An exhaustive new analysis of the 9,787 known bird species and 129 more that are now extinct has indicated that 10 per cent will disappear by 2100 if present trends continue.

Another 15 per cent will probably be on the brink of extinction, with only a handful of individuals left in the wild, leaving one in four of all birds “functionally extinct” less than 100 years from today.

The true picture may be bleaker still. Should environmental degradation and habitat loss worsen, as most researchers predict, some 14 per cent of birds will be extinct by the end of the century, and another 25 per cent critically endangered or extinct in the wild.

Even under the best-case scenario — under which conservation measures ensure that no new species become threatened — at least 6 per cent would die out and 8 per cent would be close to extinction.

The findings, from a team at Stanford University in California, raise fresh concern that biodiversity is declining at an alarming rate, and that the world is in the grip of its sixth mass extinction.

More than 3,000 animals and plants were last month added to the official Red List of threatened species by the World Conservation Union (IUCN), which declared a record 15,589 organisms at risk.

It found that one in eight bird species is currently threatened, along with one in four mammals, one in three amphibians and almost half the species of turtles and tortoises.

Several birds, including the Hawaiian thrush and the Atitlan grebe, were declared extinct in the latest Red List, with others, such as the Hawaiian crow, becoming extinct in the wild. Dozens more are critically endangered, including British species such as the slender-billed curlew, the Balearic shearwater and the sociable lapwing.

The new research, which is published in the journal Proceedings of the National Academy of Sciences, indicates that the recent extinctions are the early stirrings of a worrying trend. A loss of bird life on the predicted scale would have grave knock-on effects, disrupting natural mechanisms of plant and crop pollination, seed distribution and insect control. Even human health could be affected, as ecological changes favour animals and insects that carry disease.

“Our projections indicate that, by 2100, up to 14 per cent of all bird species may be extinct and that as many as one out of four may be functionally extinct — that is, critically endangered or extinct in the wild,” Cagan Sekercioglu, who led the study, said.

“Even though only 1.3 per cent of bird species have gone extinct since 1500, the global number of individual birds is estimated to have experienced a 20 to 25 per cent reduction during the same period.”

Human health had already been affected by falling bird populations in several parts of the world, the researchers said. In India, for example, the vulture population declined by 95 per cent during the 1990s, largely as a result of poisoning with the veterinary drug diclofenac. This increased numbers of feral dogs and rats, which have led to a rising incidence of rabies.

The extinction of the passenger pigeon in North America has contributed to the spread of Lyme disease, which can cause neurological damage and flu-like symptoms. This is carried by ticks, which thrive on the bodies of deer mice.

As passenger pigeons died out, the acorns on which they fed became available to deer mice, supporting much larger populations of the rodents and spreading the diease.

“It’s hard to imagine the disappearance of a bird species making much difference to human wellbeing,” Gretchen Daily, another researcher, said. “Yet consider the case of the passenger pigeon. Besides mail becoming a lot less fun to receive, its loss is thought to have made Lyme disease the huge problem it is today. When passenger pigeons were abundant — and they used to occur in unimaginably large flocks of hundreds of millions of birds — the acorns on which they specialised would have been too scarce to support large populations of deer mice, the main reservoir of Lyme disease, that thrive on them today.”

Under the best-case scenario considered by the team, conservation measures will be imposed that prevent any new bird species being added to the Red List. Even this means that 6 per cent of species will become extinct by 2100.

Under the intermediate scenario, in which extinctions happen at the same rate as between 1994 and 2003, 10 per cent of species will be lost.

The worst-case scenario assumes that the number of threatened species increases by 1 per cent every year — a rate the researchers described as “conservative”. This would lead to the disappearance of 14 per cent of birds by 2100.

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