New DNA tests have pinned the blame firmly on the ducks imported by the Slimbridgebased Wildfowl & Wetlands Trust in 1948. Some offspring later escaped, leading to a huge growth in numbers across Europe.

Researchers say the new DNA findings support the continuation of controversial plans to reduce duck numbers drastically. There had been claims that many of the birds arrived here naturally, like other US ducks, and so should be spared the cull designed to bring down the numbers of the colourful duck and save the white-headed duck.

But the new genetic evidence shows that the birds all have specific genetic material linking them to the Slimbridge seven.

The researchers say that control measures have generated considerable controversy in the UK, and that the ruddy duck has been welcomed as an interesting addition by many birdwatchers.

"It is not surprising that opponents to control measures against ruddy ducks have argued that the growing population in Europe has been partly established by vagrants arriving through natural dispersal, '' say the researchers from Boston University.

To test whether the natural arrival of ruddy ducks from North America was involved, the team compared the genetic diversity of ruddy duck populations from both continents, including present-day birds at the Wildfowl & Wetlands Trust which are thought to be direct descendants of the Slimbridge seven. The results show that there is considerably lower genetic diversity in the ruddy ducks from Europe compared to those from North America.

"Our results suggest that a small number of founders from North America, bearing only a fraction of the total genetic variability of the species, gave rise to a new population in Europe, '' say the researchers.

"Seven ruddy ducks, four males and three females, were brought to the WWT at Slimbridge in 1948 and approximately 90 descendants of these birds escaped between 1953 and 1973. These appear to be the founders of the present feral population across Europe and North Africa, as well as the birds still held in captivity in Europe.'' They add: "Because the ruddy duck and the white-headed duck have been separate species for perhaps one to two million years, and because we can now rule out the natural arrival of ruddy ducks from North America to Europe, ongoing efforts to eliminate introduced ruddy ducks from European countries should be continued in order to conserve the white-headed duck.''