Passenger Pigeons once migrated through Canada, the United States, and the Gulf of Mexico in numbers so huge that they darkened the sky. One flock was described as "a column, eight or ten miles in length...resembling the windings of a vast and majestic river."

In 1808, one flock of passenger pigeons in Kentucky was estimated at more than two billion birds. Today, they are extinct owing to a combination of results of human activity, including the destruction of their food sources, westward expansion, and overhunting.