Nihoa Millerbird

Acrocephalus familiaris kingi

Description

The inconspicuous Nihoa millerbird, Acrocephalus familiaris kingi, is dark gray-brown above and buff-white below; it has a dark, thin, warbler-like bill. The name millerbird is derived from its habit of preying on the larger miller moths. The Nihoa millerbird was discovered in 1923 by Alexander Wetmore. A related species, the Laysan miller-bird (A. f. familiaria ) was discovered on Laysan in 1891 but became extinct by 1923.

Behavior

The millerbird is secretive—running, hopping, or flying about in the underbrush and rarely leaving cover. It is sedentary and seldom moves more than 65 ft (19.8 m) out of its home territory, usually an area of about 0.5-1.0 acre (0.2-0.4 hectares). Miller-birds are insectivorous, gleaning prey from leaves, stems of bushes, leaf litter, and the soil surface. Although little is known of its breeding behavior, it is thought to nest between January and May and lay a clutch of two eggs. The color of the eggs ranges from pale blue with brown splotches to white with sepia brown or black spots. Nest heights average 21 in (53.3 cm) aboveground. Two adults have been observed sharing incubation, but it is unclear if these were a pair or an adult and a nest helper. Males appear to guard nests or territories with conspicuous songs delivered from the tops of large bushes.

The now extinct Laysan millerbird, a close relative of the Nihoa millerbird, retired to the shade of bushes or tall grass tussocks during the hot part of the day. Millerbird activity peaks in the morning and late afternoon.

The Nihoa millerbird appears to be sedentary and probably occupies the same territory throughout its life. Territories have populations of about 300 individuals.

Habitat

Nihoa Island has steep slopes, rocky outcroppings, well-developed valleys, and precipitous cliffs on the west, north, and east. The topography is quite rugged, and the millerbird prefers areas of dense, low shrubs where it can forage and build low nests. The millerbird is difficult to observe; it has usually been seen in dense cover near the ground.

Of the 156 acres (63.2 hectares) that comprise Nihoa Island, only 80-100 acres (32.4-40.5 hectares) are considered suitable habitat for the millerbird. Two plants, Chenopodium oahuense and Solanum, attract a great many insects, and they provide the most important food source; Sida and Eragrotis plants are also a source for insects. The millerbird preys on miller moths in all of its stages; a single bird will eat six to 10 moths a day. The millerbird also seems to require a daily portion of fresh water.

Distribution

The Nihoa millerbird is endemic to the island of Nihoa. Early estimates placed the number of miller-birds at 100-200. More reliable census techniques used during the 1960s and 1970s estimated a population range of 200-600 birds.

The Nihoa millerbird remains restricted to Nihoa Island, which has a carrying capacity of 600 birds. A 1986 estimate of the millerbird population was 577. No attempts have been made to establish the millerbird on any of the other islands of the northwestern chain.

The abundance of insects, which is the only millerbird food source, is linked directly to the vegetation on Nihoa. Adverse conditions in the weather pattern would cause a decrease in plant production and result in a lower population of plant-feeding insects, which would reduce the millerbird food supply and force a decrease in the population.

Threats

The Nihoa millerbird, like other island birds, is highly susceptible to outside disturbance of any kind. If an introduced predator such as the rat became established, it could devastate bird populations. Avian diseases, brought in from outside by migrating birds, could also pose a serious threat to the millerbird. These factors need to be monitored closely and immediate action taken to counteract their effects.

Conservation and Recovery

Because the population is small and highly concentrated, a severe hurricane or a tidal wave could decimate the millerbird on Nihoa. Therefore, a U. S. Fish and Wildlife Service (FWS) recovery team is examining the possibility of translocating the miller-bird to other islands. Necker Island has been determined to be unsuitable for the millerbirds because of the lack of suitable vegetation and the scarcity of insects. Laysan Island is the obvious choice because it was the range of the closely related Laysan miller-bird, but the factors that caused the extinction of that species may still persist.

The island of Nihoa is a part of the Hawaiian Islands National Wildlife Refuge administered by the FWS. Landing on the island or entering surrounding waters is prohibited, except by special permit. Permits are typically granted only for research purposes.

Contacts

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121
http://pacific.fws.gov/

Pacific Remote Islands Ecological Services Field Office
300 Ala Moana Blvd., Room 3-122
P.O. Box 50088
Honolulu, Hawaii 96850
Telephone: (808) 541-1201
Fax: (808) 541-1216

References

Berger, A. J. 1981. Hawaiian Birdlife. University of Hawaii Press, Honolulu.

Scott, J. M., and others. 1988. "Conservation of Hawaii's Vanishing Avifauna." Bioscience 38 (4): 238-253.

U. S. Fish and Wildlife Service. 1984. "Recovery Plan for the Northwestern Hawaiian Islands Passerines." U. S. Fish and Wildlife Service, Portland.