A flock of Rose ringed parakeets in London – their population has grown to 30,000 now and it has got to the point where they are threatening indigenous species.
The RSPB agree and a spokesman’s quote below:

Mr Webb said that, at 16 inches long, a parakeet was larger than many garden species so shooting them “should not prove too difficult”. But he added: “Of course, many people would be distressed to see any animal shot.”

They seem to be all over london and the south east devouring fruits and seeds which indigenous birds eat.

Where did they all come from?
There are three theories mentioned in the article among others, but nobody really knows, but it looks like they’re here to stay.
1) Rose-ringed parakeets started breeding in the wild in the UK is that an entire flock escaped from Shepperton studios, Surrey, during the filming of The African Queen in 1951. It is thought around 20 birds vanished from an aviary on set.
2) A pair of the birds were released by Jimi Hendrix in Carnaby Street in the Sixties as a symbol of peace,
3) They are the descendants of domestic pets which escaped their owners’ cages

They’re very noisy birds, and I’ve seen them for myself roosting in the trees of Hampstead Heath