The swan found dead from bird flu in a Scottish village may have been infected by another bird in the country.
Scotland's chief medical officer said it was thought the bird was a native mute swan, which does not migrate.
Experts believe it is likely the domestic swan contracted the virus by mixing with another infected bird.
Test results have confirmed that the swan died from the H5N1 strain of the virus, the first case seen in the UK. The H5N1 strain can be fatal to humans.
On Wednesday, an initial 1.8 mile (3km) protection zone was set up around the village of Cellardyke, where the swan was found and, on Thursday afternoon, a surveillance zone was extended to 965 sq miles (2,500 sq km).
Patrols have been stepped up by police around the zones, with vehicles being stopped and checked, along with poultry farms.
Chief Veterinary Officer Charles Milne confirmed that the swan's body had been partially eaten but added there was no evidence to suggest this had been done by a domestic animal such as a pet cat or dog.
He advised the public to report any dead birds they spotted and stressed that they should not go near them.
"They should not pick up or approach dead birds but should report it," he said.
'Lack of urgency'
Ian Thomson, bird expert from the East Lothian Ranger Service, told BBC Scotland that another bird was likely to have carried the virus to UK shores.
He said: "Mute swans are residents in Britain therefore it is very unlikely that this bird brought the avian flu here.
"It would have had to have transferred from another bird, that type of bird is open to debate."
The exact spot where the swan was found in Cellardyke harbour
The swan was discovered eight days ago by a local woman who was walking near-by.
She told BBC Scotland that she was concerned by the lack of urgency shown by the authorities initially.
Locals reportedly spotted the dead bird floating around in the water for several days, with one woman saying she had seen seagulls pecking at it.
Tina Briscoe, 68, who works at St Andrew's University, found the dead swan washed on the shore at the pier.
She said: "When I found the dead bird, I called the police, who told me to phone the RSPB, who in turn told me to phone Defra. It was over 12 hours later when someone came to collect the bird.
"It was left on the harbour all night which really worried me, I would have thought immediate action was needed. My worry was that if other birds or dogs or cats had picked at it, then they might spread something."
However Mr Milne defended the length of time it took to remove the bird.
He said: "The swan was reported after 5pm on 29 March and it was collected the next day between working hours.
"The procedures were followed fully and the timeline could not have been tighter."
Leading microbiologist Professor Hugh Pennington urged the public not to worry about any personal health risk.
He told BBC Scotland that the number of people who have died from bird flu had been very small compared to the millions of birds who have perished.
Prof Pennington said: "It is something we have been expecting as this virus has been very good at getting around the world.
"It is worth noting that the number of human cases around the world has been very small compared to the amount of virus that has been doing the rounds both in wild birds and commercial.
"It's not a virus that's a threat to the British population - you have to have extremely close contact with the birds to be at risk.
"The only people in Britain who are at risk from this virus - and that's if it got a hold of the poultry industry - would be people looking after those poultry, people going in hen houses and the vets making any diagnosis."
Rural Affairs Minister Ross Finnie told BBC Scotland that there was no need for panic.
"This is a very serious situation but it is a situation where we need to look at the evidence, deal with it rationally and put all of the planning measures that we have been rehearsing for some time into place, but without inducing a general state of panic."
The H5N1 virus does not at present pose a large-scale threat to humans, as it cannot pass easily from one person to another.
However, experts fear the virus could mutate to gain this ability, and in its new form trigger a flu pandemic, potentially putting millions of human lives at risk.