The local government of Okinawa is building a wall across the Japanese island to save a rare species of bird that is threatened with extinction by a foreign interloper, the famously fearless, snake-fighting Indian mongoose.

Introduced a century ago to eliminate pit vipers, the mongoose has turned its fangs on a far more vulnerable opponent: the Okinawan rail, a shy and flightless bird that was discovered by human beings only in 1981. Decades of assaults by mongooses have driven the rail into the far northern reaches of the island, where it faces extermination.

A campaign of hunting, with a bounty of 5,000 yen (£25) per mongoose tail, has eliminated 6,000 of the red-coated carnivores but failed to stop the slaughter.

Hence the plans to erect the barrier, which will be to the mongoose what Hadrian’s Wall was to the Picts and the Great Wall of China to the nomad hordes. In 1986 an estimated 1,800 rails lived in the Yambaru region of Okinawa island but their number has slipped below 1,000. “The game might be almost up,” said Go Ogura, of the University of the Ryukyus, in Okinawa. “We have to do something.”

Seventeen mongooses were introduced into Okinawa in 1910 as a solution to a deadly problem — the habu, or Okinawan pit viper, which bit and killed hundreds of islanders every year.

Until a few years ago, amusement parks mounted gladiatorial combats between mongooses and pit vipers for the entertainment of tourists. But unless they are deliberately confined together the two species have little to do with each other. “Mongooses are creatures of the day and pit vipers are nocturnal,” said Takashi Nagamine, a member of Veterinarians to Save the Okinawan Rail and Other Animals. “So there aren ’t many chances for them to fight.”

Instead the mongooses raid the rails’ nests, devour their eggs and chicks, and rip out the throats of the adults. The wall will be built at one of the narrowest points of the 876sq mile island. By incorporating a natural barrier created by two reservoirs, its length will be reduced to 4½ miles. A mile-long section will be built this year at a cost of Y57 million (£27 5,000).

It will be coated with slippery material to prevent mongooses scaling it and its base will be buried to prevent tunnelling. Where it is breached by roads, mongoose traps will be set.

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