“Here they come!” The shout goes out from the far side of a tulip poplar, in the backyard of a slate-roofed house in the historic district of Leesburg, Virginia, about 40 miles northwest of Washington, D.C. I look up. About a half-mile away, black specks pepper the horizon. At first it’s difficult to identify the birds. They are tiny, coal-colored filaments, like a child’s stick-figure drawings of gulls in flight, adrift in a reddening sky. But after a brief few seconds, this becomes evident: The birds are large. They are numerous. And they are flying straight toward us. In fact, those birds—mostly black vultures, with a smattering of their red-headed kin, turkey vultures—know exactly where they want to be: roosted for the night in one of the trees beside the big fieldstone house at 212 Cornwall Street. They’ve been here for weeks. At first just a few dozen settled each sunset into a pair of tall pines. Evening by evening, more joined the roost. Their numbers climbed...