A disease that is killing greenfinches and chaffinches in the UK has now spread to Europe, scientists report.
A paper in the journal Ecohealth confirms that the disease has been found in Finland, Norway and Sweden and is at risk of moving further afield.
The disease, called trichomonosis, is caused by a parasite and was first seen in finches in the UK in 2005.
Since then, the country's greenfinches have declined by 35% and chaffinch populations have fallen by 7%.
Becki Lawson, a wildlife veterinarian at the Zoological Society for London (ZSL) and lead author of the paper, said: "Trichomonosis has emerged as a very serious threat to these birds, so it is very important that vets and ornithologists collaborate to determine whether we might see further spread and to monitor the impact of the parasite on wild bird populations across Europe."
The parasite that causes the disease, Trichomonas gallinae, has long been known in pigeons and doves, and scientists believe it somehow spread from these birds into finch populations.
The study charts how the disease then moved from central and western counties in England and Wales towards eastern England in 2007, and then into Finland, Norway and Sweden (Fennoscandia) in 2008, as well as spreading further around the UK.
Molecular analysis has revealed that the same strain of the parasite was present in UK and European finches, and researchers now believe that migrating chaffinches were responsible for the spread.
Dr Lawson explained: "It looks like chaffinches left the east of England in 2008, and that spring they went to the breeding ground in Fennoscandia and took the parasite with them, which is where the outbreak occurred."
While greenfinches and chaffinches have been most badly hit, the disease has also been diagnosed in a number of other bird species, including the house sparrow and yellowhammer, both of which are already endangered.
The research team says it is now key to try to understand trichomonosis and to monitor its spread.
Mike Toms, head of garden ecology from the British Trust for Ornithology and an author of the paper, explains: "We are concerned whenever you see something like this: a sudden drop in a population.
"But the fact we have been able to pick this one up and understand why it has happened is a positive thing."
Mr Toms said: "We are looking out for birds that are fluffed up, lethargic and sitting around the bird feeders and not really going anywhere, and maybe looking a bit wet around the bill.
"Those are the typical signs of disease and if birds are coming into people's gardens, and people are seeing a number of birds with those symptoms, you should report them."
Outbreaks tend to be most severe and most frequent from August to October, he added.
While it is difficult to treat wild birds suffering from the disease, researchers say there are things that people can do to help limit the spread such as regularly cleaning all feeders, bird baths and feeding surfaces.