By Helen Briggs
BBC News science reporter
A bird spirals into the sky above a field on the Oxfordshire borders, trilling and chirruping as it flies vertically upwards, becoming a mere dot against the blue.
"It's a skylark," says Marek Nowakowski, in hushed tones. "Listen, there are two of them."
We are tramping across what appears to be little more than a matted field on the fringes of Upton Farm near Banbury.
It's actually an ancient archaeological site, where King Charles' army lined up against the troops of Essex in 1642 for the Battle of Edgehill, which marked the start of the English Civil War.
Today, the huge, flat field has been left uncultivated to preserve the site and to encourage the return of wildlife. Upton Farm is one of 80 Conservation Grade farms in the UK that "farm" wildlife as well as crops.
Some 10% of the land has been set aside to provide natural habitat and food for the animals and birds that were once widespread on British landscape.
"Conservation grade farming has really made use of about 30 years of habitat creation," says Nowakowski, an agri-environment consultant from the Farmed Environment Company.
"I suppose in its simplest form, if something sticks out of the ground and requires management, we'll call it a crop.
"Now that could be a crop of wheat, destined to be bread; it could be a crop of clover, destined to be bumblebee food.
"What we're doing in Conservation Grade is using the science that has been developed to underpin a protocol that says, 'if you sow the following habitats at the right ratios, you will have put back the tapestry of opportunities for wildlife that once existed 20, 40, 50, a hundred years ago'."
Threat to farm birds
Rob Allan, the farm and estate manager, takes us to the edge of a field that has been sown with a mixture of cereal, kale, millet, fodder radish and lined to provide winter food for birds.
The plants and pods have been pecked apart by birds foraging for food during a particularly harsh winter.
Marek Nowakowski (left) and Rob Allan
Farmland birds have declined by 50% on average in England between the mid 1970s and the early 1990s.
Conservation Grade Farming aims to encourage birds and other wildlife back to farms by providing them with food and shelter.
The incentive for farmers is largely financial - they are paid a premium for the crops grown on their land.
"Conservation grade farming means to me that, first and foremost, I can get a premium for the crops that I sell and it also brings the farm alive in terms of wildlife," says Allan.
"Ten percent of the area we commit to putting into wildlife habitats and then actively farm those to increase the numbers of species on the holding."
For the farmer, this means more work - topping margins, sowing bird food and installing boxes for owls and other birds.
But, Allan says, he has noticed the difference in terms of wildlife returning to the farm.
"We notice a lot more now because we're looking for it," he says.
"They were obviously there before but they're here now in a lot greater numbers and we have people ringing up and saying they've seen barn owls over such and such a margin. It pulls the whole thing together really."
Link with the land
Upton Farm covers 2,000 acres (800 hectares), made up of 1,200 acres (480ha) of arable land, 500 acres (200ha) of grassland grazed with sheep and the rest woodland.
It grows conservation grade crops - oats, wheat and barley - for the food company Jordans.
Bill Jordan, chairman of W Jordan Cereals Ltd, believes food processors have an inextricable link with the farming industry and should be working hand in glove to preserve wildlife, the environment and the quality of farms.
Bird food sown at the edge of a field
"A lot of farmers are realising that if they just push forward intensively they are not going to get the sort of farm in a few years' time that has got the habitat and the environment that they used to support," he says.
There is no denying the company's commercial motive in appealing to the environmental conscience of consumers. As Jordan puts it, "the provenance story of ingredients has become a hot issue for consumers".
"There are an awful lot more people nowadays who are interested in where their food came from, and how it was grown, and whether it was for the benefit or otherwise of the wildlife around us," he says.
Jordans pioneered the Conservation Grade mark, drawing on research carried out by a number of organisations, including the Royal Society for the Protection of Birds (RSPB).
Agriculture Policy Officer, Harry Huyton, says it stands alongside other environmental foodmarks such as Organic and LEAF (Linking Environment and Farming).
"We recognise that Conservation Grade can benefit biodiversity and deliver conservation benefit," he says.
"It requires farmers to do things on their farm that have been scientifically demonstrated to benefit birds and biodiversity.
"Yes, it might be that Jordans do it for commercial reasons but if consumers do buy it, more land is going into habitat for wildlife."