The Guam rail (Gallirallus owstoni), known locally and in Chamorro as ko'ko', is a small, terrestrial bird endemic to Guam in the Rallidae family. They are one of the island's few remaining endemic bird species. The species became extinct in the wild in the early 1980s when biologists captured the remaining wild population to establish a breeding program. They have since been successfully introduced to the nearby Rota and Cocos islands. In 2019, they became the second bird species to be reclassified by the International Union for the Conservation of Nature from Extinct in the Wild to Critically Endangered. Adult male and female plumage is primarily brown with barred black-and-white plumage on their underside. Their heads are brown with a grey stripe running above the eye and a medium-length grey bill. They have strong legs with long toes that help them walk over marshy ground. Their most common vocalizations are short "kip" notes, but also screeches during the breeding season. Guam rail numbers fell drastically due to predation by invasive brown tree snakes. The species is now being bred in captivity by the Division of Aquatic and Wildlife Resources on Guam, and at some mainland U.S. zoos. Since 1995, more than 100 rails have been introduced on the island of Rota in the Commonwealth of the Northern Mariana Islands, in an attempt to establish a wild breeding colony. In 2010, 16 birds were released onto Cocos Island, with 12 more being introduced in 2012.
Taxonomy and etymology The Guam rail was first described as Hypotaenidia owstoni in 1895 by Lord Walter Rothschild. Rothschild announced the specific name owstoni was "in honour of Mr. Alan Owston, of Yokohama, whose men have collected for me on the Marianne Islands." The generic name, Hypotaenidia, comes from the Ancient Greek hypo meaning "under" and tainia meaning "stripe" or "band." This refers to the banded feathers seen on their underside and wings, typically found across rail species. The Guam rail is now placed in the genus Gallirallus that was introduced in 1841 by Frédéric de Lafresnaye.
Description Guam rails are a medium-sized rail species, growing to about 11 inches (28 cm) in total length. Adults will weigh around 7 to 12 ounces (200 to 350 grams). They have an elongated body, with small wings and medium-length legs. Guam rails have underdeveloped wing muscles and are considered flightless, though they can fly about 3 to 10 feet (1 to 3 meters) at once. Their strong leg muscles make them fast runners instead. Adult Guam rails are monomorphic, meaning males and females have the same plumage and characteristics. They are primarily brown, with a grey bill and tan-colored legs. The head and back are brown. It has a grey eye stripe and throat, a dark blackish breast with white barring, and the legs and beak are dark brown.
Distribution and habitat Historically, the rail was found only on Guam. The island covers about 132,230 acres. According to a 2013 survey, about 53% of the island is forested. Rails were once distributed widely across most habitats on the island, including mixed forest, savanna, grasslands, fern thickets, and agricultural areas. Individuals observed by the Guam Division of Aquatic and Wildlife Resources were noted as preferring edge habitats that provided good cover. Today, the rail is still found on Guam, though in captivity, with wild populations introduced on Rota and Cocos Islands. Since 1985, the rails have only lived in captivity on Guam, located at the Guam Department of Agriculture.
Ecology and behavior Guam rails are secretive, fast birds. Though they are capable of a short bursts of flight, they seldom fly. It was found more frequently in savannas and scrubby mixed forest than in uniform tracts of mature forest. It was usually found in dense vegetation but it was also observed bathing or feeding along roadsides or forest edges. Its call is a loud, piercing whistle or series of whistles, usually given by two or more birds in response to a loud noise, the call of another rail, or other disturbances. Though individuals will respond almost invariably to the call of another rail, the species is generally silent.
Breeding It is a year-round ground nester and lays 2–4 eggs per clutch, and both parents share in the construction of a shallow nest of leaves and grass. They mature at six months of age, and have been known to produce up to 10 clutches per year in captivity. Their nesting habits make them highly susceptible to predators, such as the native Mariana monitor and the invasive feral pig, feral cat, feral dog, mangrove monitor, brown tree snake, black rat, and Norway rat.
Food and feeding Guam rails are omnivorous foragers known to consume gastropods, insects, geckos, seeds, and vegetable matter. A large part of their diet consists of giant African snails, which were introduced to Guam around 1945 and became an easy food source for the birds. Pieces of snail shell and coral have been found in the stomach and gizzard contents of Guam rails, suggesting use as grit. Guam rails have been observed foraging and hunting. When foraging, they peck food from the ground and eat seeds and flowers from grasses. They have also been observed hunting insects, particularly butterflies.
Parasites The species hosted a unique louse, Rallicola guami, which seems to have been a victim of conservation-induced extinction, since Guam rails taken into captivity were deloused to assist survival; it is considered most likely extinct.
Threats
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