The creature in question is a gorgeously coloured bird, not much smaller than a magpie, which screeches noisily whether flying or at rest.
The ring-necked parakeet - vibrant green all over, save for a narrow, pink and black neck band; with a long tail and a large, red beak - is a native of parts of Asia and tropical Africa. It first appeared in the wild in Britain in 1969.
According to Josephine Pithon of the University of York, who has been carrying out research on southern England's ring-necked parakeets, we now have about 2,000 of them. The birds are scattered as far afield as South Wales and East Anglia.
"The largest single population is in west London, stretching from Windsor in the west to Richmond in the east and to Reigate in the south", says Ms Pithon. "There are two smaller populations, one mainly around Margate and Ramsgate, and the other in southeast London."
Were it not for the food that they eat, you could argue that these exotic birds brighten up many a cold, damp winter afternoon in Cheam. It sounds harmless enough: a vegetarian diet of berries, nuts and fruits. But Britain's main populations of ring-necks are perilously close to some of the main fruit-growing areas of England. One or two of them can devour the plums on a garden tree in a few hours, so just think what havoc they might cause among the pear, apple, plum and other soft fruit bushes in Kent.
"Potentially, yes, they could cause a considerable amount of damage," says Sir Christopher Lever, an expert on introduced animals, including birds. "But," he adds quickly, "so far they haven't been a problem."
Josephine Pithon's research confirms this view, although she does note that they have caused damage to garden orchards by taking a peck from each fruit before letting it fall. Nevertheless, concern that ring-necked parakeets could have the makings of a major pest explains why the research was sponsored by the Ministryt of Agriculture.
No one is sure how these birds arrived in Britain. They may have come from free-flying homing birds kept as pets which failed to make it back to their aviaries. They may have been escapees from pet shops or from exotic bird farms. Or, according to the ornithological literature, they may derive from birds released by returning sailors when they realised the expense involved in a lengthy period of quarantine. Or a combination of all three.
However they came to be here, and despite their origins in hot countries, they are seemingly able to survive the cold of a British winter. This unimagined success may well be the result of the British gardener's virtual obsession with putting food out for birds in winter. But Josephine Pithon does point out that within their natural range, ring-necked parakeets also occur at high altitude in the Himalayas. "They seem to survive there quite well except for suffering from frostbite on their feet," she says.
These large birds probably have no obvious predators except for the occasional domestic moggy or a stoat that might strike lucky, and other birds such as magpies robbing their nests - but they do not appear to be in direct competition with any other bird species, so none is suffering as a result of their presence.
David Gibbons, of the Royal Society for the Protection of Birds, agrees they might compete for tree nesting holes in gardens, parks and orchards with jackdaws, owls and woodpeckers, but there is no evidence so far of any problem.
All the same it is a bird to watch, in more ways than one. For the next few years, at least, make the most of this exotic addition to your garden but do reinforce your peanut holder.