SCOTTISH farmers changing their crops are thought to be largely responsible for an ­alarming decline in the numbers of kestrels.

New research by the RSPB has found the raptor's numbers, which have decreased by 65 per cent over the past decade and a half, fall at times of change in agriculture methods.

The authors of the study, which is in its early stages, say more specifically the move away from spring-sown barley to autumn-sown wheat and oilseed rape, leaves fewer winter food sources for prey species such as voles and seed-eating songbirds. This means less food for predators like kestrels.

It says other possible factors in the species decline include climate change, increased ­competition for nest sites, a rise in rat poisons in their rodent prey and the impact of larger predators like goshawks and peregrines.

The kestrel is the most common bird of prey in Europe, but its population has fallen into steep decline north of the Border. In 1995 there were roughly 10,000 breeding pairs in Scotland, but by 2013 that had been more than halved to roughly 4,500 pairs - the biggest decline of any monitored bird species in the country.

Staffan Roos, senior ­conservation scientist at RSPB Scotland, said: "It's really sad to see kestrels suffering such large drops in numbers in recent years. Research into what is causing the decline is vital because once we know what factors are having an impact we can offer advice on how to increase the populations of this charismatic bird of prey.

"For example, we can speak with farmers about increasing field margins to boost vole numbers, which would give kestrels a more abundant food source.

"We are already supporting practical steps that we believe will help us to understand the population decline better. We support the Scottish Raptor ­Monitoring Scheme (SRMS), which recently has increased the monitoring of kestrels and ringing of kestrel chicks throughout Scotland. This will provide useful information on the survival and movements of kestrels. We also provide nest boxes, including on some of our reserves, to reduce the risk of competition for suitable nest sites."

Across Britain in 1995 there were roughly 40,000 breeding pairs of kestrels, but by 2013 that figure had dropped to between 30,000 and 35,000, a far smaller proportionate drop than that seen in Scotland.

They are still one of easiest raptors to spot as they hover near roads and motorways, but RSPB Scotland's conservationists are now carrying out urgent research to help understand why numbers of this popular species are diminishing so rapidly.

The figures come from the latest Breeding Bird Survey, which is carried out each year by more than 2,800 volunteers across the UK. In Scotland, 471 sites were monitored in 2013, but kestrels were observed in only about 35 of them.

Brookfield Drinks, the brewer of Kestrel Lager, supports the RSPB in its efforts to identify the causes of the kestrel population's decline.

The money the company provides is used to fund the research programme set up to help save the kestrel, increased monitoring of kestrels in Scotland through SRMS and to advise landowners, crofters and farmers on wildlife-friendly practices.

The Breeding Bird Survey is the main scheme for monitoring the population changes of the UK's common breeding birds, providing an important indicator of the health of the countryside.

It is run by the British Trust for Ornithology and is jointly funded by the trust, the Joint Nature Conservation Committee and the RSPB.