No one knows when and how parrots native to India and sub-Saharan Africa arrived in Britain, but their exponential growth in population in and around London has caused much concern among residents and officials.
Killing them is one of the measures agreed by the Department for Environment, Food and Rural affairs (DEFRA).
Kept as pets in Indian homes and known for the ability to reproduce words used by human beings, flocks of parrots over London are increasingly seen as pests.
Imperial College London, which is carrying out the first scientific inquiry into the bird's numbers and impact, has established that from an initial nesting in London in 1969, there are now nearly 32,000 birds, most of them crowded within London and South-East.
The birds have multiplied in such large numbers that they are threatening native birds such as robins and woodpeckers.
They have colonised parks and gardens, particularly in London, Surrey and Kent. Their annual growth rate - 25% a year - has caused much alarm in official circles. The aviation industry has also been affected by the population, with reports of bird-hits.
The rose-ringed parrot is protected under the Wildlife and Countryside Act, but from January 2010, it is legal to kill them under general licence if it is causing damage to crops or wild birds.
Areas in London that are affected adversely by flocks of parrots are Perivale, Wormwood Scrubs, Stanwell (close to Heathrow airport), Sutcliffe, Brockwell park, Hither Green, Redhill, Wast Ewell, Hersham, and Ramsgate in Kent.