The largest bird known to have taken to the skies would have been a remarkable glider, scientists say.
A North American team has studied the flight abilities of Argentavis magnificens, which lived six million years ago in Argentina.
With its seven-metre (23ft) wingspan, the animal must have been an expert at riding thermals and updrafts.
But, the team tells PNAS journal, at 70kg (155lbs) it might have struggled to get airborne by flapping its wings
See a size comparison for the bird
Instead, the group believes, Argentavis probably used the same technique to get into the air as that employed by modern hang-gliders - by running downhill or by launching from a perch to pick up speed and lift.
"How to get airborne was the problem," explained Professor Sankar Chatterjee, curator of palaeontology at the Museum of Texas Tech University in Lubbock, US.
"But once it was on a thermal, it could easily rise up a mile or two without any flapping of its wings - a free ride, just circling. Then at the top, the bird could simply glide to the next thermal and in this way it could certainly travel 200 miles a day," he told BBC News.
On the runway
Professor Chatterjee and colleagues estimated the flight parameters of fossil Argentavis bones and plugged the information into computer flight models.
The results indicate the bird - which could have rivalled some light aeroplanes for size - would probably not have had the muscle power to lift itself into the air from a standing take-off or even maintain continuous flapping flight.
However, Argentavis certainly had all the makings of a high-performance glider.
Its giant wings would have extracted the maximum energy from rising air forced up by the slopes of the rocky Andes or the warming atmosphere above the grassy pampas, the Argentine plains.
"Like an albatross or a hanglider, Argentavis needed a little sloping surface; and he needed to run a bit, and headwind would have helped. Using this trick he could take-off but after that he didn't need to do much flapping of the wings," Professor Chatterjee said.
Writing in the Proceedings of the National Academy of Sciences (PNAS), the team adds: "Because the fossils of Argentavis are found from the foothills of the Andes to the pampas, it is likely that it used primarily slope-soaring over the windward slopes of the Andes and thermal-soaring over the open pampas."
With its powerful beak and big clawed feet, Argentavis would have made a fearsome predator, swooping down to snatch unsuspecting rodents.
The kori bustard (Ardeotis kori) has a claim to being the heaviest modern flying bird at about 18kg (40lbs).
The wandering albatross (Diomedea exulans) is widely recognised as the modern avian with the longest wingspan, at more than 3m (10ft).
The biggest flying animals known to science were pterosaurs. The flying reptiles that lived more than 65 million years ago had wingspans exceeding 10m (30ft).
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