A swan with the H5N1 bird flu strain probably came from the Bay of Montrose about 30 miles north of where it was found dead, scientists in Scotland say.
AREA ON ALERT
Poultry owners within wild bird risk area must keep birds indoors or, if not possible, ensure they are kept away from wild birds
Bird transport within 6 mile (10km) surveillance zone will be curbed
Poultry within 1.8 mile (3km) protection zone must be kept indoors and will be tested
Experts are still testing other birds found near Cellardyke harbour in Fife - but so far all have proved negative.
Meanwhile, a UK helpline has had at least 5,500 reports of dead birds.
Government chief scientific adviser Sir David King has said there is a "very low" chance of the virus mutating to a form that spreads between humans.
Rural Affairs Minister Ross Finnie told BBC News the "preponderant" view of ornithologists was the swan was part of the native population found around the Bay of Montrose.
"But they did enter one big caveat... that population is added to by somewhere between 10 and 20 birds each year from mainland Europe," he added.
The swan's DNA was being tested to determine "whether it was absolutely part of the local population", Mr Finnie told BBC News.
"Although it is proving very difficult because of the highly decomposed nature of the swan."
The H5N1 virus cannot pass easily from one person to another and therefore currently does not pose a large-scale threat to humans.
Experts 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.
But Sir David King told ITV1's Jonathan Dimbleby programme suggestions of an inevitable global human pandemic were "totally misleading".
He added: "We have got a virus in the bird population that has gone on since 1996, and in Asia particularly there has been a lot of contact between human beings and the birds that have got that virus."
Despite that, a human virus had not developed, he said.
Sir David said bird flu was "absolutely not" present among poultry, and said he was "fairly optimistic" it was absent in wild birds.
He stressed that so far one dead bird had been washed ashore with H5N1, which may have come from a previously infected part of Europe.
"The one swan doesn't mean it has arrived here," he said.
The infected swan found in Fife had a "very similar" strain to one which infected more than 100 birds in Germany, tests showed.
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 of 965 square miles (2,500km) has also been established which includes 175 registered poultry premises, containing 3.1 million birds, 260,000 of which are free-range.
And Cobra, the government's crisis management committee is due to meet.
BIRD FLU FACTFILE
Bird flu viruses have 16 H subtypes and nine N subtypes.
Four types of the virus are known to infect humans - H5N1, H7N3, H7N7 and H9N2
Most lead to minor symptoms, apart from H5N1
H5N1 has caused more than 100 deaths in Cambodia, China, Indonesia, Thailand, Turkey and Vietnam
The World Health Organisation says not all H5 or H7 strains are severe, but their ability to mutate means their presence is "always a cause for concern"
A health spokesman said: "This is still a disease of birds, not humans."
These sentiments were echoed by John Oxford, a professor of virology at Queen Mary's School of Medicine, in London.
He told BBC Radio Five Live: "We're not expecting a human case from this swan - nor are we really expecting human cases from chickens."
He said the focus of attention in future would be more likely to be southeast Asia, not Fife.
But shadow health secretary Andrew Lansley said the government's pandemic plans left questions unanswered, such as whether schools should be closed in the first wave of an outbreak and whether the public should be advised to avoid public transport.
"The economic and human consequences of these decisions are immense," he said.
He said there should be an open public debate the issues prior to the onset of any pandemic.
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.