IT IS dusk, and the parakeets are coming in to roost. Just a few at first, brief flashes of green against the glowing sky at the end of a blistering late-summer day.

Steadily the numbers increase, then suddenly the sky seems to turn emerald as thousands swoop down, cawing and cackling, to cluster, jostling, in the tall trees. The poplars are now alive with raucous parakeets, chattering, nibbling, hanging upside down. The noise is deafening.

We might be in the forests of India, except that we are in the grounds of Esher Rugby Club in suburban Surrey, witnessing the latest, and loudest, evidence of global warming: an astonishing mass invasion of these exotic little parrots.

The wild parrot population has exploded in Britain. The colony at the edge of the rugby ground in Esher is the largest in the country, with an estimated 7,000 Indian ring-necked parakeets (Psittacula krameri).

The parakeets, which originated in northern India, have thrived in the recent string of warm summers and short, mild winters.

According to the latest scientific survey, the British parakeet population is increasing by 30 per cent a year, and is likely to reach 100,000 by the end of the decade. The parrots love it here. Pining for the Himalayas? “Mate, this parrot wouldn’t leave if you put four million volts through it.”

The parakeets have brought a splash of tropical colour to the London suburbs, but in other ways they are yet further, disquieting proof of shifts in the natural world: ornithologists fear that the parakeets — robust, adaptable and aggressive — will impinge on the habitats of indigenous species such as starlings, kestrels and little owls. The potential damage to crops can be seen in the upper branches of the poplars, which have been stripped bare by the voracious birds.

No one is quite sure how the parakeets got here. One story holds that the ancestors of the flock escaped from Shepperton Studios in 1951 during filming of The African Queen with Humphrey Bogart and Katharine Hepburn.

Other theories are that they escaped from an aviary during the storm of 1987, or that Jimi Hendrix brought them to Britain and released them to bring some psychedelic colour to London’s skyline, man. In fact, tiny populations of parrots have been spotted in Britain since 1855, but only in recent years have the numbers started to increase sharply.

The explanation is not solely climatic: the ring-necked parakeet lays two eggs a year, has no natural predators in Britain and lives for up to 34 years. The parakeet also seems to reach sexual maturity early, but that’s city life for you. The British parakeet population is now so established, and so well adapted, that even the toughest winter would probably have only limited effect on their numbers.

Chris Butler, a biologist at the Edward Grey Institute in Oxford, has studied the parakeet population and now believes that they can no longer be controlled. Already, they outnumber barn owls, nightingales and kingfishers. They have also been spotted in Wales, and as far north as Glasgow, and they are by no means the only exotics to be found in Britain’s warming climate.

Monk parakeets have been spotted in Hertfordshire and Devon, alexandrine parakeets have set up home in Merseyside, and orange-winged parakeets, originally from the Amazon, have settled in Weybridge.

So far, there is no firm evidence that the parrots are muscling out other species, in the manner of the invasive grey squirrel and the mink, but Mike Toms, of the British Trust for Ornithology, said that they tended to nest in the same places as native birds, and earlier. “We need to be aware of the possible impact,” he said. “They nest in quite large cavities so they would be competing with birds like tawny owls and jackdaws.”

In India, the parakeets are a serious agricultural pest, and of the 20 non-native birds to have settled in Britain, the ring-necked variety is much the most likely to cause significant damage. One business that has experienced the destructive side of the parrots is Painshill Park vineyard. Its crop was reduced to 500 bottles of rosé from an expected 3,000, after the parakeets flew in and devoured the grape harvest.

But Europeans have always felt affection for parrots, ever since Alexander the Great imported a pair to Greece after his conquests in India. In parts of London they are already referred to as “posh pigeons”. The debate over the parakeets mirrors the arguments over human immigration, with some applauding the new arrivals for adding to the diversity of metropolitan life, while others complain that they take up space and resources that should be reserved for the natives. “They come over here, nick our nests, eat all our nuts, and they’re protected by the Government . . .”

Andy Bonczik, one of a handful of people who have assembled at Esher rugby ground to watch the riotous dusk assembly of parakeets, said: “It’s a wonderful sight. It’s just so incongruous. We could be in the jungle.”

Danny, an 11-year-old in football kit whose home abuts the ground, is far less impressed. “I can’t get to sleep before eleven when they finally shut up, and my dad says they’re ruining his garden,” he said. With that, he hurled his football up at the trees, with a volley of abuse.

Which raises another issue. The highly intelligent ring-necked parakeets are among the best mimics in the parrot world, able to master a vocabulary of up to 200 words by the age of two. In their chosen home of Esher Rugby Ground, they may be picking up some rather ripe language. The residents of the stockbroker belt welcome the parakeets today but in a few years’ time, if flocks of green parrots take to the suburban skies chanting filthy rugby songs, they may feel rather differently.

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