The brilliant green ring-necked parakeet, an increasingly familiar sight in the Home Counties, could become "the grey squirrel of the skies", according to Tony Jun-iper, one of Britain's leading wild parrot experts. It may threaten crops and edge out British birds for nesting space, with green woodpeckers particularly at risk.
The grey squirrel was introduced from America in the late 19th century. In the past few decades its numbers have exploded, driving out virtually all native red squirrels south of the Lake District.
The ring-necked parakeet, Psittacula krameri, which is the world's most widespread wild parrot species, was first recorded successfully breeding in the wild in England in 1969. A population is believed to have been established with birds that escaped from avi-aries and others released by sailors returning from the Tropics, who found that to keep them would need a long and expensive quarantine period. The first detailed national census in 1996-1997 found 1,508 birds, concentrated in the southwest and southeast of London. They gather for the night in huge roosts: one at Esher Rugby Club in Surrey, has on occasion hosted more than 1,000.
Iridescent green, fast flyers and loud callers, they are now a well-known sight in the likes of Richmond Park and Kew Gardens. (the recently retired director of Kew, Professor Sir Ghillean Prance,encountered them on his first stroll in the gardens in 1988. "I heard this screech and thought that can't be a parrot - this is Kew.")
Although their growth has been steady, it could follow that of the grey squirrel and suddenly become explosive, said Mr Juniper, joint author of a recent guide to parrots of the world. "The ring-necked parakeet is an aggressive, adaptable and very clever species which can exist in a very wide range of habitats and climates over a very wide geographical area. "It has an all-purpose beak, which means it can eat most things, and like the grey squirrel it is cute, which makes pest control controversial. To see these flights of green arrows charging up and down squealing really is something. But there is potential for a problem."
Mr Juniper also fears for British birds that nest in holes in trees, such as stock doves, jackdaws, owls and green woodpeckers. Parakeets also must have nesting holes but cannot make their own and may drive other birds out.
A spokesman for the Royal Society for the Protection of Birds, Chris Harbard, said: "Parakeets need holes and there aren't that many natural holes around. I think we will have to watch the expansion carefully."