A giant "terror bird" deemed to have been one of Earth's top predators after the demise of the dinosaurs was in fact probably a plant-loving herbivore, German scientists reported on Thursday.

Palaeontologists have wrangled for years about Gastornis, a huge flightless bird that lived between 40 million and 55 million years ago.

Up to two metres (6.5 feet) high, Gastornis had a massive beak with a hooked top -- a feature that looks so ferocious that many experts conclude the creature must have been a meat-eater, hence the "terror bird" monicker.

"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," said University of Bonn geochemist Thomas Tuetken, who took part in a new assessment.

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

"The terror bird was thought to have been a top predator at that time on land."

Tuetken and his colleagues, though, reassessed the bird's diet. They began to forge a pro-veggie opinion after they measured calcium isotopes in fossilised Gastornis bones found in a former open-cast brown-coal mine in Saxony-Anhalt, eastern Germany.

The residual signal of these isotopes is a telltale of what proportion of the animal's diet came from meat versus plants.

The indicator gets "lighter" as the isotope passes down the food chain.

In theory, the bones of a supposed apex predator like Gastornis should have sent a strong signal.

But the team found the isotope levels were weak, similar to those of herbivorous mammals and dinosaurs held in museum collections. Carnivores, including the Tyrannosaurus rex, had a far stronger signature.

The work was showcased on Thursday at an annual international meeting of geochemists called the Goldschmidt Conference taking place this year in Florence, Italy.

The vegetarian view of Gastornis has had other recent backing from evidence in the United States.

Footprints from the American cousin to the Gastornis suggest that its feet did not have sharp claws, which are characteristic of predators.

And its big size meant that it may have been quite unable to hunt for small and nimble early mammals, which for carnivores were the food du jour, say some experts.