By environment correspondent Alex Kirby

Conservationists say the fate of many UK farmland birds depends on how British agriculture responds to climate change.

The Royal Society for the Protection of Birds (RSPB) says farmers may seek to take advantage of a future warmer climate to grow more crops.

If they do, it fears, some bird species could suffer further stress.

But in other ways climate change could be a positive benefit, especially to smaller birds.

Using research it has commissioned which is to be published in full later this year, the RSPB has issued a report, Climate Change: UK farmland birds in the global greenhouse.

Higher yields

Its publication marks the climate conference in The Hague, called to finalise the workings of the Kyoto Protocol, the international agreement on tackling global warming.

Song thrushes are already in trouble

The RSPB says: "Changing rainfall patterns, high evaporation rates, crop-wrecking storms, milder winters and higher carbon dioxide levels in the atmosphere may encourage farmers to aim for higher yields and to plant winter crops.

"This would mean further intensification of agriculture, and possibly heavier applications of pesticides and herbicides, with potentially disastrous consequences for farmland birds."

With climatologists predicting heavier rain for most of the UK during the nesting season, the report says that newly hatched chicks are especially vulnerable to heavy spring rains while they remain in the nest.

Studies dating back four decades show that species such as the capercaillie, the grey partridge and the cirl bunting can suffer serious losses in a wet spring. And the report says migratory species may also be at risk from climate change.

Losers and winners

"If different parts of the world warm at different rates and at different times", it says, "this will cause real problems for migrating birds who could get out of synch with food sources en route."

Skylarks may lose nesting and feeding sites

But the RSPB believes some birds may hope to do better as the climate warms, because milder winters may enhance their survival prospects.

This is especially likely for smaller species like wrens and sparrows, which often suffer population crashes during very cold winters.

Some birds may also be able to breed for longer and to produce more chicks.

And if farmers grow new spring-sown crops, the report says, this will leave stubble fields in the winter where birds can feed.

Otherwise, it foresees hazards throughout the year:

• heavier rains and floods in spring
• intense summer droughts bringing dry soils, parched vegetation and starving birds, and an increased fire risk
• autumn storms killing birds and disrupting migration
• stubble fields lost in winter, denying birds an important food source.

Among species at particular risk, the RSPB says, are linnets, skylarks, ptarmigans, lapwings and song thrushes, whose numbers have fallen by more than half in the last 30 years.

And the world's most southerly breeding population of twite, which live in the Pennines of northern England, could lose the few remaining hayfields there on which they depend.

The report says migrants like swallows "face the triple whammy of British summer droughts, unpredictable climate change in their tropical wintering grounds, and the perils of finding their way home as winds alter and food sources dry up".

The RSPB wants farmers to set aside more land for birds, and says the government should revise its rules on the timing of farm operations to protect birds nesting earlier.