The swan found to be infected with a strain of avian flu may have caught it from another bird.
Scotland’s chief veterinary Charles Milne has said the bird found at Cellardyke, Fife is thought to be a mute swan.
This raises questions about the source of the infection because mute swans are generally not migratory.
A spokesman for the Royal Society for the Protection of Birds said it was possible that some birds have migrated across the North Sea from Europe because of cold weather.
But he said there was also a strong possibility that the swan could have caught the virus from another migratory bird.
The alternative – but this is much less likely – is that the swan picked up bird flu from infected domestic poultry or poultry products.