The Guam rail, or ko‘ko‘, (Rallus owstoni) is a flightless bird endemic to the island of Guam, a U.S. territory at the southern end of the Mariana Islands chain. It formerly occurred island-wide in most nonwetland habitats .
Though habitat loss, hunting and DDT spraying may have reduced its population level to some degree, the rail was estimated at 80,000 birds in the 1960s  and was still common as of 1969 . Thereafter it suffered what may be the most rapid, catastrophic population decline of any U.S. bird species. The decline began in southern Guam in 1971, resulting in its extirpation there in the mid-1970s . In northern Guam the decline began in 1972, resulting in the complete extirpation of the species from the wild by 1986 [1, 5].
Between 1976 and 1982, roadside rail counts declined by 99.8 percent (80.4/100 km to 0.2) .
The Guam rail’s rapidly shrinking range mirrors the expanding range of the brown tree snake (Boiga irregularis), a native of Indonesia, New Guinea, the Solomon Islands and northern Australia . It was positively identified on Guam in the 1950s, but may have been seen in the 1940s, and was likely transported from the Admiralty Islands by the U.S. military during a large World War II buildup. The rail disappeared from areas where snake densities reached high levels, until the very last low-snake density area in extreme northwest Guam was overrun in the late 1970s. It was not alone: The brown tree snake had a devastating effect on all of Guam's native forest birds, resulting in all 12 being listed as endangered by the territory in the 1970s and eight of them by the U.S. Fish and Wildlife Service in 1970, 1977 and 1984.
Though extirpated from the south, the Guam rail remained well distributed and fairly common in the north in 1976 and 1978 . By 1981, though, it was reduced to about 2,300 birds, with the largest concentration on Andersen Air Force Base [2, 5]. By 1983, it was confined to two isolated populations totaling fewer than 100 birds: the Northwest Field (a military landing strip not used since 1949) and the flightline of Andersen Air Force Base [3, 5, 8].
By March 1984, rails were extremely rare in the Northwest Field and the flightline population, though the largest remaining, was reduced to 70 acres [4, 5]. The latter was also threatened by a land-clearing proposal, prompting the U.S. Fish and Wildlife Service to emergency-list the Guam rail as an endangered species in April 1984  and finalize the listing in August 1984 .
Following its listing, the Guam rail was seen twice in the wild: one bird in 1985 and another in 1986 [5, 16].
CAPTIVE BREEDING AND REINTRODUCTION
By 1983 the rail’s plight was so dire that a captive-breeding program was initiated by the Fish and Wildlife Service and the Guam Division of Aquatic and Wildlife Resources. Between 1983 and the cessation of captures in 1985 owing to the disappearance of wild birds, 21 rails were brought into captivity [5, 11, 15, 16]. These are the sole founders of all rails in existence today.
Beginning in 1984, captive rails were transferred to mainland facilities  and now occur in at least 17 facilities on Guam and the mainland [6, 11]. The first successful reproduction in captivity occurred in 1984 in the Division of Aquatic and Wildlife Resources facility .
In 1983 there were 21 birds in captivity , in 1985, 61 , in 1989, 113 , in 1994, 173 , in 2005, 140  and in 2008, 158 .
GUAM - ANDERSEN AIR FORCE BASE
Guam rails have twice been released on Andersen Air Force Base, but neither effort resulted in a persisting population . In 1998, 16 birds were released in Area 50, a 60-acre zone surrounded by a snake barrier and managed internally with snake trapping. Rails bred in the wild, but their population was extirpated by feral cats and other predators sometime before 2008.
In 2003, 44 rails were released into a snake-reduced zone within the Air Force Munitions Storage Area . Feral cats killed 80 percent of radio-collared birds. Efforts to eliminate the cats were hampered by lack of permission. The population is believed to have been extirpated sometime before 2008 .
GUAM - COCOS ISLAND
On November 15, 2010, 16 captive-bred ko‘ko‘ birds were released into the wild on Cocos Island, on lands managed by the Cocos Island Resort and the Guam Department of Parks and Recreation . No brown tree snakes exist on the island. Rats were eradicated prior to the release.
