DUCK numbers are rising and hunter numbers are falling in the United States but that's ironically bad news for the birds.

A new US study has found that with fewer Americans purchasing duck-hunting licenses, the government receives less revenue to spend on duck conservation programs.

Dr Mark Vrtiska, from the Nebraska Game and Parks Commission, said America's waterfowl population was reaching record levels that meant "these should be glory days for duck hunting".

But his study, published in Wildlife Society Bulletin, found sales of "duck stamps" – the Federal Hunting License – had fallen from more than 2.1 million annually in the 1970s to as low as 1.3 million in 2008.

Dr Vrtiska estimated the decline cost the government $126 million between 1995 and 2008, which could have funded 42,495ha of protected wetlands.

"You may think the fall in hunters would be good news for ducks, but ironically it is leading to less money for the conservation of their habitat," said Dr Vrtiska.

"Federal funding for conservation is dependent on the revenue raised by selling the duck stamps, a unique dynamic for wildlife managers in the United States. Up to 98 per cent of money raised by the duck stamps is used to purchase or lease habitat within the National Wildlife Refuge system."

The study forecast that hunter numbers would continue to fall due to a "cultural shift and changing attitudes" towards what has been an American tradition.

"The resulting fall in funding is impacting all those involved in habitat conservation which is only made more important by the dramatic rise in duck numbers," Dr Vritska said.