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.
Just a few hundred yards away, around dusk, a scene unfolds that feels distinctly less Belgian.
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.
One explanation for the thousands of parakeets in Brussels today is that they are descendants of a much smaller group of birds that were released in the 1970s from a small zoo and theme park, Meli Park Heysel, in the city.
According to local lore, the zoo’s director, Guy Florizoone, released the birds because he wanted to add a splash of color to the city. “The start of the Brussels populations was without doubt the release of several tens of birds from the zoo,” said Diederik Strubbe, an environmental scientist at Ghent University in Belgium who studied parakeet populations for his Ph.D. thesis.
When reached by telephone, Mr. Florizoone, now 80, said he had released 40 to 50 parakeets in the early 1970s as part of an experiment that he called “Birds in Freedom,” so that visitors could see them out and about. Most of the parakeets returned, he said. A few did not.
However, he said his experiment “has little connection” with the enormous population growth of parakeets in Belgium and across Europe, including Britain. “It’s impossible that such numbers would have flown over the Channel,” he said. “They’re not capable of that.”
(Mr. Florizoone’s wife, Marie-Claire, is less convinced about her husband’s role in the spread of parakeets in Brussels: “The only thing I know is that my husband is not responsible,” she said, “although people keep on thinking that.”)
Mr. Florizoone said that warmer weather in Europe had only accelerated the population growth of parakeets, a link confirmed by the ParrotNet project at the University of Kent in England, which studies how parakeets affect ecosystems.
In addition to milder winters, parakeets benefit from a lack of predators and plentiful food supplies in cities like Brussels, ornithologists said.
“Urban areas are like an all-you-can-eat restaurant,” Dr. Strubbe said.
Parakeets have been destructive not only to crops but also to other animals, including bats.
Jimmy Foucault, a journalist who was walking by the tree full of parakeets on a Sunday evening in September, said the abundance of tropical birds in Brussels was worrying. “These kind of species in Belgium — it’s just weird,” he said.
But in Brussels, they have lived harmoniously with other species because of the city’s preservation of old trees that are perfect for cavity-nesting birds like parakeets, said Jean-Yves Paquet, a director at Natagora, an organization focused on environmental preservation in Brussels. (In London, the ancient parklands of Hyde Park and Richmond Park are also popular places for parakeets).
“There is enough space for everybody, in fact,” Mr. Paquet said.
The authorities have asked the public not to feed the birds, but are not actively trying to limit their population growth, he said. Having “really cool-looking wildlife” can bring positive mental health benefits, said Jim Groombridge, the ParrotNet chair and a professor of biodiversity at the University of Kent.
While some people love the parakeets, others view them as noisy menaces. When they took up residence outside the former NATO headquarters in Brussels, the birds were so disruptive that officials tried a number of methods to encourage them to relocate, including playing recordings of falcons and hawks from loudspeakers mounted among the trees.
Still, Matthew Klimow, the former deputy assistant secretary general of NATO, remembered the birds fondly. “The parakeets were part of the charm of urban living in a city adorned by acres of big leafy trees,” he wrote in an email from Turkmenistan, where he is now the U.S. ambassador.
On an evening in September, Brigitte Dufour, a human rights lawyer, paused to admire the birds as they gathered at dusk, chirping loudly. “For me, they just bring joy,” said Ms. Dufour, on a walk with her dog, Roméo. She said she loved waking up to the sounds of parakeet chirps each morning, giving her the feel of being surrounded by nature, rather than in a big city. “I think that if they can live alongside with the other species here, why not.”
Claire Moses contributed reporting.