Every night between roughly October and April, hundreds of thousands of American Crows roost in the trees and on the buildings in Rockville, Maryland. The exact location changes from winter to winter, but generally they gather near Montrose Road near the White Flint Metro. At dusk and dawn, the sight is eerie and reminiscent of Alfred Hitchcock's "Birds."
According to a Washington Post article, the crows had been congregating in Rockville for 20 or 40 years. Since the article was published in 1993, that suggests that the Roost has been active for at least 50 years if not 70 years.
Crows are intelligent and social animals. In the winter months they huddle together for warmth and safety. The bright lighting of the city at night and the safety in numbers also might make it easier to spot owls, one of their main predators.