Last weekend at Cape Cod I saw a swirling flock of tree swallows at their staging area.
Staging: Designating a stopping-place or assembly-point en route to a destination -- from The New Shorter Oxford English Dictionary.
Tree swallows (Tachycineta bicolor) breed as far north as the tundra/tree line in Canada and Alaska and spend the winter from Florida to Central America. Their departure from western Pennsylvania is barely noticeable but on the East Coast they gather in salt marshes in huge flocks of a hundred thousand birds. Their interim stops on migration are called staging areas.
In the evening tree swallows funnel down to the marsh in a tornado of birds. At dawn they burst up from the roost, as shown in the Central Florida video above.
Last Saturday I saw thousands of tree swallows flying in tight formation at West Dennis Beach. Though sunset was two hours away they flew low across the salt marsh, hovered and touched down on bushes, swirled up and around and away.
At the height of their swirling I took some photos but couldn't capture their magic. However, this picture shows why they flew so fast and so close. There's a falcon in the upper right corner with a swallow in its talons. Perhaps it's a merlin. I would never have noticed without this photo.
Here's an audio description of the tree swallows' fall migration at Connecticut salt marshes at Living On Earth: BirdNote®: Roosting Tree Swallows
It's worth an autumn visit to the East Coast to see this.