© East News / Rex FeaturesA family of Starlings has chosen a post box for the third year running in an Essex seaside town to raise their young brood.
People have been listening to skylarks singing in Britain for 10,000 years. But now they, and many other much-loved species, are vanishing fast.

The B1042 that winds from the Bedfordshire town of Sandy towards the village of Potton is a difficult road to cross. Fast and twisty, there are several blind bends where pedestrians must take their lives into their hands. That is trickier than it sounds, for most pedestrians who cross the B1042 already have a pair of binoculars in their hands.

The road separates the grand headquarters of the RSPB, home to hundreds of birdwatchers, from some unkept fields, home to hundreds of watchable birds - hence the regular skips across the tarmac.

The skips, though, are now less regular for many RSPB staff, for the star attraction of the neighbouring fields has flown. Until a year ago, a clutch of woodlark nested there, one of Britain's rarest birds with just 1,000 or so thought to remain. Then their home was ploughed up and replaced with a giant field of swaying hemp plants. The woodlark have not been seen since.

It is not just the professional birdwatchers of the RSPB who have seen their local landscape transformed. Across Britain, and with little fanfare, the face of the countryside has subtly changed in recent years. Farm fields that stood idle for years under EU schemes to prevent overproduction, such as the one across the road from the RSPB, have been conscripted back into active service. The uncultivated land, previously a haven for wildlife, has been ploughed, and farmers have planted crops such as wheat and barley, with occasional hemp for use in paper and textiles.

As a result, the amount of land available for birds such as the woodlark has halved in the last two years. Without efforts to stem this loss of habitat, conservation experts warn that the countryside of the future could look and sound very different.

Starved of insects in the spring and seeds through the winter, the metallic-sounding corn bunting and plump grey partridge, formerly one of the most common birds on UK shores, are on the brink. And the skylark, whose twittering has provided the soundtrack to millions of countryside walks and inspired Percy Bysshe Shelley, in Ode to a Skylark, to praise its "profuse strains of unpremeditated art", is struggling and could soon vanish from many areas. Numbers fell 53% from 1970 to 2006. "This is not just about birdwatchers. These birds are part of our common heritage," says Gareth Morgan, head of agriculture policy at the RSPB.

Government figures show that populations of 19 bird species that rely on farmland have halved since serious counting started in the 1970s - a decline conservationists blame on intensive farming methods, with insecticide and herbicide sprayed on to monoculture fields shorn of vibrant hedges. The unmistakable yellow hammer, which likes to sing while perched as a dash of color on hedges and bushes, has steadily disappeared with the hedges and bushes. And a startling 80% drop across England in 40 years has diluted the shifting Rorschach blots painted on the dusk sky by massed flocks of starling - though urban changes are blamed for this too.

Farmland birds may sound a niche problem, and you may think that the rest of the countryside is doing OK, but for most people, farmland is the British countryside. About 75% of Britain is farmed, and about half of that is arable fields. Take a train between two UK towns, particularly in eastern counties, and almost all of the countryside you see is farmland.

As Simon Gillings of the British Trust for Ornithology (BTO) puts it: "For most people, farmland is the countryside and farmland birds are the birds they see." If birds are struggling, then it is a fair bet that other wildlife is too. "Birds are indicative of other things," Gillings says. "If birds are declining then what does that say about the plants and insects they rely on? It's all linked together."

Despite the long-term decline, things had been looking better for many farmland birds until recently. To understand why, we must enter the shadowy domain of the EU and its infamous Common Agricultural Policy.

During the 1980s, farmers were paid a guaranteed price for crops such as wheat and barley, a policy that led to massive oversupply and the infamous grain mountains. Faced with the soaring costs of storing all the spare food, EU officials worked out it would be cheaper to pay the farmers not to produce it in the first place. Thus began the voodoo economics of the "set-aside" scheme, under which subsidies to arable farmers across Europe were paid only if they agreed to leave some of their land unfarmed - between 8% and 15% each year. Amid repeated hostile jibes at farmers being paid not to do anything, the requirement continued for two decades until 2007, when the EU dropped it after poor harvests and rising food prices. Food was needed, lots of it, and farmers across the UK exploited the rule change and pressed fields of set-aside land back into production. The change was sudden and dramatic. From 424,000 hectares left unfarmed in 2007, just 159,000 hectares remained a year later.

