Wow, those parakeets get everywhere! How far North in Europe are you? We get them occasionally in East Anglia but with greater frequency. In London I can show you dozens in some of the parks. They have even been seen as far North as Durham.
I am in Düsseldorf, which i think is pretty much the northern border for the parakeets along the rhine for now.
Rumor says that 50 years ago about 20 of them escaped from an aviary in cologne and they spread out to the cities along the rhine around cologne. There's about 220 breeding couples in and around my city (2016 data). They are all indian Rose-ringed parakeets.
London is 51.5° North and Düsseldorf is 51.2° so basically the same, but your winters are a lot colder than ours I imagine, meaning our parakeets can live in places further North, like Durham.
There are many rumours about how they got loose in the UK. One is a music video/film with Jimmy Hendrix at Elstree studios in Borehamwood where they were released as part of the production. There are quite a lot of them in that area so it may be true 🫨
Winters are a lot milder here than they used to be, although if they've been around for 50 years their ancestor made it through colder winters than i have seen i guess. Interesting that they seem to be the same kind of parakeets in London!
I feel like the population grew a lot this century, like @[email protected] said, they surely weren't as present when i moved here. Then again from what i read, the population is only slightly growing.