Before the moa went extinct the flightless bird reshaped New Zealand's ecosystem.
Researchers from several New Zealand and Australian museums made the discovery using state of the art 3D modelling.
It now appears the large bird's diet depended on its size.
"The giant moa had a very strong bill … the ability to bite through thick twigs and sticks, but some small moas probably were grazing very much like sheep," Canterbury Museum curator Professor Paul Scofield said.
The study also dispelled the theory that ostriches and emus are closely related to moas.
The species went extinct about 500 years ago.