An outbreak of bird flu has been confirmed at Abbotsbury Swannery in Dorset.
Nine dead swans have tested positive for the H5N8 strain of the disease.
The swannery said about 80 birds had died so far this winter when it would normally expect about 30 to 40. The cause of most of the swans' deaths is not known.
The virus, which is different to the H5N1 strain that affected the colony in 2008, is considered low risk to humans.
Tourism general manager John Houston said: "It is spread through bird droppings. It has been in Europe on a wide scale for the last few months so we have expected it because you can't stop migratory birds crossing from the continent.
"This time of the year is a very difficult time of year for wild birds anyway and we would normally expect to see 30 to 40 [swan deaths] over the winter period but, as of Saturday, we have had 80 so far which is above the norm and is distressing for the staff who are very close to these wonderful birds."
There are about 600 birds at the site, the world's only managed colony of nesting mute swans.
The attraction, which is closed for winter, said it routinely sent any bird that died to the Department for Environment, Food & Rural Affairs (Defra) for testing.
In December, poultry keepers were ordered keep their birds inside to protect them from the strain.