This story was updated July 2018.
Imagine, if you will, a cruise boat full of vacationers enjoying a cool summer evening on Lake Murray. The sun has turned into a blazing orange ball as it slowly slips below the horizon. With Jimmy Buffett playing over the speaker, the visitors savor the last moments of daylight.
Then, out of nowhere they begin to appear. Graceful little swallows, swooping playfully through the air. Three or four birds soon become a dozen. After a few minutes, they're coming from every direction in the sky. Tens of thousands of purple martins inexplicably brought together as if driven by some unseen force of nature.
Could these unsuspecting sightseers be embarking on a trip into the Twilight Zone?
No. They're headed to Bomb Island, the largest purple martin sanctuary in North America. Some 800,000 to one million birds fly up to 160 miles every evening to roost in the trees on this uninhabited spit of land.
"It really is a phenomenon," said cruise boat owner Jay Downs, who has been narrating the avian exhibition for years. "They fly over the island for several minutes as if they're fluffing their pillows."
From the first week of July through the end of August, the purple martins show up every afternoon at sundown on the lake just outside Columbia. On their way to Bomb Island, they often nosedive straight down to the water for a "drink on the wing." Aerial insectivores, they also eat on the fly.
The birds began passing us as we cruised within a half mile of Bomb Island. By the time we joined the other bird-watching boaters circling the island, the sky was dark with purple martins. The closer we got, the more eerie it looked. Scenes from Alfred Hitchcock's "The Birds" kept flashing in my head.
But it was clear these people-friendly swallows wanted nothing more than to hunker down for the night. No one is allowed on the island during the summer roosting season. Then again, who would want to walk under a canopy of trees filled with well-fed birds?
Refreshed and rested, the purple martins take off at daybreak in mass. It has been reported by local weather forecasters that the radar image of the morning migration is larger than Hurricane Hugo.
If you've got binoculars, bring them with you. But don't go watching "The Birds" before you take the tour.