Zoologists have shattered the belief that sheep and deer are exclusively vegetarian.

Grass may satisfy them most of the time but in places where their normal diet is low in essential minerals they have been seen eating grouse and seabird chicks. Dr Niall Burton recorded the carnivorous activity on Muggleswick Common in Co Durham and included it in a report by the British Trust for Ornithology in the journal British Birds.

He explained that the moor, north of Stanhope in Weardale, was grazed by sheep and managed for grouse. He saw a brood of eight chicks, less than a week old, foraging in the patchwork of heather and close-cropped turf. The grouse moved on to the short turf, becoming highly visible, when one of three nearby sheep, "ran forward, picked up a chick and ate it whole".

He said: "The alarmed female grouse quickly removed her remaining chicks into the heather, but the sheep was prevented from taking a second only by my intervention."

He also referred to instances, reported by Dr Bob Furness from Glasgow University, of sheep eating live Arctic tern and Arctic skua chicks on Foula, Shetland, and red deer eating live Manx shearwater chicks on Rum in the Inner Hebrides.

However, unlike his observation in Co Durham, only heads, legs or wings of the chicks were eaten in those cases. His theory was that the meat-eating was "probably a means of alleviating a mineral deficiency, perhaps of calcium, in the mammals' diet".