By Molly Daly

PHILADELPHIA (CBS) — A rare bird was spotted at the John Heinz National Wildlife Refuge on Sunday during an annual daylong birding event called the Big Sit. Birders flocked from neighboring states to see the avian visitor.

It was a hot, gray, muggy day, and Big Sit organizer Debbie Beer says avian traffic was slow.

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“There were only twelve birds after 9:30 in the morning,”  she said.

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But species number 65 was worth the wait: a White-winged Tern. Birding tour guide Adrian Binns spotted the bird, normally found in Eastern Europe and Asia.

“I’ve actually seen it on the wintering grounds when I lead tours to Africa,” Binns said.

Beer and the others quickly took to social media.

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(credit: Adrian Binn, WildsideNatureTours.com)

“We knew how much other people wanted to see it,” she explained, “so that was really our goal, to spread the word as fast as possible, because birds can disappear in a minute.”

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But the bird cooperated, sticking around till dark. Beer says seeing the rarity felt like winning the lottery.

“It’s like looking for buried treasure,” she said. “And when it happens, that one in a million, it is amazing.”

The White-winged Tern has only been spotted in the state once before — two months ago, in Tioga County. Binns says it’s possible the bird seen Sunday could be the same one, now molting out of its black, white, and gray breeding plumage.

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The White-winged Tern is the 310th bird species observed at the Heinz Refuge.