The government has exaggerated the amount of land needed to reverse a decline in farmland bird numbers, experts have claimed.

A document being drawn up by three conservation groups suggests the decline could be reversed by placing just 1-2% of arable land under voluntary environmental management – less than half the area DEFRA says is required.

But ministers will press ahead with a compulsory set-aside replacement that will see up to 5% of arable land taken out of production across England unless more farmers embrace wildlife measures under environmental stewardship.

DEFRA remains determined to retain the environmental benefits of set-aside amid evidence that farmland bird populations have halved since 1970. Bird numbers were a barometer for the health of the wider environment, it said.

But a different approach is being drawn up by the Linking Environment and Farming (LEAF), the Game and Wildlife Conservation Trust (GWCT), and the Farming and Wildlife Group (FWAG). It will be presented to farm leaders next Friday (27 March).

LEAF project manager Camilla Harris said: “If it is targeted and managed correctly alongside the right in-field options, we believe 1-2% will deliver benefits way above and beyond anything achieved by a compulsory blanket approach.”

The paper will be presented to the NFU and Country Land and Business Association (CLA). Both organisations have just 10 weeks left to argue their case for a voluntary rather than compulsory approach towards a set-aside replacement.

GWCT project manager Alastair Leake will use the meeting to urge the NFU and CLA to show leadership on the issue. “We shouldn’t set ourselves impossible targets but we should at least aspire to deliver more than a mandatory scheme,” he said.

Farm leaders should encourage more farmers to sign up to entry-level stewardship and choose in-field options that benefited farmland birds, said Dr Leake. “It is beholden on us as a farming community to look at the scheme and implement it better.”

A DEFRA spokeswoman it was not simply a question about how much land should be taken out of production. It was also important how that land was managed and distributed. A government consultation on the issue closes on 27 May.