Farmland bird numbers fell 5% last year to new record lows, official figures showed today.
The latest statistics covering the fortunes of 19 key species which breed or feed on farmland in England showed populations had more than halved since records began in 1966.
Conservationists said the year-on-year fall between 2008 and 2009 could be the result of a cold winter and the loss of "set-aside" land which farmers were paid to leave fallow, an EU policy which was abolished in 2007.
With another harsh winter this year, the figures for 2009-2010 could be worse still.
The single-year decline is part of a trend in falling farmland bird numbers which has seen populations drop 53% below where they were when monitoring first started in 1966.
Experts at the RSPB said the figures made "depressing reading" and they were most concerned about the fortunes of threatened species including lapwings which saw a 12% decline, corn buntings whose numbers fell 7%, and the grey partridge, which plummeted by 23%.
Lapwings and grey partridges, both "red listed" species because of concerns about the declines they are suffering, were among six birds which saw significant falls in numbers between 2008 and 2009, along with kestrels, skylarks, starlings and greenfinches.
In total 12 species saw numbers fall, while seven farmland birds including jackdaws and goldfinches saw populations increase.
The RSPB said farmers were having success in reversing declines in birds such as lapwings on land which benefits from advanced environmental subsidies, known as the higher level stewardship scheme (HLS), designed to boost numbers of threatened species.
But just 1% of farmland is covered by the higher level scheme, which is paid for by the EU and the Government, and the wildlife charity fears the programme is under threat from coalition budget cuts.
The basic package of environmental subsidies - known as entry level stewardship (ELS), which was designed to reverse the overall trend in falling farmland bird numbers - does not appear to be working as well it should, the RSPB said.
Today's figures showed a five-year decline in bird numbers of 10% up to last year, even though 56% of farmland in England is covered by the basic subsidy scheme.
The RSPB said the longer term decline suggested the different options for managing land under the ELS scheme were not being used in the right ways.
The charity is backing the voluntary Campaign for the Farmed Environment by the agricultural industry that aims to replace the benefits to wildlife of set-aside, and urged more people to join the scheme to help farmland birds.
Dr Mark Avery of the RSPB said it was hard to draw any "hard and fast conclusions" from the one-year fall in bird numbers, but added that the five-year decline was a real cause for concern.
As to why numbers fell last year, he said: "The winter before last was a moderately cold one which could have impacted on birds' ability to find food.
"We may also be seeing the knock-on effect of set-aside being abolished in 2007, removing valuable foraging and nesting habitats for wild birds in the farmed countryside."
And he said: "Lapwings - known to some as the 'farmer's friend' - are particularly vulnerable and their populations have been steadily falling for more than 30 years, so a decline of 12% in one year across England is really bad news.
"Those farmers who are helping to save this beautiful, threatened bird through the Higher Level Stewardship are achieving some great results.
"So to cut this important environmental scheme now could be disastrous."