But the most likely scenario, experts say, is that the birds escaped and were deliberately released from homes and pet stores. Once free, the parrots began breeding.
Until recently feral parakeets in Britain confined themselves to urban areas, principally London. Yet Butler's study shows the birds are now acquiring a taste for the countryside.
No longer dependent solely on bird feeders, the parakeets are increasingly feeding on wild and cultivated fruit, nuts, berries, seeds, and tree buds.
The birds are now turning up in many parts of Britainfrom Wales in the west to England's east coast and as far north as Glasgow in central Scotland.
Some ornithologists fear the wider presence of parakeets could potentially spell disaster for a number of native birds, including owls, woodpeckers, and falcons. Like parakeets, these native birds nest in tree holes.
"I have met several birders who don't like parakeets at all and feel they are causing declines in native species," Butler said. "There is the potential for parakeets to have a negative impact on native species, because they begin breeding in early March, which is earlier than other cavity-nesting species."
Chris Perrins is a professor at the Edward Grey Institute of Field Ornithology in Oxford. He said native British birds that could lose out include the little owl, green woodpecker, and kestrel.
"Parakeets don't excavate their own holes so are dependent on finding a natural split in a tree or finding a hole made by another bird," he said.
Another concern is the possible impact of marauding flocks of parrots on farms and orchards.
In Britain rose-ringed parakeets have been observed feeding on fruits such as apples, grapes, pears, plums, raspberries, and strawberries, as well as cereal crops like barely and corn.
The worry is that as parakeets colonize rural regions, more farms will be targeted.
"I've met people who've had their apple or pear orchards wrecked," Perrins said. "Damage is on a tiny scale at the moment, but that may not be the case when numbers go up to a hundred thousand."
Fruit crops near southwest London have suffered numerous raids in recent summers.
One vineyard, for instance, was stripped only weeks before the grapes were to be harvested. Enough were left to produce just 500 bottles of wine instead of the several thousand forecast.
Feral parrot populations are also on the increase in many other western countries, especially in urban areas.
The monk parakeet, a native South America specie, is now well established in Brussels, Belgium, for example. In Barcelona, Spain, rose-ringed parakeets are now ranked the fourth most numerous bird.
Bakersfield, California, has more than a thousand rose-ringed parakeets. The population is believed to date back to a hurricane-force storm that destroyed an aviary in 1977. The birds have been observed fighting with woodpeckers over nesting holes.
In southern Florida, utility companies now grapple with monk parakeets and the massive, nettlesome nests they build atop transmission poles.
In Britain, however, it might be too late to control booming populations.
"With the current rate of increase, you'd have to have a pretty big blitz to keep numbers from rising," Perrins said. "It would be difficult to get rid of them anyway, because they retire into towns, where it's not so easy to pursue and shoot them."
Love them or loathe them, it appears that in Britain, at least, these exotic invaders have already flown the coop.
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