A Eurasian crane that was part of a group that was released in Somerset in September has gone missing.
The birds were hatched at Slimbridge Wetland Centre in Gloucestershire as part of the Great Crane Project, which aims to reintroduce cranes to the UK.
A Royal Society for the Protection of Birds spokesman said the female bird, named Howard, was last seen on Friday.
A member of the public from Highbridge said he thought he may have spotted it earlier.
David Ball said: "I went down to get into my car this morning, looked up and there was a bird on the house across the road.
"It was a tall bird with long legs, a long neck and a long beak - one I'm quite sure I've never seen before and I've been watching birds all my life."
He said he called the RSPB to report the sighting.
The Great Crane Project is a partnership between the Wildfowl and Wetlands Trust, the Royal Society for the Protection of Birds (RSPB) and Pensthorpe Conservation Trust.
A number of crane eggs were brought to the Wildfowl and Wetlands Trust at Slimbridge from the wild population in Germany.
They hatched in April and the 21 cranes were reared at the centre before they were transferred to Somerset.
An RSPB spokesman said they had started to explore the Somerset Levels and Moors.
He said the tall waterbird was carrying distinctive yellow blue and red identification rings.
Conservationists have been unable to trace it through its radio tracker, which means it could have flown out of range.
Earlier on Wednesday afternoon, an RSPB spokesman said the bird was still missing.
Project manager Damon Bridge appealed for anyone who saw it to call 01458 254414.
He said: "It's odd that it's not returning to the group in the evening as normal - but then this individual has always been a bit of a loner.
"Naturally everyone on the Great Crane project team is hoping the bird is found safe and well.
"Sadly though, if she is not found soon we may be left fearing for the worst."