The cause is the Microclimate, it differs from those in the surrounding areas, often with a slight but sometimes with a substantial variance
On the Königsallee in Düsseldorf, for example, the “Halsbandsittich” or Rose-Ringed Parakeet (Psittacula krameri) has now become a part of everyday life.
The bright green birds can be seen at feeding stations, parks and balconies and sleep on trees in the middle of the city. There are around 1000 ring-necked parakeets in the greater Düsseldorf area, estimates a bird expert. There are perhaps twice as many in Cologne. The cute green birds have been spreading a tropical flair in Bonn for decades. The parrots are actually at home in Africa and Asia. The triumphant appearance of the strange bird began in 1967.
At first, “Halsbandsittiche” were found in the wild in Cologne, the Federal Agency for Nature Conservation (BfN) in Bonn said, and it is very likely that they escaped or were released. According to current estimates, there are now around 8500 ringed parakeets now living in the wild, mainly in Düsseldorf, Cologne, Bonn, Wiesbaden and Heidelberg
Per Wikipedia: “The rose-ringed parakeet is sexually dimorphic. The adult male sports a red and black neck ring, and the hen and immature birds of both sexes either show no neck rings, or display shadow-like pale to dark grey neck rings. Both sexes have a distinctive green colour in the wild, and captive bred ringnecks have multiple colour mutations which are turquoise, olive, white, blue, violet, grey and yellow. Rose-ringed parakeets measure on average 40 cm (16 in) in length, including the tail feathers, a large portion of their total length. Their average single-wing length is about 15 to 17.5 cm (5.9 to 6.9 in). In the wild, this is a noisy species with an unmistakable squawking call. Captive individuals can be taught to speak. They are a herbivorous and non-migratory species”
Atmospheric conditions made it possible for these tropical birds in these cities to survive and prosper. The cause is the Microclimate, it differs from those in the surrounding areas, often with a slight but sometimes with a substantial variance.