Scientists had previously believed the giant bird was a carnivorous predator or scavenger but the absence of raptor-like claws in footprints uncovered in a landslide in 2009 supports the theory Diatryma was not a meat-eater, they said.
Measuring 7 feet tall with a huge head and beak, the giant flightless bird is commonly portrayed as a fierce predator and was thought of as "the bird that replaced dinosaurs as the top predator," researcher George Mustoe of Western Washington University in Bellingham told the BBC.
"Let's be honest: scary, fierce meat-eaters attract a lot more attention than gentle herbivores," he said.
The researchers studied a set of footprints made 55.8 million to 48.6 million years ago and preserved in sandstone in northwest Washington, the only known footprints of the giant bird.
"[The tracks] clearly show that the animals did not have long talons, but rather short toenails," researcher David Tucker said. "This argues against an animal that catches prey and uses claws to hold it down. Carnivorous birds all have sharp, long talons."
Diatryma also did not have a hook on the end of its beak, a feature common to all raptors that allowed them to hold prey and tear flesh from carcasses.
"A more likely scenario [than being a carnivore] would be a gentle Diatryma that used its beak to harvest foliage, fruits, and seeds from the subtropical forests that it inhabited," Mustoe said.