Between 1989 and 2008, 918 Guam rails were introduced to the island of Rota . Though Rota is near to, and has essentially the same habitat as, Guam, it is outside the rail’s historic range. The Fish and Wildlife Service very rarely introduces an endangered species outside its historic range, but was driven to do so because of the ubiquitous presence of brown tree snakes throughout Guam and the dominance of feral cats in the few places where snake levels have been reduced. If successful, establishment of wild rails on Rota will help preserve a wild genetic makeup under natural selection pressures and potentially provide a source of wild-born rails for a future Guam reintroduction.
Reproduction in the wild in Rota was first documented in 1995 . In 2007 the population consisted of 60-80 birds in the Duge and Apanon areas. Predation by feral cats, however, is the population's primary cause of mortality, requiring cat control and continued introduction of captive bred birds to maintain the population .
 Jenkins, J.M. 1979. Natural history of the Guam rail. Condor 81:404-408.
 Engbring, J. and F.L. Ramsey. 1984. Distribution and abundance of the forest birds of Guam: Results of a 1981 survey. U.S. Fish Wildlife Service FWS/OBS-84/20.
 Aguon, C.F. 1983. Survey and inventory of native land birds on Guam. IN: Guam Division of Aquatic and Wildlife Resources, Annual Report, FY 1983. Department of Agriculture, Guam.
 Beck, R.E., Jr. 1984. Survey and inventory of native land birds on Guam. IN: Guam Division of Aquatic and Wildlife Resources, Annual Report, FY 1984. Department of Agriculture, Guam.
 U.S. Fish and Wildlife Service. 1990. Native Forest Birds of Guam and Rota of the Commonwealth of the Northern Mariana Island Recovery Plan. United States Fish and Wildlife Service, Honolulu, HI. Available at: http://ecos.fws.gov/docs/recovery_plans/1990/900928b.pdf
 U.S. Fish and Wildlife Service. 2009. Ko‘ko‘ or Guam Rail (Gallirallus owstoni) 5-Year Review, Summary and Evaluation https://ecos.fws.gov/docs/five_year_review/doc2529.pdf
 Aguon, C.F. 1982. Survey and inventory of native land birds on Guam and Northern Marianas Islands. IN: Guam Aquatic Wildlife and Resources Division Annual Report, FY 1982. Department of Agriculture, Guam.
 U.S. Fish and Wildlife Service. 1984. Determination of endangered status for seven birds and two bats on Guam and the Northern Mariana Islands. July 27, 1984 (49 FR 33881). http://ecos.fws.gov/docs/federal_register/fr875.pdf
 U.S. Fish and Wildlife Service. 1984. Determination of endangered status for the Guam rail. April 11, 1984 (49 FR 14354). http://ecos.fws.gov/docs/federal_register/fr814.pdf
 Guam Division of Aquatic and Wildlife Resources. 2012. Guam Rail / Ko‘ko‘. Website http://www.guamdawr.org/learningcenter/factsheets/birds/rail_html. Accessed May 10, 2012.
 Haig, S.M., and J.D. Ballou. 1995. Genetic Diversity in Two Avian Species Formerly Endemic to Guam. The American Ornithologists Union. 112(2):445-455.
 Guam Division of Aquatic and Wildlife Resources. 2005. Guam Comprehensive Wildlife Conservation Strategy. http://www.guamdawr.org/Conservation/gcwcs2/GuamCWCS%20Chapter3.pdf
 Lint, K.C. 1968. A rail of Guam. Zoo Nooz 41:16-17
 Pratt, H.D., P.L. Bruner and D.G. Berret. 1979. America's unknown avifauna: The birds of the Mariana Islands. American Birds 33(3):227-235.
 Haig, S.M., J.D. Ballou and S.R. Derrickson. 1990. Management options for preserving genetic diversity: Reintroduction of Guam rails to the wild. Conservation Biology 4(3):290-300.
 Haig, S. M., Ballou, J. D. and N. J. Casna. (1994) Identification of kin structure among Guam rail founders: A comparison of pedigrees and DNA profiles. Mol. Evol., 3:109-119.
 U.S. Fish and Wildlife Service. 2010. Guam Rail / Gallirallus owstoni / Ko‘ko‘. Website: http://www.fws.gov/pacificislands/fauna/guamrail.html. Accessed May 10, 2012.
 Enbring, J. and T.H. Fritts. 1988. Demise of an insular avifauna: the brown tree snake on Guam. Transactions of the Western Section of the Wildlife Society 24:31-37.