For 15 years, whooping crane chicks have been hand-raised by scientists wearing white whooping crane costumes in Maryland, shipped to Wisconsin and taught to migrate by other white-robed people, before they are led south to Florida by costumed volunteer pilots flying ultralight aircraft.

It was one of the most quirky, beloved and interventionist American conservation efforts, meant to build a migratory population of endangered whooping cranes in eastern North America without making them used to humans. This month, the final graduating class of six young birds was released — yes, by people in white costumes — at St. Marks National Wildlife Refuge in northern Florida.

The program is ending, largely because whooping cranes have turned out to be lousy parents.

“They can establish pairs, they know how to mate, they can copulate, and they know how to lay eggs,” said Peter J. Fasbender, a U.S. Fish and Wildlife Service field supervisor for Minnesota and Wisconsin. “They’re just incapable of parenting.”

In the past 11 years, the eastern population has grown to about 100 birds, but it has managed only to fledge 10 chicks. The culprit, government biologists think, is what made the program the fascination of legions of schoolchildren who followed it: the disguised people and the aircraft leading cranes. Though they never spoke to the birds and also directed them with crane puppets, Fish and Wildlife decided that humans were still too involved in teaching the cranes how to live. In January, the agency announced that this year’s would be the last ultralight-led migration.

The cranes, five feet tall and ornery, did not always struggle. Thousands probably lived in North America before Europeans arrived. But hunting and building caused their numbers to dwindle to 16 by 1941, according to the Fish and Wildlife Service. Now, there are about 500 cranes in the wild.

A migrating group that winters in Texas seems to have learned how to parent, Fasbender said.

But the eastern population has shown little inclination to self-sustain. The birds wander from their eggs, sometimes never to return. The reason may be partially because of the habitat, experts say — poor food supply or too many predators. But the decision to rethink the program stems mostly from a hunch that the cranes simply aren’t catching on because they’re not learning how to be parents from other cranes.

“After 15 years, it’s the same show,” Fasbender said. “They just don’t get it.”

It is time, he said, for a paradigm shift that jettisons the people from the process. What that means is now being hashed out. But one part of the plan, Fasbender said, will involve chicks first being reared by captive adult birds at Patuxent Wildlife Research Center in Maryland, then released to wild adult pairs in Wisconsin that have displayed some proclivity for parenting.

The hope is that the chicks might learn to become functioning adults and that the adults might learn to become better moms and dads.

“We’re looking at it from A to Z, eliminating as much of the human intervention that we can,” Fasbender said. “We’re just trying to have birds raising birds.”

The federal government’s decision is a disappointment for Operation Migration, the organization that since 2001 has been leading the whooping cranes from Wisconsin to Florida. It was a journey that typically took about three months, because both the cranes and the aircraft won’t fly in some kinds of weather.

On stopovers, the caravan was usually greeted by a crowd of ardent whooping crane fans, curious onlookers and excited schoolchildren, many of whom followed the migration’s path online.

“It’s one of those projects that appeals to birders, certainly, and naturalists, and adventurers, and aviators, and people who dream about running away with the circus, because that’s what it’s like,” said Joe Duff, Operation Migration’s co-founder, and one of its pilots. “When  you take an ultralight aircraft and lead an ancient bird south, it creates a lot of attention.”

Thanks to funky El Niño weather, this year’s migration was the longest ever: It began in September and ended Feb. 6 “with a whimper,” Duff wrote on the organization’s blog. Instead of flying the cranes into the refuge, strong winds forced the group to box them up and drive them in.

“So that’s it,” he wrote. “Our career as avian aviators has ended.”

Duff said he’s sure the organization’s work was central to the eastern population’s growth, and he does not agree with the federal government’s decision to end the flights. But he said Operation Migration will remain involved.

“It’s about the birds, not about our own entertainment,” he said.

The group’s three aircraft, Duff said, will probably end up in museums.

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