Parakeets are multiplying across southern England so quickly that they could start to threaten native bird species and agricultural crops, according to experts.

From just 2,000 ten years ago, there are now an estimated 30,000 parakeets in Britain, concentrated in London and South East England. Numbers could reach 50,000 within two years.

Parakeets nest in holes in trees and there is concern that this could pose a threat to many indigenous hole-dwelling species such as nuthatches, kestrels, jackdaws, starlings and tawny owls, John Tayleur of the British Trust for Ornithology told BBC Wildlife Magazine. Parakeets nest earlier in the year and could hijack the best sites, leaving native species out in the cold.

Evidence from a study in Belgium showed a significant fall in nutchatch numbers in areas where parakeets numbers had increased, possibly because of competition for nesting sites.

Although there is no definitive evidence of the same thing happening in Britain, Mr Tayleur said it was "likely that this effect could be seen in the UK". A study in 2003 by Christopher Butler, formerly of Oxford University, showed that parakeet numbers in London were increasing at a rate of 30 per cent a year.

For many years parakeets have been a common sight in southwest London and Surrey, where they are nicknamed "posh pigeons", and there have been recent sightings as far away as Brighton and Scotland.

Although the bright green birds are a popular with visitors to many of the parks around London, they have attracted a reputation for damaging fruit trees — another of their nicknames is "flying secateurs".

The RSPB said that they are "on the cusp" of becoming a problem for crops and in India they are a serious menace to fruit and cereals. Dr Butler has warned that the birds could become "a serious agricultural pest" with "the potential of inflicting serious economic and ecological damage".

The speed and aggression of parakeets means they are not easy prey for raptors. There have been sightings of them seeing off sparrowhawks and even squirrels.

It is unclear how the rose-ringed parakeets, native to Africa and India, were introduced to the UK, but they have been present in large numbers since 1969. They are thought to have escaped from aviaries — though one legend has them escaping from the set of the African Queen in 1951. Some of it was filmed in London. Another legend has Jimi Hendrix releasing a pair into the wild during a tour in the 1960s. They were recognised as an established exotic species in 1983.

Farmers have already started to complain. Steve Gallimore, of Garson's Farm in Surrey, said that the birds had destroyed 20 per cent of his apple crop in 2006. Though they are tempted to take drastic action, the birds are, for now, a protected species.

The July issue of BBC Wildlife Magazine is out tomorrow.

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