The disappearance of the biggest breeding colony of rare little terns from a Norfolk beach is baffling bird experts.
The little tern is one of Britain's rarest sea birds and an endangered species
The colony on Great Yarmouth's North Denes beach is traditionally the Britain's biggest nesting ground.
It usually attracts 200 breeding pairs - about 8% of the country's little terns, an endangered species and one of Britain's rarest sea birds.
Two weeks ago when the first terns arrived a fence was put up around the site to protect the birds.
Now the colony has taken off, leaving Royal Society for the Protection of Birds officers puzzled.
Last year at the height of the breeding season vandals ripped out electric fences and wrecked two-thirds of the 90 nests in an overnight attack, scattering the breeding birds.
That incident has sparked speculation that the attack has deterred the birds from using the site for breeding this year.
RSPB warden Brian Woodden said it was very difficult to explain what had happened.
"Clearly there are no birds here now. There may be many reasons for that.
"Visitors often ask if it is due to the vandalism last year- and the answer to that is we don't know.
"I wish I knew the answer but I don't."
He said one factor could have been the helicopter search of beaches for missing seven-year-old Daniel Entwistle which may have disturbed the birds.
"They appear to have chosen to go elsewhere," he said.
Another breeding colony further up the coast at Winterton is much bigger than it has been in previous years, he added.