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.

After Elizabeth Carlen, a biologist at Fordham University, caught pigeons with a net gun and took their blood samples during a series of road trips across the region, she discovered that birds all the way from Virginia to southern Connecticut show genetic signs of interbreeding. And in a paper published this month in Evolutionary Applications, she and a co-author also report that another separate, distinct pigeon supercity begins in Providence, R.I. and continues to Boston.