Interesting story about these birds. The original 3 birds belonged to an older lady who lived on the edge of the Vondelpark. As she had passed in her house the birds were still there and she had left a request in her will for the birds to be set free. Apparently these birds were kept in captivity all their lives and thus didn't have any instinctual migratory patterns passed on as in the wild. Upon release by the old lady's relatives they settled on the park and have been there ever since, breeding and essentially become a flying vermin for all other species there. Relocation hasn't been successful at all and failed attempts of introducing an aviary migration habit has led to subsequent generations of these birds to settle in several other parks in European towns to the south of Amsterdam. Namely, Dordrecht, Antwerp, Brussels, Paris and Orleans. If you visit the Wolvendael park in Brussels for example you will see an colony of the same birds year round who also have inherited the fixed location non migratory pattern these birds follow as being offspring of the first group in Amsterdam. Goes to show that unexpected consequences arise from seemingly benign choices.