In the lively Brussels neighborhood of Flagey, you can be sure of two things: People will be lined up for fries at Frit Flagey, and pigeons will be nearby, pecking at scraps. Hundreds of electric green parakeets, more commonly associated with the tropics of West Africa or India than gray, rainy Brussels, flock to a tree beside a pond. They slumber there for the night, turning the tree into a brighter shade of green, and take flight at dawn. The number of rose-ringed parakeets in Brussels has swelled from just a few in the 1970s to some 10,000 today, becoming one of the most common birds in Brussels, after pigeons and sparrows. As populations of wild parakeets have grown — not only in Brussels but also in London, Paris and more than 100 other cities in Europe — researchers are trying to understand how a tropical bird has flourished in cold climates.

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Locations: Brussels, Flagey, West Africa, India, London, Paris, Europe