By Rebecca Morelle
BBC News science reporter
Bird migration experts say it is still unclear how the H5N1 virus arrived in the UK.
Birds have started to migrate back to their breeding grounds
Tests have confirmed that a swan found in the harbour of the village of Cellardyke in Fife, Scotland, carried the deadly strain of the virus and contingency plans to halt future outbreaks are underway.
But experts are looking into how the bird caught the disease, and to see whether it came in recently from another country or if the virus has already been in the UK for some time.
The Scottish Executive said it believes the bird to be a mute swan, and have taken the precaution of a implementing a 2,500 sq km (960 sq mile) "wild bird risk area" around Cellardyke in case other birds are infected.
Hard weather movement
According to bird migration experts, the bird may not be native, and is a mute swan from the Baltic region or Black Sea that journeyed to the UK to escape a cold snap.
"This swan could have been part of a hard weather movement," Grahame Madge, from the RSPB, told the BBC News website.
"This is where you get hard weather in the Balkans or the Black Sea and water freezes over, meaning birds that need access to fresh water need to move - sometimes a few miles, sometimes 100 miles."
Scotland has implemented a "wild bird risk area"
Mr Madge believed that a hard weather movement could have been responsible for the spread of H5N1 to France and Germany confirmed in February.
"The possibility is that this swan could have been part of the tail end of this movement, and might not have been a British-born bird at all."
Martin Fowlie from the British Trust for Ornithology (BTO) also suggested that this might be the case, and also raised the possibility that the swan may not have even died in the UK.
"It might have been a mute swan from Eastern Europe that had flown west and might have died in the North Sea and had been washed in," he said.
A domestic swan?
However, there is the possibility that the mute swan is from the UK.
Mute swans that are native to the UK do not migrate, and in the part of Scotland where this swan was found they tend to remain in roughly the same spot.
"Mute swans in Fife occur at very low densities and all the information we have on mute swans is that they do not move more than 2-3km in that area," said Mr Fowlie.
If the swan is a native one, then it must have caught the virus from another bird, he said.
One explanation, according to Mr Madge, is that a bird migrating out from an infected country in central Europe may have passed though the area before continuing its journey further north to its breeding ground.
There is also a lot of movement of birds out of the UK at the moment as they back to their breeding grounds.
Experts are unsure whether the bird was native to the area
"We are getting birds that have spent the winter here now pouring out of Britain to their more Arctic winter grounds, heading north to Greenland, across to Iceland, Scandinavia, Arctic Russia and eastern and central Europe. These movements include millions of birds," said Mr Madge.
Some of these birds travel over Scotland, and if any carried the virus they may have passed it on to the swan in Fife.
However this would mean the virus is already in the UK, and regular tests of birds have been carried out throughout the country and no other cases of H5N1 have been seen.
But as swans, geese and ducks leave the UK, other birds are also coming in from sub-Saharan Africa, northern Africa and southern Spain.
"There are not many wildfowl associated with these birds - they are mainly small songbirds," explained Professor Graham Martin, from the Centre for Ornithology in Birmingham.
"There is just one species that comes in to breed, the gargeney, but that is very rare, you only get a few each year. They tend to be fresh water rather than coastal, so it seems very unlikely that a gargeney has been near this swan."
Martin Fowlie also said it was important not to put all of the emphasis on migratory birds.
"The initial spread for South East Asia up to China and Russia can probably be assigned to poultry. It's a long trade route, and at the time it went against the direction the birds migrated," he explained.
He also said that Nigerian and Turkish outbreaks were probably because of domestic poultry not migratory birds, although this was probably not the case for the spread across Europe.
Professor Martin agreed: "There are huge movements all the time right across Europe of poultry that we know very little about. People bring poultry backwards and forwards all the time, and its not logged."
"We know more about wild bird movement than we do about domestic bird movement."