A flightless “terror bird” that lived in England 50,000 years ago may have been a chicken at heart despite its fearsome reputation.

Gastornis stood over six feet tall, had short stubby wings and resembled a giant dodo with attitude.

Its size, frightening appearance and huge, curved beak have led experts to believe it was a ruthless top predator in the prehistoric forests of western and central Europe.

But new biochemical evidence suggests that Gastornis may have been vegetarian.

Analysis of its fossilised bones shows a calcium composition similar to that of plant-eating mammals and dinosaurs.

Carnivores have a different calcium profile, because of the way the element becomes “lighter” as it passes through the food chain.

Lead scientist Thomas Tutken, from the University of Bonn in Germany, said: “The terror bird was thought to have used its huge beak to grab and break the neck of its prey, which is supported by biomechanical modelling of its bite force.

“It lived after the dinosaurs became extinct and at a time when mammals were at an early stage of evolution and relatively small.

“Thus, the terror bird was thought to have been a top predator at that time on land.”

Tutken led a team of researchers who studied fossilised remains of terror birds from a former open-cast coal mine in Geiseltal, Germany.

They presented their findings at the Goldschmidt geochemist conference in Florence, Italy.

Suspicions about Gastornis were first raised when footprints believed to belong to the bird’s American cousin did not show the expected imprints of sharp claws.

Some experts have also pointed out that the creature’s sheer size and inability to move fast may have hampered its ability to prey on small, quick mammals.

The scientists hope to confirm their results with further studies on other fossils.

If Gastornis had not been a bird, more conclusive evidence could have been obtained from teeth, Tutken said.

“Tooth enamel preserves original geochemical signatures much better than bone, but since Gastornis didn’t have any teeth, we’ve had to work with their bones to do our calcium isotope assay,” he said.

“Because calcium is a major proportion of bone – around 40 per cent by weight – its composition is unlikely to have been affected much by fossilis-ation. However, we want to be absolutely confident in our findings by analysing known herbivores and carnivores using fossilised bone from the same site and the same time period.

“This will give us an appropriate reference frame for the terror bird values.”