WatchList Species Account for Nihoa Millerbird (Acrocephalus familiaris)

Qualifies for the list as a Red List Species

Photo: Jack Jeffrey

The Nihoa Millerbird and Laysan Millerbird are regarded as subspecies of the same species. The Laysan Millerbird became extinct around 1923 as a result of the introduced European rabbit’s depredations on the vegetation of Laysan Island. The Nihoa form is endemic to the remote island of Nihoa, only 63 ha in size, of which 40 are vegetated, the rest being rock or bare soil. The island is part of the Northwestern Hawaiian Islands National Wildlife Refuge and can be visited only with access permits. It is steep and rocky, with low, scrubby vegetation, rising to 277 m. There are steep cliffs on 3 of the island’s 4 sides and landing is difficult. As a result, save for yearly population estimates made by U.S. Fish and Wildlife biologist, the species has not been the subject of any systematic studies.

Population estimates since the 1960s have ranged from 31 to 731, with an average of 381; no long-term trends are evident. The Millerbird is found in all the vegetated portions of the island, where it gleans insects from shrubs, bunchgrass, and other vegetation. Some observers have mentioned that the Millerbird seems to prefer habitats near the few freshwater seeps on the island. Even within this small island, the birds are apparently very sedentary and probably remain on the same territories (0.19-0.40 ha) their entire lives.

The Nihoa Finch, the only other passerine on the island, may sometimes eat Millerbird eggs; severe drought or storms are also population bottlenecks for the bird. There is a threat from unauthorized boat landings and the risk of the further introduction of pest plants and animals. Already several alien species of plants and insects are established on the island; the only control for the alien plants is that visiting scientists make an effort to control them by pulling by hand.

Though Nihoa Millerbirds have never been translocated elsewhere, there is discussion of replacing the extinct Laysan Millerbird with Nihoan Millerbirds on Laysan Island. If translocation is attempted, it should be only of birds of the year, and during high population years, so that the population on Nihoa is not jeopardized.