Winter surveys carried out across Britain by the Wildfowl and Wetlands Trust have shown a steady fall for several years, with numbers dropping by 40 per cent in the past decade.
The mallard population is now at its lowest level since the present recording system began in the Sixties.
With their beautiful plumage - the males have glossy green heads bordered by a white neck ring - mallard are one of the most agreeable and familiar sights, not only of the countryside, but of parks and any city spaces where there is water.
They are difficult to count on a national basis, however, because their population is very diffuse, and tends to be spread out in small numbers on rivers and ponds across the country, rather than gathered together in vast flocks on estuaries and reservoirs like many other wildfowl species.
It is the annual Wetland Bird Survey, a partnership scheme of Britain's leading ornithological bodies run by the WWT, which has picked up the decline. The survey involves 3,000 volunteers counting birds at more than 3,000 wetland sites in the winter months.
Last winter, according to its report published today, the survey found that the peak count for mallard was 140,213 in November, compared with 192,100 in the winter of 1987-88. In between there has been a continuous fall.
"We're as confident as we can be ... that this is a real decline," said the WWT's Peter Cranswick, who runs the survey. "It's tricky with so many mallard on small sites like rivers, but over the last 10 years the numbers have gone down every year, and have now dropped to the lowest levelsince the mid-Sixties."
No-one knows why they have declined so sharply. It even is possible the problem is not located in Britain. During the winter mallard fly in from as far away as Iceland and Russia and British birds also migrate abroad.
"To be honest we don't really know," said Chris Harbard of the Royal Society for the Protection of Birds. "Perhaps our wintering birds prefer it in Holland. Or perhaps they really aren't breeding as well. But a 40 per cent decline in 10 years is certainly very worrying."Reuse content