A dead swan found in Fife which tested positive for the deadly H5N1 strain of bird flu was a whooper swan, DNA tests by government scientists have found.
The breed originates from outside the UK but it was unclear whether the dead bird picked up the disease abroad.
A number of migratory whooper swans have recently been checked in the UK and all results have been negative.
Some experts have suggested the swan could have died in another country and been washed up on the coast.
No other birds have tested positive for H5N1 since the discovery in Cellardyke on 29 March.
'One-off'
Whitehall sources told the BBC a "working hypothesis" is the bird could have died in another country and been washed up on the Scottish coast.
Early test results suggested the swan found in Scotland had an almost identical virus to birds found in Germany, which saw an outbreak of H5N1 in Ruegen last month.
The RSPB is hopeful that this, combined with the lack of any further cases, means the Cellardyke swan could be a "one-off".
The society's spokesman Andre Farrar said: "The likeliest scenario - and this has to be in the realms of speculation - is that this bird may have set off on its journey northwards, got part of the way across the North Sea, felt grotty, and landed on or fell into the sea, died and was washed into Cellardyke."
The whooper swan is known to migrate from Iceland, Scandinavia and northern Russia to spend winters in the UK, the Low countries and the south Baltic Sea.
Dr Martin Fowlie from the British Trust for Ornithology backed the theory but said another possibility was that the swan contracted the virus from another bird in the UK before it intended to move north.
Microbiologist Professor Hugh Pennington said confirmation of the swan's identity made it more likely that the case was an isolated one.
"This raises the likelihood that it had no contact with any native birds and that this case of H5N1 on our shores was a one-off," he added.
Earlier, professor Pennington told Radio 4's Today programme that if the swan caught the virus locally, birds would still have the disease but not been detected.
But he said bird flu would not be "a foot-and-mouth situation - the virus is not going to go on the rampage".
8,000 calls
The H5N1 virus cannot pass easily from one person to another and therefore currently does not pose a large-scale threat to humans.
Experts, however, fear the virus could gain this ability if it mutates. They say it could trigger a flu pandemic in its new form, potentially putting millions of human lives at risk.
BIRD FLU FACTFILE
Tests on birds found near Cellardyke are still being carried out, and a UK helpline has received 8,000 reports of dead birds.
A six-mile (10km) surveillance zone and 1.8 mile (3km) protection zone in place around Cellardyke will remain for at least 30 days from the day the swan was found.
A wild bird risk area has also been established which includes 175 registered poultry premises, containing 3.1 million birds, 260,000 of which are free-range.
Ministers, meanwhile, are considering bringing in targets to regulate the time between reporting a dead bird and tests being completed, according to the sources.
If you find a dead swan, goose or duck; or three or more dead wild or garden birds in the same place, you should call the Defra helpline on 08459 335577.
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