When a French pensioner died of a brain haemorrhage during a walk in the Pyrenees this summer, vultures started circling low over the body.

“His three friends were really frightened,” said a local resident. “They were convinced the vultures were going to attack. They shouted and waved their arms and, in the end, they managed to scare the birds away. But they were in a complete panic.”

There have been reports from across the French Pyrenees this year of a radical change in the way the region’s vultures behave. A programme to incinerate animal carcasses in Spain has deprived “les vautours” of food, causing them to become aggressive. Where once they scavenged, now they hunt, according to farmers.

Alain Larralde, a cattle breeder in Ilharre in the French Basque country, said that in May he saw dozens of birds circle and kill a cow. “There were so many of them that they covered the entire meadow,” he said. “Then I saw the cow slumped on the ground in the middle being devoured. It really hurt. You can’t image what it’s like to see an animal being eaten alive.”

So far this year officials have registered 42 demands for compensation from breeders who say their livestock has been attacked by vultures. There were 33 requests last year.

But ornithologists say it is collective hysteria. Denis Vincent, of the French Bird Protection League, said: “For the most part these stories don’t stand up. It’s impossible for vultures to fly off with animals bigger than them, as people have claimed, especially when those animals are alive.” He said that farmers were blaming vultures for killing sheep and cattle when, in fact, they were eating carcasses. Jean-Louis Venant, who collects birds of prey, added: “The habits of vultures haven’t changed for thousands of years.”

But vultures may never have been hungrier. A 2006 European Union directive forced Spain to ban the practice of leaving carcasses in open trenches. In upper Aragon, on the French border, they ate an estimated 8,000kg (17,600lb) of rotting meat every day. These carcasses are now burnt and Aragon’s 10,000 or so vultures must look elsewhere. In June, 200 were spotted in Belgium, where bird-lovers put 200kg of pork out for them “to build up their strength” for the journey back to Aragon.

In the French Pyrenees, the issue has become highly sensitive, with officials afraid that tourists will be driven away by what Le Nouvel Observateur magazine described as “the mutant vultures”. The birds have been a protected species since 1976. Now, amid claims that farmers are shooting at the birds, vets are to be paid to carry out an autopsy on all animals said to have been killed by vultures to separate fact from fiction.

Didier Hervé, director of the Upper Béarn Heritage Institute, said that panic was spreading. “A mother told me that a flock of vultures settled next to her by the village fountain when she was with her children,” he said.

Jean-Pierre Pommiès, a shepherd and mountain guide, reported an attack after taking tourists into the mountains. “The vultures were chasing one of my sheep and pecking its leg,” he said. “I chased them away but if I’d been five minutes later they would have cleaned up. They say the shepherds are inventing these stories, but this time there were 30 tourists who witnessed what happened.”

Birds of a feather

— Griffon vultures breed across southern Europe, the Middle East and southern China. In winter they migrate to northeast Africa, the Arabian peninsula and north India

— They are up to 43 inches (110cm) long, with wingspans of up to 110 inches. They weigh up to 22lb (10kg) and can live for up to 40 years

— They are traditionally thought to survive exclusively on carrion

— A griffon vulture will spend about 8 out of 24 hours in the air, during which time it is likely to cover 200 to 300 miles (320 to 480km)

Sources: Hawk Conservancy Trust; Wild Natures; www.birdcheck.co.uk; www.birdlife.org

Times Recommends

• LA profits from Michael Jackson stimulus
• Berlusconi's absent wife causes G8 headache
• Chimps that took a punishing step for Man