Okinawa Rail Hypotaenidia okinawae


Justification of Red List Category
This species is listed as Endangered because it has a single, very small population within a very small range on just one island, and both its range and population are undergoing continuing declines as a result of introduced predators and the loss of forest to logging, infrastructure development, agriculture and construction of golf courses.

Population justification
Surveys conducted between 1996 and 2004 estimated 717 individuals at the end of the period. Repeat surveys in 2006 found no further decrease (K. Ozaki in litt. 2006). On this basis the population is estimated at c.720 individuals, equivalent to c.480 mature individuals.

Trend justification
Hanawa and Morishita (1986) estimated the population to be c.900 pairs or 1,500-2,100 birds. Ozaki et al. (2006) reported a reduction to 717 individuals by 2004 (with no further decline by 2006). Hence, a decline of 37% occurred over ten years and the negative population trend is therefore best placed in the band 30-49%, and considered on-going.

Distribution and population

Hypotaenidia okinawae is endemic to Okinawa Island, Japan, where it is confined to Kunigami-gun (=Yambaru) in the northern quarter of the island, north of Shioya and Higashi-son. Surveys from 1996 to 2004 found that numbers declined dramatically from c.1,800 individuals in 1986 to an estimated 717 individuals, with its range contracting northwards by 40% since 1985-1986 (Ozaki et al. 2006). Surveys in 2006 did not detect any further contraction in range size, suggesting that measures to control introduced Javan Mongoose Herpestes javanicus have had some success (K. Ozaki in litt. 2006). Low genetic diversity in the species's population is suggestive of a recent population bottleneck, with genetic diversity likely to be negatively affected by habitat fragmentation (Ozaki et al. 2010). 


It occurs in a variety of habitats, including primary and secondary, evergreen and broadleaf subtropical forest, often with a dense undergrowth of ferns, near to streams, pools or reservoirs and cultivated areas close to forest. It is almost, but not completely, flightless and feeds on the forest floor, and occasionally in shallow water, where it takes mostly land snails (Kobayashi et al. 2018), as well as invertebrates and lizards. Nests are made on the ground during the breeding season, from May-July. The usual clutch-size is two to three eggs.


Logging, dam construction and associated road-building, agricultural development and golf course construction are causing forest loss and fragmentation. Recent research (Ozaki et al. 2006) indicates that its range on the island has contracted to the north (resulting in a 25% decrease in extent of occurrence). The research strongly implies that introduced Javan Mongoose Herpestes javanicus are the cause of this contraction, although feral dogs, cats, indigenous snakes, Weasels Mustela itatsi and introduced Large-billed Crows Corvus macrorhynchos are also possible predators (K. Ozaki in litt. 2006; Taylor and Sharpe 2020). The Brown Tree Snake Boiga irregularis has also been observed on the island which may have potential future implications on the species's roosting sites (Taylor and Sharpe 2020). Road kill was the cause of mortality in 70% of the 22 recorded deaths during a five year study from 1998 to 2003 (Kotaka and Sawashi 2004) and at least 19 deaths in 2011 (Shimpo 2011). Mortality was worst during the breeding season, suggesting that road kill lowers recruitment to the adult population and may be causing localised declines.

Conservation actions

Conservation Actions Underway
It is legally protected in Japan. Yonaha-dake and parts of Mt Ibu and Mt Nishime are designated as protected areas. Conservation organisations have been promoting the protection of Okinawa's forests since 1970 and have bought some sites on Mt Yonaha to establish private wildlife reserves. In 1996, Yambaru was designated as a national park. The species was the focus of several surveys conducted between 1996 and 2006. Some areas of fencing and a trapping regime have been employed to reduce predation on the species by introduced mammals (Ozaki et al. 2006). For example, strict mongoose eradication between 2000-2007 had led to significant recovery of the species in the following years (Kotaka et al. 2020). Traffic calming measures have been taken in some areas to reduce the number of road kills. Japan's Ministry of Environment plans to undertake a captive breeding programme for the species, aiming to obtain a captive population of 200 individuals. Genetic analysis of 20-25% of the population has indicated that minor haplotypes persist, suggesting that conservation measures have been implemented in time to preserve most of the species's genetic diversity (Ozaki et al. 2010).

Conservation Actions Proposed
Establish monitoring and research programmes and support captive breeding programmes. Provide full protection for the whole area of Yambaru. Continue to expand research and actions to control introduced predators. Instigate a conservation education programme for Okinawa using this species and Okinawa Woodpecker Sapheopipo noguchii as flagship species. Develop measures to further reduce road kills. Continue fencing areas to reduce predation. In 2016 plans were announced by the Japan's Environment Ministry to designate a new national park in the northern part of the island to help this species (JiJi Press 2016).


30cm. Medium-sized, nearly flightless, short-tailed rail with long, strong legs. Dark olive-brown upperparts. Black sides of head and underparts, barred with white from lower neck to undertail-coverts. White loral spot and line from rear base of eye down side of neck. Red bill and legs. Juvenile has paler upperparts with mottled white underparts. Voice Highly vocal. Calls include a loud kyo, a kwi kwi kwi ki-kwee ki-kwee, often answered by a ki-ki-ki and a kyip kyip kyip given by a pair. Other calls include a rising pig-like squeal and a deep bubbling gu-gu-gugugugu and gyu-gyu-gyagyagya.


Text account compilers
Fernando, E.

Benstead, P., Bird, J., Ozaki, K., Pilgrim, J., Takashi, H. & Taylor, J.

Recommended citation
BirdLife International (2022) Species factsheet: Hypotaenidia okinawae. Downloaded from http://www.birdlife.org on 07/07/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 07/07/2022.