Like the first snowflakes of an approaching blizzard, a small flurry of tiny birds flitted across the slow-moving water of the Connecticut River earlier this week, just as the sun began to dip below the western bank in Old Saybrook.

“Here they come!” Cristina Negrón cried excitedly. Cristina and her husband, Amby Burfoot, were paddling a tandem kayak alongside Amby’s brother Gary and me in single kayaks that we launched earlier from Old Lyme as part of an impromptu flotilla to witness one of nature’s most amazing local phenomena: the nightly “murmuration” of tree swallows.

From late summer through early fall hundreds of thousands of these migrating birds swarm at sunset and swirl for about 15 minutes before roosting among the reeds and phragmites on Goose Island just south of Lord Cove in Old Lyme, within sight of the Baldwin Bridge.

A few dozen other vessels, including kayaks, canoes, small power boats and even a tour boat loaded with passengers, drifted with an ebbing tide as the 6:45 sunset approached.

The first cluster of cheeping swallows skimmed and swooped toward the island before disappearing in gathering darkness. I raised my brows as if to ask, “Is that it?” but Gary had advised, “Be patient.”

Sure enough, 15 minutes later they came thick and fast, huge waves of birds, eventually forming an enormous, amorphous, swirling cloud hundreds of feet high. We laughed delightedly.

“Amazing!” I exclaimed. But I still hadn’t seen the best part.

We watched, mesmerized, for another 10 minutes, and then – WHOOSH!

As if sucked by a giant vacuum, the birds formed a tornado-like column and descended to Earth in a vortex. In seconds the sky was empty.

“It’s like an improvisational dance performance – always different, and always pretty spectacular,”  Bill Yule, educator/naturalist with the Connecticut River Museum in Essex, said when I called him later.

Yule explained that the swallows, which migrate from points as far north as Canada to as far south as the Gulf of Mexico, feed on flies, gnats, midges and other insects found along the shore. The tall grasses and tidal wetlands at the mouth of the Connecticut River draw these insects, now at peak activity, which in turn attract swallows and other birds.

Nobody knows for sure how long the swallows have been returning to Goose Island – “It could be a thousand years,” Yule said – but their presence didn’t become widely reported outside birding circles until fairly recently. What’s more, the birds that show up one night may move on and be replaced by a new flock the next, he added.

“The migration is not really understood,” he said. Sometimes the swallows, which can zip along at 35 mph, swirl like a tornado, other times they veer in various directions; their descent can be rapid or slow, or in layers.

“It’s really quire amazing,” he said.

Their mass roosting is part of a natural instinct called the “selfish herd effect,” Yule said, observed in animals as varied as water buffalo in Africa or white martins in Venezuela, in which large groups bed down or roost together to ward off predators.

The swirling just before roosting is called “murmuration,” he added.

The Connecticut River Museum (ctrivermuseum.org) is among a handful of organizations that offer nightly boat rides in season to observe the swallows. Others include Connecticut River Expeditions (ctriverexpeditions.org) and Connecticut Audubon Society's EcoTravel in Essex.

Or, you can experience the show the way my friends and I did, by kayak. There’s a launch site on Pilgrim Landing Road off Route 156 just north of I-95 with a small parking lot but no room for trailers.

Goose Island is less than a mile north. Farther north is another launch at the end of Ely’s Ferry Road, which requires a longer paddle south to Goose Island. There also are other launch sites across the river in Old Saybrook and Essex.

Since you’ll be returning in the dark, make sure you bring lights.

The lower Connecticut River also is home to osprey, great blue heron, egrets and other shore birds.

You have a few more weeks to catch the swallows. Soon, migrating hawks will be flying through, and then in midwinter bald eagles will arrive.

In short, the area is an ornithological paradise, and I’m thrilled that for at least part of the year such an abundant variety of feathered creatures call it home.