A government-funded drive to wipe out the US interloper is not only expensive but doomed to failure, say wildlife activists

A controversial UK cull of ruddy ducks, a US native that has been compared to a "feathered lager lout" for its displays of thuggish and amorous behaviour, has cost the British taxpayer more than £740 for each dead bird.

Figures from the Department for Environment, Food and Rural Affairs (Defra) show that shoots of the chestnut-coloured bird have cost taxpayers £4.6m, yet only 6,200 have been killed.

The disclosure has sparked an outcry from ornithologists and animal activists who have protested since the cull began five years ago. They say that the bird, ­targeted because it had interbred with the threatened white-headed duck in Spain, should have been left alone. The cull is due to end in August.

Lee Evans, the founder of the British Birding Association, said that the cull should be abandoned. "It's appalling and pointless, a complete waste of taxpayers' money. What's the point of it all? Our ruddy ducks don't go to Spain, but the French ducks do, and the French are not culling their birds. These marksmen are getting away with murder," he said.

Sir Peter Scott, the respected conservationist, has been blamed for accidentally introducing Oxyura jamaicensis to Britain more than 50 years ago. After escaping from Scott's Gloucestershire sanctuary, the species soon gained an unfortunate reputation: its mating call sounds more like a belch, it boasts a penis half the length of its body and, after mating, it ignores its partner.

By the 1990s the population was estimated to be in the thousands and the bird had spread to Spain, where it had cross-bred with its white-headed cousin. This has resulted in hybrid youngsters that threaten the purity of the species and its very existence, according to some experts.

Birdwatchers say the plan to cull the birds is pointless because other European countries have not implemented a similar plan.

Hampshire-based estate manager John Levell claimed to have witnessed one shoot. He wrote on his website: "The Defra gunships turned up today in their attempt to obliterate the poor old ruddy duck. The cost of eight gunboats chasing half a dozen ruddys about must run well into the tens of thousands of pounds, and I can't imagine this government has money to burn in such a fashion."

At Pitsford Water, Northamptonshire, seven marksmen arrived in a four-by-four each towing a trailer and a boat, according to one birdwatcher.

The scheme has been run by the Food and Environment Research Agency (Fera), part of Defra. However, other European countries such as Holland and France have resisted a cull, with a resultant rise in the population.

A spokesman for the RSPB said it stood by its decision to support the cull. "We appreciate that members may feel great concern over birds being killed, and eradicating ruddy ducks from the UK is clearly not an action that anybody approaches with enthusiasm. However, we have long felt that it would be wrong to stand by, do nothing and allow the white-headed duck to become extinct. It is important that eradication from the UK is achieved as soon as possible."

In a statement a Defra spokesman said the ducks "threaten the survival of the globally endangered white-headed duck. Eradicating the ruddy duck in the UK is part of the European Life Project to conserve and protect the white-headed duck and is supported by conservation organisations such as the RSPB, BirdLife International and Wildfowl & Wetlands Trust. Extensive research has shown that culling ruddy ducks by shooting is the most humane way of eradicating them, and this is carried out by trained expert marksmen."