By John Ostapkovich

PHILADELPHIA (CBS) — Today, September 1st, 2014 marks 100 years since the passenger pigeon went extinct, something thought unthinkable until it actually happened.

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Passenger pigeons were once common as dirt. “In the 18th and 19th Century, the passenger pigeon was actually the most abundant bird species in all of North America,” said Ya-Wei Li of Defenders of Wildlife.

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Li says hunting and habitat loss cut into the birds’ numbers and when people finally began to notice the decline, a peculiarity of the passenger pigeon made breeding in captivity impossible.

“The passenger pigeon only initiates courtship and reproduces when they gather in large numbers, so if you have just a few passenger pigeons they don’t breed successfully,” Li said. “They couldn’t reproduce in zoos any more because they just didn’t have the numbers to initiate reproduction.”

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Li says the loss of this bird species is an object lesson in maintaining the Endangered Species Act, that extinction is forever.

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