When Do They Call an Animal Extinct?They used to wait 50 years.

Coelacanths of the bird worldA paper published in Science magazine on Thursday reported multiple sightings since 2004 of the ivory-billed woodpecker, a large bird that was thought to be extinct by most ornithologists. Who declared the ivory-billed woodpecker extinct in the first place?

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No one ever did, officially. The U.S. Fish and Wildlife Service and the World Conservation Union—which keep track of endangered and extinct species in America and around the world—said the birds were critically endangered, but neither labeled them with the other E-word. Even so, many in the scientific community believed that the woodpecker would never be seen again. Experts had never been able to confirm repeated sightings by amateur bird-watchers over the years, and many of them considered the bird at least unofficially extinct.

For the scientific community to believe that the ivory-billed woodpecker was still around, several credible ornithologists had to see the bird with their own eyes. An initial sighting last year by Gene Sparling of Hot Springs, Ark., was not sufficient until it had been corroborated by an official research team led by Cornell University. The team collected video images of the woodpecker and recorded its distinctive "double-raps." But even without documentary evidence, that fact that a group of "experts" all said they'd seen the bird would have established its existence.

It's much harder to prove that an animal is extinct. The World Conservation Union used to operate under the 50-year rule, which held that an animal could be declared extinct only if it had not been seen in more than 50 years. In the 1990s, though, the rules were tightened and clarified: Today, the World Conservation Union will label a species extinct only if "there is no reasonable doubt that the last individual has died." In general, scientists must now show that repeated efforts to survey a species' known habitat failed to turn up any individual sightings or evidence of its continued survival.

With this procedure in place, certain species are more likely to be declared extinct than others. A large mammal that lives in open grasslands will be easier to survey than a small animal that lives underground. The swamp forest habitat of the ivory-billed woodpecker made it especially difficult to track and may account for the paucity of sightings—and for the World Conservation Union's reluctance to call the woodpecker extinct.

A significant loss of an animal's known habitat can also be used as evidence for extinction, but only when scientists can show that the animal would not be able to survive in another environment. Locals provide another source of evidence: A USFWS-funded study of the Sampson's pearlymussel from the early 1980s used accounts from commercial clammers and the unsuccessful offer of a reward for specimens to conclude that the mussel was extinct.

The Fish and Wildlife Service (which is required by law to survey the status of any animal listed under the Endangered Species Act at least once every five years) will declare an animal extinct only after a lengthy review process involving three independent experts and a period of public comment. Only a handful of endangered species, such as the Tecopa pupfish, the longjaw cisco, and the dusky seaside sparrow, have ever been taken off the list because they went extinct.

Bonus Explainer: Does a species ever re-emerge from official extinction? It happens. The Fernandina rice rat and the Vietnamese warty pig, for example, were both declared extinct by the World Conservation Union in 1996. Evidence of living rice rats and the discovery of a fresh warty pig skull over the next few years led to a retraction. And one of the most famous species rediscoveries took place in 1938, with the identification of the coelacanth—a giant, prehistoric fish presumed to have been extinct for 65 million years.

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Daniel Engber is a senior editor at Slate. He can be reached at .

Print of ivory-billed woodpeckers by Robert Havell/Academy of Natural Sciences of Philadelphia/Corbis.

Correction, March 9, 2006: The credit in this piece originally termed the image of the ivory-billed woodpecker a painting by Robert Havell. It is a print done by Robert Havell after a painting by John James Audubon.

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