please email BirdLife

To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.

Key facts
Current IUCN Red List category Critically Endangered
Family Sylviidae (Old World warblers)
Species name author (Rothschild, 1892)
Population size 250-999 mature individuals
Population trend Stable
Distribution size (breeding/resident) 2 km2
Country endemic? Yes
Links to further information
- Additional Information on this species