Some of the islands of this EBA (covering the US dependent territories of the Northern Mariana Islands and Guam) are the peaks of submerged and still-active volcanic mountains, and others, notably Guam, are uplifted coral formations.

Vegetation is varied, with lowland rain forest (mostly man-modified and much reduced) found on limestone areas and old lava flows; fire-adapted grasslands are extensive in southern Guam and on some of the northern islands. Leucaena leucocephala, a weedy tree from the American tropics (locally called tangan-tangan), was seeded widely after World War II, and is now ubiquitous on the larger southern islands where it forms dense thickets.

Most of the restricted-range species utilize forest and many appear to have adapted to secondary and/or introduced vegetation including tangan-tangan. Some are (or were, as most species have been extirpated from Guam) found on all islands, but a number are confined to the southern ones and thus have tiny world ranges. Several species occur more widely in other Micronesian Endemic Bird Areas (EBAs 190-192).

The most important threat to the birds of this EBA is the possible spread of the introduced brown tree snake Boiga irregularis (native to Australasia), a most effective colonizer and bird predator (see, e.g., Savidge 1987, Conry 1988, Fritts 1988, Jaffe 1994). Although the snakes on Guam have declined with the demise of their bird prey, introduced skinks have proved an alternative food source on which they can persist indefinitely (T. Fritts verbally 1995).

A series of recovery plans has been developed for the birds of this EBA (e.g. USFWS 1990) and efforts are under way to establish an experimental population of Gallirallus owstoni on Rota (Witteman et al. 1990, Haig et al. 1993). However, all bird populations on Rota, and on nearby Tinian and Saipan, are potentially threatened by future snake introductions, as cargo arrives on these islands via Guam.

Despite the absence of brown tree snakes on Rota, some birds, e.g. Zosterops rotensis and Corvus kubaryi, have also declined on this island, and other threats may be operating (H. D. Pratt in litt. 1994), such as other introduced predators, competitors or diseases; the introduced Black Drongo Dicrurus macrocercus has been implicated in the recent rapid decline of Z. rotensis (Craig and Taisacan 1994).

In 1993 a National Wildlife Refuge was established on Guam to preserve remaining forest and ensure the continued existence of Corvus kubaryi; protection of this habitat will also permit re-establishment and recovery of a variety of bird species should snake control ever take place (Wiles et al. 1995). Four remote northern islands (Farallon de Pajaros, Maug, Asuncion and Guguan) are wildlife sanctuaries important for some restricted-range birds and for seabirds (Reichel 1991, G. J. Wiles in litt. 1993, D. W. Stinson in litt. 1994).

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Mariana Islands. Downloaded from https://datazone.birdlife.org/eba/factsheet/189 on 23/11/2024.