Groundbreaking research for the Nihoa millerbird

Endangered Species Bulletin, Spring, 2009 by Ken Foote

Found only on the small Hawaiian island of Nihoa, the critically endangered Nihoa millerbird (Acrocephalus familiaris kingi) teeters on the brink of extinction. Its single, small population is highly vulnerable to chance events such as severe storms and droughts, accidental introduction of alien species and diseases, and population fluctuations. But new research provides hope that a second population can be established.

Fish and Wildlife Service biologists and other scientists are concerned that the Nihoa millerbird could meet the fate of its close relative, the Laysan millerbird (Acrocephalus familiaris familiaris). In 1923, the same year that the Nihoa millerbird was scientifically described by Alexander Wetmore, the Laysan millerbird, endemic to Laysan Island, was declared extinct. Habitat destruction by introduced rabbits (Oryctolagus cuniculus) led to the demise of the Laysan millerbird and several other terrestrial bird species found only on that island. With the near-complete devegetation of Laysan Island, the millerbird's insect prey disappeared, along with the bird's nesting habitat.

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Nihoa Island does not have rabbits; however, in the 1980s, a non-native grasshopper (Schistocerca nitens) arrived on the island. This particular insect is prone to population bursts that result in major damage to the island's vegetation and the millerbird's habitat. Although millerbirds eat insects, and may add grasshoppers to their diverse diet, they can't keep this abundant invader under control. The island's remote location and rugged terrain make management of this pest a difficult challenge.

In 2006, owing to the urgency of protecting the Nihoa millerbird, the Service commissioned a ranking of potential translocation sites for this species and two other endangered birds of the Northwestern Hawaiian Islands, the Nihoa finch (Telespyza ultima) and Laysan finch (Telespyza cantans). This effort ranked Laysan Island at the top of the list for a second population of Nihoa millerbirds. Having once had its own millerbird species, Laysan was a logical choice. Moreover, a comprehensive restoration plan for Laysan Island, completed in 1998, includes a call for introducing the Laysan millerbird's closest relative, the Nihoa millerbird, in order to replace a missing component of the island's ecosystem.

The translocation of the Nihoa millerbird from Nihoa to Laysan will thus accomplish two goals: establishing a second population of a critically endangered species (thereby reducing the threat of extinction) and contributing to the restoration of Laysan Island. Before a translocation is feasible, however, critical data on millerbird life history and habitat requirements are needed.

Mark MacDonald, a graduate student from the University of New Brunswick in Canada, leads a team that is working with the Service to collect information needed for translocations. From July through September of 2007, MacDonald and his team captured and banded Nihoa millerbirds, collected body measurements, assessed body fat and breeding condition, identified individual territories and analyzed vocalizations, conducted feeding experiments, collected fecal samples, observed behavior to determine diet composition, noted the presence and abundance of non-native grasshoppers, and sampled the insect community on both Nihoa and Laysan to assess the millerbird's potential prey base.

MacDonald's study estimated the Nihoa millerbird's population at approximately 800 individuals--a relatively high number in 40 years of low and fluctuating numbers. He believes that this could be attributed not only to high numbers of birds present during the survey period but also a larger survey area, the use of more experienced observers, or (most likely) the greater visibility of the birds during the late summer, when vegetation cover is most limited.

Using mist nets, 85 Nihoa millerbirds (60 males and 25 females) were captured and banded. Banding permits identification of previously captured birds and reduces stress that can be caused by multiple captures. Most importantly, however, banding allows individual birds to be identified in the field and enables biologists to identify pairs, map their territories, and track individual survival from year to year through repeat sightings. Photographs and measurements of wing and tail feathers were taken from each individual, as well as small feather samples for genetic analysis. Growth bars visible on the tail feathers can help scientists determine the age of the bird, and comparison of photographs and measurements with results of lab analyses will aid in finding a way to sex Nihoa millerbirds in the field. Development of these methods will ensure that the right numbers of male and female birds are moved to Laysan.

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Several Nihoa millerbirds were placed in a temporary enclosure and presented with a selection of island insects. The purpose was to identify millerbird dietary preferences and see if the birds would eat in captivity. Preliminary results showed that the birds fed readily from a plastic container of prey items. Of the choices offered, they left behind only lady bugs, sow-bugs, and ants. One bird was quick to chase down fast-moving cockroaches before taking smaller, slower insects such as spiders and beetles. Another test with a male and female showed that, after a brief adjustment period, the pair fed together without hesitation.