All 18 young whooping cranes led south from Wisconsin by ultralight aircraft last fall were killed in storms that hit Florida, dealing a devastating blow to a project to create a second migratory flock of the endangered birds in North American, a spokesman said.

The cranes were being kept in an enclosure at the Chassahowitzka National Wildlife Refuge near Crystal River, Fla., when the storms moved in and intensified Thursday night, said Joe Duff, senior pilot and co-founder of Operation Migration, a nonprofit organization coordinating the project.

"The birds were checked in late afternoon the day before, and they were fine," Duff said.

But the area of the enclosure was unreachable by workers at night, and all the birds were found dead Friday, he said.

Duff, contacted by telephone, speculated that an unprecedented storm surge drew the tide in and overwhelmed the birds. The cause of the deaths wasn't immediately known, but Duff said it may have been drowning.

The thunderstorms and at least one tornado that hit central Florida caused widespread damage and killed at least 19 people.

For the crane project, Duff described it as an "unavoidable disaster" that ironically followed a milestone.

For the first time in six years, an entire group of young birds reared at the Necedah National Wildlife Refuge in central Wisconsin made it to the Florida refuge without the loss of a single crane.

The project's previous losses all involved individual birds killed by predators or fatally injured in accidents.

"It's a fluke. It's an unforeseen thing," Duff said. "So many birds and they were such good birds. It was our hardest migration and our most difficult one to fund."

The various groups and agencies working on the project had seen the size of the flock grow to 81 birds with the latest arrivals, but the loss of the young cranes drops the total back to 63, and there could have been other losses as well.

Duff said there was no way of knowing whether other whooping cranes that winter in the area had survived the storm.

Operation Migration is part of the Whooping Crane Eastern Partnership, which, like Duff, said the work would continue.

"Our thoughts also go out to those in central Florida who suffered personal losses as a result of these storms," said John Christian, co-chair of the partnership.

By coincidence, members of the whooping crane recovery team were meeting in Louisiana, going over the past year's progress and setting goals for this year, when they learned what had happened, Duff said.

"We were all excited we'd reached such a great milestone," he said. "We led all 18 to Florida and had not lost one."

After the initial shock, "it just reinforced the support and determination to get this done," he said.

For the past six years, whooping cranes hatched in captivity have been raised at the Necedah refuge by workers who wear crane-like costumes to keep the birds wary of humans.

Ultralight aircraft are used to teach new groups of young cranes the migration route to Florida. From then on, the birds migrate north in the spring and south in the fall on their own.

In another milestone reached in 2006, a pair of the whooping cranes produced offspring in the wild. One of the two chicks survived and migrated with the parents to Florida.

The whooping crane, the tallest bird in North America, was near extinction in 1941, with only about 20 left.

The other wild whooping crane flock in North America has about 200 birds and migrates from Canada to the Texas Gulf Coast. A non-migratory flock in Florida has about 60 birds.