One rough estimate put the population in Britain at 30,000 a few years ago, up from only 1,500 in 1995. Researchers at Imperial College London are now trying a more scientific census through its Project Parakeet, which enlisted volunteer birders around the country for simultaneous counts on a recent Sunday evening.

“I was delighted when I first saw one in my yard, but when you have a flock of 300, it’s a different matter,” said Dick Hayden, a retiree who was volunteering at Long Lane Park. “They eat all the berries. They ate all the food from my feeder in one day; it was ludicrous. I had to stop putting it out because it got too expensive.”

There is wide agreement that the Adams and Eves behind the current population boom did not fly here from Asia or Africa but escaped from British pet cages or were intentionally released by their owners. The great mystery is what allowed the parakeets to procreate with such phenomenal success just in the past decade.

Throughout much of the 20th century, there have been occasional sightings around Britain of escaped parakeets, which are hardy enough to survive the foothills of the Himalayas. But their numbers remained low, and most scientists assumed that they were not adapted well enough to breed readily.