A flock of some 100 vultures killed a cow and her newborn calf, the latest in a series of attacks in which carrion-eaters get so hungry they set upon on live animals, a Spanish farmer's union said Thursday.

The attack occurred last weekend in the Mena Valley, an area in northern Burgos province that is home to many cattle ranches, the Spanish Interior Ministry office in Burgos said.

It said a rancher alerted police after seeing the birds swarm on a cow that had just given birth and kill both animals.

"There was nothing he could do to stop them," the office said in a statement.

The phenomenon is on the rise in that valley and elsewhere in Burgos, said Jose Manuel de las Heras, president of the local chapter of a union called the Farmers and Ranchers Coordinator.

In the past two months there have been three or four attacks in the valley and several others elsewhere.

Traditionally, farmers and rural officials designate areas to dump the carcasses of farm animals like mules so vultures could feed on them.

But there are fewer and fewer of these places, in part because of mad cow disease: it is now illegal to dump cow or any other ruminant remains at such feeding troughs, de las Heras said.

The result is that vultures are so hungry they have even shed their wariness of humans to swoop down near farms and feast on live animals like cattle and pigs, de las Heras said.

"We have seen them land 100 meters from people. They are not afraid of anything," he said in a telephone interview.