Millerbird

Acrocephalus familiaris

Craig Rowland, US Fish & Wildlife Service
  • SYLVIINAE
  • 380

The small, drab Millerbird is the only Old World warbler (subfamily Sylviinae) known to have colonized the Hawaiian Archipelago. Scant information exists on this bird because the population that occurred on Laysan Island was little observed by naturalists prior to its extinction in 1923. The population that remains on Nihoa Island is seldom observed due to the extreme inaccessibility of the island and concerns about human activity negatively impacting fragile seabird colonies and endemic species. It is unknown whether these two populations represented two separate species or forms of one species.

Identification
This brown and white warbler is the only thin-billed bird on Nihoa Island. Males frequently sing, especially during the breeding season, and their song (on Nihoa Is.) has been described as thin, metallic, and energetic.

Distribution and Population Trends
Known only from two islands within the Hawaiian Archipelago. The Laysan Island population was considered abundant prior to the introduction of rabbits around 1903. Somewhere between 1916 and 1923 the Laysan population disappeared as rabbits consumed all living plants that provided food, shelter, and nest sites for this little warbler. Another population was discovered on Nihoa Island in 1923, the same year that the Laysan population was confirmed extinct. The logistics of accessing Nihoa and conducting accurate surveys have made it difficult to track population changes or dynamics, but it seems that the population has remained stable within its extremely small range (40 ha of vegetation on a 63 ha island). Over the last 30 years of annual surveys estimates of population sizes have ranged from 31 to 731 birds, while the carrying capacity of the island has been estimated at around 600 birds.

Ecology
Very little is known about the Laysan Millerbird, while limited and difficult access to Nihoa Island has significantly reduced opportunities to learn about the species from its extant population. Birds on Laysan Island were characterized as energetic and confiding, often seen around buildings or camps searching in crannies for insects or even hopping around and landing on visitors. On Nihoa, Millerbirds are known to prefer dense cover near the ground, where they search for insects and larvae and build nests in dense shrubs. The first nest on Nihoa was discovered in 1962, and the nesting season is now suspected to run from January to September. Their food consists entirely of insects and larvae, especially moths and caterpillars.

Threats
As with any limited population, random events could have catastrophic effects. The tiny exposed island of Nihoa is especially susceptible to weather events, and weather events probably account in part for changes in Millerbird population estimates over the past 30 years. The fragile nature of this tiny ecosystem and the chance that human visitors could introduce an alien species are an ongoing cause for concern. At this time only 3 alien plants are thought to be established on Nihoa Island, and disease-bearing mosquitoes that have decimated other Hawaiian endemic birds have not become established on the island. Rats, mosquitoes, or new plants are all possible threats.

Conservation
The species was federally listed as Endangered in 1967. The island of Nihoa is part of the Hawaiian Islands National Wildlife Refuge and access is strictly controlled through a permitting process. While visiting scientists make an effort to pull invasive weeds, no other substantive efforts on behalf of the Millerbird have been undertaken. The option of translocating Millerbirds to other islands (including Laysan, Necker, and Kaho'olawe) has been seriously considered, with the option of returning Millerbirds to Laysan receiving the most serious attention. Computer models show that the current population has an unacceptably high probability for extinction unless efforts are made to establish supplemental populations either on other islands or in captivity.

What Can You Do?
The Hawaiian Islands National Wildlife Refuge provides essential habitat for Millerbirds. Unfortunately, the refuge system is often under-funded during the U.S. government's budgeting process. 

To support U.S. National Wildlife Refuges, consider purchasing a duck stamp which helps to fund wetland habitats in refuges: http://www.fws.gov/duckstamps/

The Endangered Species Act has helped protect Millerbirds and made it possible to learn critical information about its biology. Audubon continues to work to ensure that this vital legislation is being used to protect our publicly-owned wildlife resources. Check out http://policy.audubon.org/ to learn of the latest news about the Endangered Species Act and how you can help. To learn more about other species protected under this legislation, visit: http://endangered.fws.gov/

Join Hawaii Audubon Society. A chapter of National Audubon, the Hawaii Audubon Society works to protect and educate people about Hawaii's birds. For more information visit http://www.audubon.org/chapters/hawaii-audubon-society

References
Bailey, A. M. 1956. Birds of Midway and Laysan Islands. Denver Mus. Of Nat. Hist., Mus. Pict. 12: 1-130.

BirdLife International. 2000. Threatened Birds of the World. Lynx Edicions and BirdLife International, Barcelona and Cambridge, UK.

BirdLife International (2006) Species factsheet: Acrocephalus familiaris, Millerbird http://www.birdlife.org/datazone/speciesfactsheet.php?id=7614

Ehrlich, P.R., D.S. Dobkin, and D. Wheye. 1992. Birds in Jeopardy. Stanford University, Stanford, CA.

Marie, P. M., et. al. 1997. Laysan and Nihoa Millerbird (Acrocephalus familiaris). In The Birds of North America, No. 302 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologist's Union, Washington, D.C.

Pratt, H. D. et. al. 1987. A Field Guide to the Birds of Hawaii and the Tropical Pacific. Princeton UP, Princeton, New Jersey.