These have become common urban birds all over Europe, and each city has its own story about how they came to be there - somebody's escaped pets, a zoo that went broke, a break-in at a pet shop that released some ... These are probably mostly urban myths. My guess is that they were popular pets for a brief period, but then people found out how loud they were and decided to simply release them. I haven't heard that they have displaced endemic species, so perhaps not much harm done. They are thriving. And they are a popular sight. Still, they don't really belong here.
