Milwaukee ? All 18 endangered young whooping cranes that were led south from Wisconsin last fall as part of a project to create a second migratory flock of the birds were killed in storms in Florida, a spokesman said.

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

The area of the enclosure was unreachable by workers at night, and all the birds were found dead, Duff said.

“It’s very traumatic to the whole team who put so much time and effort into these birds,” he said Saturday.

He speculated that a strong storm surge drew the tide in and overwhelmed the birds, or they were electrocuted from lightening strikes reported in the area. The official cause of the deaths was not immediately known.

The thunderstorms and at least one tornado that hit central Florida caused widespread damage and killed at least 20 people.

For the past six years, whooping cranes hatched in captivity have been raised at the Necedah National Wildlife Refuge in central Wisconsin 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. Then the birds migrate north in the spring and south in the fall on their own.

Duff described the loss as an “unavoidable disaster” for the whooping cranes project. Ironically, for the first time in six years, an entire group of young birds reared at the Necedah refuge had made it to the Florida refuge without the loss of a single crane.

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 may have been additional losses.

Operation Migration is part of the Whooping Crane Eastern Partnership. Partnership officials and Duff said the work would continue. Chicks are expected to hatch in two months, he said.

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 nonmigratory flock in Florida has about 60 birds.