13 January 2017
The trade in caged birds poses a risk to native species if the pets escape into the wild, UK researchers say.
They identified almost 1,000 species of bird introduced into new areas by human activity over the past 500 years.
More than half of these arrived after 1950, probably driven by the trade in exotic birds.
Global demand for parrots, finches, starlings and other exotic birds has soared.
"Areas that are good for native birds are also good for alien birds," said Prof Tim Blackburn, of University College London and the Zoological Society of London, who worked on the study.
"It's a worry because aliens may threaten the survival of native species."
Alien birds Image copyright SPL Image caption The ring-necked parakeet is now a common sight in London
Ring-necked Parakeets: introduced from the Asian subcontinent, these birds are now common living wild in and around the south-east of England, where they can compete with native species for food and breeding sites
Ruddy ducks: a cull was ordered in the UK after ruddy ducks from North America were found to be breeding with native European ducks
The first wave of introductions happened in the mid-19th Century as Europeans, predominantly the British, deliberately moved game birds such as duck, geese, grouse and pheasants into new territories.