Yet, food is not the only thing that grows on farmland. For those two decades, something stirred in the set-aside. Spared regular drenchings in toxic chemicals and disturbance by machinery, weeds, grubs and bugs all thrived. Stubble from harvested cereal crops left to rot over the subsequent winter offered seeds. Set-aside land became, totally by accident, a giant bird table. The effects were not universal: some areas and different set-aside options assisted some species more than others; the small, graceful yellow wagtail appeared to gain the least because they sometimes nest in winter-grown cereal, which was a common casualty of set-aside. But, in general, the uncultivated land was a conservation godsend.

The 2007 EU decision to scrap set-aside threatens to undo much of the good work. Last winter, the BTO surveyed some 2,000 fields across the country and found that 72% offered no winter stubble for birds to feed on. In some areas, the figure was as high as 80%. When the same fields were checked in 2003, some 60% were found to offer seed-giving stubble. It is too early to know what impact the shift has had on birds and other wildlife, but it is unlikely to be positive. The BTO is trying to find out and is expected to publish the findings later this year.

The decline in cereal prices last year seems to have calmed the rush to crop previously unfarmed fields, and the amount of land left uncultivated in Britain is expected to rise slightly this year - but with the EU set-aside requirement removed, another price spike could see it ploughed up and cropped, with serious consequences for vulnerable wildlife.

The government is so concerned that in February, the environment secretary, Hilary Benn, announced plans to reintroduce set-aside requirements as a condition of farming subsidies. The department for the environment, Defra, is consulting on whether such a move should be compulsory. The consultation, which ends this month, has exposed traditional divisions - while conservation groups such as the RSPB are pushing for a mandatory 4-5% of farmland to be kept out of production, farmers are lobbying hard to be given the choice.

"The best people to understand conservation work on farmland are farmers. If you make farmers do things then they don't really take it to heart," says Guy Smith, who runs a 500-hectare farm in Essex. "I really think farmers can be trusted with conservation work. I can't see farmers going back to planting wall-to-wall wheat. We're not hedge grabbers, we're hedge planters. The callouses on our hands come from setting rabbit guards around new hedges, not shoveling wheat."

More than 60% of farmland in Britain is managed under environmental schemes to protect field margins, streams and ditches, he says.

The National Farmers' Union wants the government to give farmers a few years to show they can provide conservation benefits, with compulsory set-aside regulations only tied to subsidy payments as a last resort.

Anyone in doubt that farmers can rise to the challenge should visit the farm of John Cousins in Hadleigh, near Ipswich. Cousins, also head of agricultural policy for the Wildlife Trusts, has 135 hectares and grows wheat and barley on two-thirds of it. But some 100 acres is not cropped. And those 40 hectares are currently the most profitable. "I go for all the [environmental] grants I can," he explains as we drive around the scruffy edge of a giant field planted with barley. "You paid for that fence. Well, the taxpayer did. And that one."

Cousins gets up to £240 each year from the public purse for every acre he dedicates to conservation. Growing wheat, he estimates, would yield just £130 profit per acre at today's prices. As a result, his farm is a showcase of the environmental measures possible on farmland. New hedges divide fields, and strips of wild set-aside land run alongside. The chatter of birdsong fills the air. Skylarks flutter overhead, while deer and a weasel dart from the undergrowth. Cousins has joined up unfarmed areas to provide a wildlife corridor across his land, while giant plots of ploughed earth wait for nothing more than mustard, sunflower and millet that will be planted purely to die and offer seeds for birds over the next winter. "Farmers get public money and I think we should give something back," he says. Those who are opposed to conservation conditions imposed by the government are under no obligation to accept the attached subsidy, he points out.

Other farmers question the wisdom of cordoning off fertile land for wildlife as the world faces what the government's chief scientist, John Beddington, has called a coming "perfect storm" of problems, including food shortages. Smith, over in Essex, says: "If we just abandon great swaths of Britain and leave it to the birds, that is not an efficient or moral use of land." Conservation measures, he says, must be done in a "measured manner that is species specific." Rather than needing to keep 5% of land unfarmed, he suggests, 1% could offer the same benefit if it was selected and targeted properly.

Morgan at the RSPB says the food shortage argument is "threadbare", although he accepts that the situation could worsen in future. He says: "The problem is far more complicated than saying some people don't have enough to eat so we need to farm every square inch of the UK right now." Food is being turned into biofuel, he says, while grain is fed wastefully to animals. "We're just not that short of food."

The RSPB, the Wildlife Trusts and others insist that we have more pressing problems on our plate. "People have been listening to skylarks singing in Britain since 10,000 years ago," Morgan says. "And they should be able to do so in 50 years' time. At what point do we say it is not acceptable for people to walk and not hear a bird that people have been writing about for hundreds of years?" As the poet Shelley added: "We look before and after, and pine for what is not".