By Richard Black
BBC Environment Correspondent
Farmers can help birds such as skylarks by leaving stubble in their fields over the winter, a UK study suggests.
Intensive farming methods have altered the prospects for many birds
It found delaying ploughing until the spring increases the breeding success of several farmland bird species, some of which are classified as threatened.
The study was carried out by scientists at the British Trust for Ornithology (BTO), using data collected by volunteers over a 10-year period.
The BTO says increasing stubble land could halt the decline of some species.
The widespread adoption of modern intensive farming methods across Britain since the 1970s has altered prospects for birds such as the skylark and the yellowhammer.
Numbers of each have fallen by more than 50% since 1977; studies have shown that changes in farming practice are largely responsible, particularly the adoption of winter crops, meaning that fields are ploughed in the autumn.
BTO's head of terrestrial ecology, Dr Juliet Vickery, said: "Stubble fields are like giant bird-tables across the countryside; they contain seeds spilled as the crop was harvested, and also weeds.
"So if the stubble fields disappear, so does a key food source."
The research, published in the scientific journal Proceedings of the Royal Society, uses data from two long studies - the Breeding Birds Survey, run jointly by the BTO, the Royal Society for the Protection of Birds (RSPB) and the Joint Nature Conservation Committee (JNCC); and the Winter Farmland Bird Survey, run by the BTO and JNCC.
The BTO says 3% of lowland farmland is currently left to stubble
Both surveys rely on volunteers to collect data from different regions of the UK, including counting sightings of various bird species in specified areas one square kilometre in size.
The researchers analysed this information according to whether the specified areas contained fields of winter stubble.
In those that did, they found many of the bird populations either recovered faster, or declined less rapidly over the 10-year period, than in comparable areas without stubble fields.
A significant difference was seen for the tree sparrow, skylark, yellowhammer, lapwing, stock dove, pied wagtail, and mistle thrush; in the east of England, populations of wren, linnet, chaffinch and greenfinch also appeared to benefit.
The only species apparently reduced by the presence of stubble was the rook, although the scientists believe this is not a real effect - rooks are particularly difficult to count accurately, they caution, and suggest this is probably a statistical blip.
Last month, new regulations came into place enabling many more farmers to gain financially from taking action to conserve wildlife than in previous years.
One of the actions rewarded by the Entry Level Scheme is leaving fields of winter stubble.
National Farmers' Union spokesman Dale Atkinson said: "I suspect these new measures will increase the amount of land left to stubble."
"It's part of this overall process of farmers taking on a dual role, of producing food and being custodians of the countryside."
His optimism was shared by Dr Vickery, who told BBC News: "We're hopeful that we will see more stubble in the future; whether it will be enough is another matter."
A more detailed analysis of data for the skylark and yellowhammer has enabled the BTO team for the first time to say how much stubble is needed.
"Currently about 3% of lowland farmland is left to stubble," said Dr Vickery.
"We can now say that increasing that to 10% would be enough to halt the decline in these species; and increasing it to 20% could enable populations to recover."