It's either the best-kept secret or the latest bucket list item, or perhaps both, among local birdwatchers: the annual fall swarm of tree swallows along the southern Connecticut River.

While the river is synonymous with eagles and other raptors during the spring thaw, every fall the migratory aerial insectivores steal the show. Between now and early October, up to a half-million tree swallows collectively roost each evening on a phragmites-filled island in the southern Connecticut River. While the number of birds is astounding, what's spectacular are the evening rituals they go through.

For several years, Connecticut Audubon Society's EcoTravel, in Essex, and Captains Mark and Mindy Yuknat of the RiverQuest vessel have put on swallow spectacular cruises that allow humans to experience the frenzy. The three-hour cruises leave from Eagle Landing State Park in Haddam on Wednesday and Sunday evenings, timed to get down the river while the sun is setting and be in place at dusk to watch the swallows.

"It's a must-do," says Priscilla Wood, one of the Eco-Travel tour guides. "Every evening is different, but usually for 20 or 30 minutes, there's a mass migration to watch as the birds fly in and collect for the night."

Connecticut's Roger Tory Peterson, ornithologist and artist, wrote that, for sheer drama, the tornadoes of tree swallows eclipsed any other avian spectacle he had seen, according to Connecticut Audubon. The birds fly up to 35 miles an hour and are natural aerial acrobats as they grab a last drink from the river and snack before bed. Some nights, they put on an aerial show, a ballet of sorts. Other nights, it's a more intense funnel down into the marsh.

Connecticut Audubon avoids naming the island to keep away hoards of boaters or island invaders that might drive away the birds or have worse consequences on them. But it also doesn't want to preclude people from experiencing the spectacle or developing a greater appreciation for these minute birds, which weigh less than an ounce. The species summers in the Northern U.S. and Canada and winters in Florida and Mexico.

Populations of aerial insectivores have declined by 85 percent in Connecticut over the past 40 years, according to the Connecticut Audubon Society's most recent state of the birds report. This includes chimney swifts, barn and tree swallows and night jars; flycatchers have also decreased.

Despite these alarming declines, the numbers of tree swallows stopping off on the Connecticut River each fall seems to be increasing, or are at least being observed more by humans. There's also a bit of mystery as to who these birds are and where they are on an annual migration.

"One thing we haven't been able to determine is if every single night is a brand new swarm of tree swallows, and they are moving on, or if these are the neighborhood birds," she says. "Every year, the swarms have grown, though."

Wood admits that sometimes it gets a bit Alfred Hitchcock-ish when the birds come in. Tree swallows are much more handsome than they get credit for, she says, because we usually see them in fast flight. The males have an iridescent deep blue back and clean white front.

"There's safety in numbers, so the swarming is in part for survival," she says. The swallows are often spooked by hawks or other predator birds. It's called a "fly-out" when tens of thousands of swallows head down to the marsh and then shoot back up to the sky.

For more on the cruises and the state of Connecticut's flying insect-eaters, go to ctaudubon.org or call (860) 767-0660. Advance reservations are recommended, and note the earlier starting times later in September and October. Pack a picnic dinner and beverage of choice, bring binoculars or borrow a pair from the boat. Tickets are $40, children 10 years and older are welcome. The boat can hold up to 45 people. See ctriverquest.com for more public and charter river cruises.

Suzanne Thompson hosts a weekly CT Outdoors radio show on WLIS 1420 AM & WMRD 1150 AM at 12:30-1 p.m., or listen to archived show in the On Demand section of wliswmrd.net. Reach her at suzanne.s.thompson@sbcglobal.net.