More than a million albatross, shearwater and petrel chicks, some goose-sized and weighing more than 20lbs, are being eaten alive every year - by mice.
The mice are descendants of the British house mouse, probably taken to Gough Island, south of Tristan da Cunha, by ships in the 19th century - but have grown to double the size of their ancestors. They have also become carnivores and learnt to attack big seabird chicks which, having evolved over millions of years on an island with no natural predators, do not know how to defend themselves.
The mice attack at night, singly or in groups, gnawing into the chicks' bodies when they sit on the nest, and eventually kill them through blood loss or destruction of vital organs.
The scale of the killing has astonished ornithologists who discovered it. It is thought it might eventually drive some of the 22 species of birds that breed on Gough to extinction.
Now the Royal Society for the Protection of Birds (RSPB) has been awarded a grant of more than £60,000 by the UK government's Overseas Territories Environment Programme to fund additional research on the Gough Island mice and a study of how to deal with them.
The slaughter was discovered by Richard Cuthbert, a research biologist for the Royal Society for the Protection of Birds (RSPB), and his colleague, Erica Sommer, from Cape Town University when they spent a year working on Gough in 2000-2001. It has been confirmed by video footage of the attacks taken by Ross Wanless, a PhD student from the University of Cape Town's Percy FitzPatrick Institute, and his colleague Andrea Angel.
"The albatross chicks spend eight months sitting waiting for food from their parents," Dr Cuthbert said. "They are nearly a metre tall and 250 times the weight of the mice, but are largely immobile and cannot defend themselves. For a carnivorous mouse population on one of the wettest and windiest places on earth, it is an easy meal of almost unimaginable quality. The result is carnage."
Gough, a UK Overseas Territory and a dependency of St Helena, is one of the most remote and hostile environments in the world, a mountainous island without permanent human inhabitants, in the Roaring Forties - an area of extreme winds.
But it is also a World Heritage Site hosting more than 10 million seabirds, including 99 per cent of the world's Tristan albatross and Atlantic petrel populations - the birds most often attacked. Just 2,000 Tristan albatross pairs remain.
"Gough Island hosts an astonishing community of seabirds and this catastrophe could make many extinct within decades," said Geoff Hilton, another RSPB research biologist. "We think there are about 700,000 mice, which have somehow learnt to eat chicks alive - much like blue tits learnt to peck milk bottle tops. The albatross chicks weigh up to 10kg and, ironically, albatrosses evolved to nest on Gough because it had no mammal predators - that is why they are so vulnerable. The mice weigh just 35 grams; it is like a tabby cat attacking a hippopotamus."
Mr Wanless said: "There are mice on other South Atlantic islands but Gough is the only site where this is known to be happening. Once one mouse has attacked a chick, the blood seems to attract others. They gnaw into the chick's body, create a gaping wound and the chick weakens then dies over several days."
Scientists suspect that the mice are also eating the eggs and chicks of the rare, ground-nesting Gough bunting, a small finch found nowhere else in the world. Researchers think the finch has been forced from the best nesting sites into less suitable upland areas. "This species is one of the most worrying because there is no other population in the world," Dr Hilton said.
The mice on Gough could be killing far more than one million chicks each year, the researchers estimate. About 1,000 Tristan albatross chicks are killed annually by mice and in 2000 and 2001, 60 per cent - 700,000 - of Atlantic petrel chicks died before fledging, probably because of mice. Many of the chicks of the one million great shearwaters are also killed by mice. .
The Gough mouse, typically weighing between 27g and 40g compared to 15g for a house mouse in the UK, is one of 2,900 non-native species damaging native wildlife on the 17 UK Overseas Territories and Crown Dependencies a review by the Joint Nature Conservation Committee (JNCC) has found.
Vin Fleming, head of the international unit at the JNCC said: "Non-native species are a major cause of the loss of biodiversity globally and their impacts are especially severe on island ecosystems typical of our Overseas Territories."