A wild crane chick has vanished from sight at the Slimbridge Wetland Centre reserve in Gloucestershire.
Wildlife experts hoped the chick would be the first wild crane to survive in western England for 400 years has disappeared.
Conservationists said as there were many risks to a young bird, they were assuming it had died.
Its parents Chris and Monty were hand reared as part of the Great Crane Project project to reintroduce cranes to western England, and experts said they were "gutted" the chick had died.
The parent birds were released on the Somerset Moors and Levels, where the birds were commonplace before disappearing in the 1600s as a result of hunting and loss of habitat.
The pair first nested at Wildfowl and Wetlands Trust (WWT) reserve at Slimbridge last year and managed to hatch one egg but the chick died in bad weather before leaving the nest.
This year they hatched two chicks, but one disappeared in May, a few weeks after hatching, and conservationists said it now seemed its sibling had also perished.
Nigel Jarrett, head of conservation breeding at WWT, said: "The risks to parent birds and their young are substantial; we do not know exactly what has caused this last crane chick to disappear but judging by the parents' behaviour that was witnessed, it was likely to be a predator.
"I am gutted; we all are. We had such high hopes for these magnificent parents and their baby.
"We could see the parents were feeding the chick throughout the day and brooding it at night. The chick was growing before our eyes."
He added: "The crane family has given a great deal of joy to our visitors and followers - many felt part of their amazing journey."
Tony Whitehead, speaking for the RSPB in the South West, said eggs in two crane nests in Somerset had also been predated, but that the cranes were likely to learn from the loss to nest in places that were harder for predators to reach.
"And as these are long lived birds, with a lifespan of up to 20 years, there is still plenty of time for them to raise successful broods and secure a self sustaining wild population," he said.
Cranes were lost from the UK as a breeding bird in the 16th century as a result of hunting and the drainage of large areas of wetlands.
The Great Crane Project, a partnership between WWT, the RSPB and Pensthorpe Conservation Trust, with major funding from Viridor Credits Environmental Company, aims to restore healthy populations of wild cranes throughout the UK.
Norfolk-based Pensthorpe Conservation Trust has been working with Eurasian cranes for more than a decade and has a small population of wild cranes in its 500-acre reserve in the Wensum Valley.