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The population of North America's endangered whooping crane is on the rise.

The endangered whooping crane population is making a comeback but it still has a long way to go before it's out of danger, biologists say.

For the first time since the 1800s, there are more than 500 whooping cranes in North America, biologist Tom Stehn told CBC News in a recent interview.

The species hit an all-time low in the early 1940s when only 15 birds remained.

The cranes nest inWood Buffalo National Park, which straddles the N.W.T.-Alberta border, and then migrate to Texas where they spend the winter.

There are 237 cranes in the flock that summers in Canada and another 280 that live in captivity or as part of a flock reintroduced in theeastern U.S., says Stehn, wholooks after the population in the U.S.

"I think it's always going to be a species that's going to take a lot of watching and a lot of effort to keep those numbers increasing and keep a population," Stehn said.

Biologist John French, who works at Maryland'sPatuxent Wildlife Research Center, says they breedsome birds in captivityjust in case something happens to wipe out the Wood Buffalo flock.

"The danger or the risk of that is that some environmental event or catastrophe will befall them and, localized in one area, could wipe out all the wild birds," he said.

Although the population is on the rise, the biologists say there will have to be more than 5,000 birds before the species can be considered fully recovered.

They expect that to take at least another 30 years.