Image copyright Thinkstock Image caption There is a growing trade in exotic birds

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

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.

This is reflected in a high number of alien bird species in the mid-latitudes, including former British colonies.

After World War Two, there was a second wave, which continues today, most likely driven by growth in the pet trade.

More birds have been introduced into the wild in the 20 years between 1980 and 2000 than in the 400 years from 1500 to 1900.

"We've been able to map alien species richness for an entire group of organisms for the first time in such detail that we can locate populations and the historical processes that led to their introduction," said lead researcher Dr Ellie Dyer, of UCL and ZSL.

"It has given us valuable insights into the different stages of species invasion - humans play a key role, but so too do environmental factors that allow alien bird species to thrive in new locations."

Growth area

"An enormous increase in global trade and areas around the world that are growing in their wealth and disposable income has led to a much greater demand for birds as pets and so there's a large trade in birds and lots of species are moved around the world," Prof Blackburn told BBC News.

"For a variety of reasons, those species can get out into the wild and they can establish populations in areas where they haven't naturally occurred.

"That trade has become so large that it's now a really significant driver of the establishment of new bird populations around the world."

The research, published in PLOS Biology, was carried out by international scientists based at UCL, ZSL, the University of Adelaide, the University of Cambridge, the University of Exeter, the University of Queensland and Imperial College London.

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