At any given time there are "at least ten billion birds in North America ... and there could be as much as 20 billion—and almost half die each year due to natural causes," said ornithologist Greg Butcher, director of bird conservation for the National Audubon Society in Washington, D.C.

But what causes dead birds to fall from the sky en masse? The Arkansas case points to two common culprits: loud noises and crashes.

Beginning at roughly 11:30 p.m. on New Year's Eve Arkansas wildlife officers started hearing reports of birds falling from the sky in a square-mile area of the city of Beebe. Officials estimate that up to 5,000 red-winged blackbirds, European starlings, common grackles, and brown-headed