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The Connecticut River is home to a tree swallow phenomenon

People watch from boats as tree Swallows gather over Goose Island in Old Lyme, Conn., as they make their way to their roosting spot on the island on Sunday, August. 31, 2025.
Raquel C. Zaldívar
/
New England News Collaborative
People watch from boats as tree Swallows gather over Goose Island in Old Lyme, Conn., as they make their way to their roosting spot on the island on Sunday, August. 31, 2025.

It’s September and that means everyday tree swallows are putting on their spectacular autumnal avian display near the mouth of the Connecticut River.

Hundreds of thousands of the birds come from all over New England and the Canadian Maritimes to roost in the marshes just south of Essex before they head south for the winter.

These tiny native birds, with an iridescent dark blue top and white underbelly, weigh about 17 grams. Matthew Bell from the Connecticut Audubon Society said that’s about the weight of three quarters.

Len Calbo, center, looks through binoculars to Goose Island in Old Lyme, Conn., as tree swallows make their way to their roosting spot on the island on Sunday, August. 31, 2025.
Raquel C. Zaldívar
/
New England News Collaborative
Len Calbo, center, looks through binoculars to Goose Island in Old Lyme, Conn., as tree swallows make their way to their roosting spot on the island on Sunday, August. 31, 2025.

Bell says it’s a good idea to use binoculars to really appreciate the show which “looks like TV static or spilled cracked pepper all over your counter.”

Once they gather by the hundreds of thousands over the marshes, the tree swallows then quickly descend to pick an individual reed where each bird will spend the night.

"Sometimes it's a plummet where every bird disappears in 30 seconds, the sky just empties out completely, other nights, it looks like a dramatic tornado,” Bell said.

He says scientists can’t say exactly why the tree swallows pick this spot every year to roost right before their migration south.

"We assume that they pick it in part, strength in numbers. Most of their predators probably can't swim out here to get them, raccoons, skunks, stuff like that,” he said.

Pat Decarli, right, looking through binoculars, and Dennis Decarli, to her left, look out to Goose Island in Old Lyme, Conn., as tree swallows make their way to their roosting spot on the island on Sunday, August. 31, 2025.
Raquel C. Zaldívar
/
New England News Collaborative
Pat Decarli, right, looking through binoculars, and Dennis Decarli, to her left, look out to Goose Island in Old Lyme, Conn., as tree swallows make their way to their roosting spot on the island on Sunday, August. 31, 2025.

Boaters and kayakers gather in the river daily right before sunset to watch the birds arrive. The Connecticut Audubon Society and the Connecticut River Museum offer boat tours to experience the event but tickets often sell out quickly.

The bird numbers peak in mid-September and disappear in late October when the swallows head south for the winter.

Jennifer Ahrens is a producer for Morning Edition. She spent 20+ years producing TV shows for CNN and ESPN. She joined Connecticut Public Media because it lets her report on her two passions, nature and animals.