Almost one in five Americans lives in the megacity that is the Northeast, a sprawling cluster of paved surfaces, bitter sports rivalries and clashing opinions about which city’s drivers are rudest.
Each city on the road up Interstate 95 from Washington, D.C., to Boston prides itself on its uniqueness. But it turns out parts of the animal world have their own senses of geography.
At the genomic level, a new study finds, most of the Eastern Seaboard’s pigeons are all mixed up. That means those birds shuffling through Central Park, clucking on the National Mall, hanging out in Baltimore’s Inner Harbor or getting chased down a Philadelphia back alley by Gritty? All one interconnected population, denizens of an unbroken avian super-metropolis.
Except New England pigeons, that is, which seem to keep to themselves.
“That was really weird to think about,” Ms. Carlen said. The suburbs of Connecticut don’t just mark the boundary between Yankees and Red Sox fans, or tomato-based and cream chowders: They also seem to block pigeons.
Discovering two distinct pigeon megalopolises was itself a surprise to Ms. Carlen.
“My original hypothesis was that each city was going to be a separate population,” she said. Previous studies had shown that pigeons are homebodies, staying within hundreds of feet of where they were born.
But it’s possible that all it takes is one intrepid pigeon finding an out-of-town mate every generation to explain how cities like Baltimore, Washington, Philadelphia and New York can have so many genetic similarities, she said.
Less than 200 miles separate New York and Boston, but something between the rival towns forms an uncrossable moat for city-slicker pigeons. A look at satellite maps of artificial light at night highlights the gap between the two metropolitan areas — a break in what is otherwise continuous urban sprawl. Although the birds can fly, all that rural green space might dissuade them from attempting a crossing.
The result adds to the growing field of urban evolution, the study of how wildlife changes in and around some of the least natural environments on the planet.
“I mean, who pays attention to pigeons?” said Jonathan Losos, an evolutionary biologist at Washington University in St. Louis, who did not participate in the research. “To find out that there’s an interesting biological phenomenon about how they might be evolving is quite fascinating.”
Previous studies have already shown how some animal populations disperse into remaining scraps of habitat like city parks, Dr. Losos said. A second phase of research has begun uncovering the kinds of adaptations animals come up with to adjust to cities.
Compared to those two approaches, “this paper is particularly significant because it marks what I would call a third stage of understanding how species respond to urban landscapes,” he said. Pigeons don’t just thrive in cities — they seem to cling to them when they disperse. And in turn, that sets up urban “islands” where pigeons could evolve differently in the future.
Future work will dig into just how closely pigeons are related within cities, and between them.
“They’re not elephants, they’re not pandas, they’re not tigers. They’re this bird that we interact with on a daily basis,” she said. “You might imagine that we have a lot of answers about it, but we don’t.”