Tests conducted in a British laboratory have confirmed that six swans found dead in a Croatian nature park had been infected with H5N1.
The birds had already tested positive for the H5 subtype of bird flu last Friday and the further tests were carried out at the laboratory in Weybridge.
After the six dead swans were found, experts later found 13 more dead swans that were believed to have belonged to the same flock in a nearby fishpond.
Two of those 13 also tested positive for the H5 subtype, but their samples have yet to be examined to determine whether it was the H5N1 strain.
Croatia is a major migratory route for birds, and about 1,500 migratory swans arrived in eastern Croatia a few days ago.
British authorities also said today that a second parrot held in quarantine may have had the H5N1 virus and that 32 others that died in captivity were now being tested.
Authorities said on Sunday avian influenza was found in a parrot that died in quarantine east of London. They said its tissue sample had been pooled with that from another bird in the testing process, making it possible, but not certain, that a second bird had also been infected.
The Ministry of Environment, Food and Rural Affairs said the birds were among two batches from Surinam in south America and Taiwan that were kept together.
Experts are working on the theory that the virus came from Taiwan and was transferred during quarantine.