Guam Rail

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Guam Rail

Rallus owstoni

ListedAugust 27, 1984
FamilyRallidae (Rails and Coots)
DescriptionLarge flightless bird with dark brown head, neck, back, rump, tail, legs, feet, and bill; grey throat, upper breast and stripe; and barred black and white wings, lower breast, abdomen and under tail coverts.
HabitatCropland, pasture, and evergreen and mixed forests.
FoodGastropods, skinks, geckos, insects, and carrion, as well as seeds and palm leaves.
ReproductionClutch of three to four eggs.


The Rallus owstoni, (Guam rail) is a large flightless rail with dark brown head, neck, back, rump, tail, legs, feet, and bill. The throat and upper breast and superciliary stripe are grey. The wings, lower breast, abdomen and under tail coverts are barred black and white. Plumage of both sexes is similar, but males are larger than the females. This species defends its territory, including both nesting sites and foraging areas.


The monogamous Guam rail nests all year round, although it was suggested that breeding peaks during the rainy season from July to November, based on data gathered from the number of broods seen during roadside counts. Both sexes share in the construction of the shallow ground nest. The typical clutch consists of three to four eggs in the wild and two to five eggs in captivity that are white to pinkish with small spots of pink or blue concentrated at the large ends. Incubation duties are shared by both sexes. Eggs do not hatch synchronously and the young are highly precocial, leaving the nest 24 hours after hatching. The eggshells are consumed by the adults. The Guam rail is an omnivorous feeder, however, it appears to prefer animal over vegetable food. It is known to eat gastropods, skinks, geckos, insects, and carrion, as well as seeds and palm leaves. The Guam rail is considered diurnal and concentrates on feeding during the day in the early morning and late evening. Nothing is known of their nocturnal activities, although captive rails are known to feed at night.


R. owstoni was distributed over much of Guam in all habitats except wetlands, although both savanna and mature mixed forest are considered marginal habitat. As Guam was probably mostly limestone forest before the arrival of man, the rail may have been more common after much of the mature forest had been converted to scrubby second growth or mixed forest. In the past, R. owstoni was also observed in association with rural areas including residential areas, golf courses, cropland (vegetables, melons, and fallow), pasture, and evergreen and mixed forests. R. owstoni appears to require forest or scrubby areas in which to build its ground nest. The Guam rail builds a nest of interwoven loose and rooted grass and leaves elevated 1-2 in (2.5-5 cm) above the ground.


The Guam rail was widely distributed in forest, scrub, and agricultural areas until 1968 when it entered into decline because of the spread of the accidentally introduced brown tree snake through the island. In 1981 the population was estimated at 2,000 birds and by 1983 it was less than 100. By 1987 it was extirpated from the wild and currently only survives in a captive breeding program in Guam and in 16 zoos in the U.S. Efforts are underway to establish a self-sustaining, experimental population on the nearby snake-free island of Rota in the Northern Mariana Islands (1993).


Predation of eggs, young, and adults by the introduced nocturnal brown tree snake, Boiga irregularis, is thought to be responsible for the present decline of the Guam rail and other forest birds of Guam. In the past, hunting has stressed the Guam rail, but there is no evidence to suggest that it was responsible for the Guam rail's recent decline on Guam.

Conservation and Recovery

The present decline of the Guam rail is apparently due to predation by the brown tree snake, B. irregularis. Other limiting factors affecting the Guam rail before the apparently accidental introduction of the brown tree snake are not known. This species will likely be extirpated from the wild on Guam in the very near future. However, owing to the high reproductive potential demonstrated by Guam rails breeding in captivity on Guam and at several mainland zoos in a program set up by the Aquatic and Wildlife Resources Division, Guam Department of Agriculture in cooperation with the American Association of Zoological Parks and Aquariums, the potential for successful recovery of this species is high providing that predation by the brown tree snake can be controlled. The establishment of an experimental population on the Island of Rota was attempted. Of the initial 22 rails released, 13 have died and the status of the other nine is unknown.


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


U. S. Fish and Wildlife Service. 18 January 2000. "Guam Rail /Gallirallus owstoni /Ko'ko'." Threatened and Endangered Animals in the Pacific Islands.