Why don't they die?

Dear Polly,
The Rose-Ringed Parakeet population of Istanbul survives some harsh winters for a bird that is mainly from SE Asia. Why don't they die from the cold?
Polly Says
Psittacula krameri also flourishes in England, Germany, Netherlands, Belgium and other European locales with harsh winters. In 2006, London Natural History Society's bird report stated 6000 of the Rose Ringed Parakeets lived wild in London. They seem to be able to flourish in cold, urban locations.

"They actually originate from the foothills of the Himalayas, so they don't need it to be that warm to live comfortably," says Andre Farrar of the Royal Society for the Protection of Birds.

http://news.bbc.co.uk/2/hi/uk_news/magazine/6478911.stm