BioEd Online

Scientists have modelled the flight of the world's largest flying bird — a 70-kilogram ancient avian behemoth with a 7-metre wingspan. The new models show that Argentavis magnificens, which was approximately the size of a modern-day Cessna 152 light aircraft, flew by gliding rather than by flapping its giant wings in the skies above Argentina 6 million years ago.

Taking off and landing would have been challenging for the beast, they conclude, but not impossible.

Argentavis is thought to be a forerunner to large modern birds such as storks and some vultures. Fossils show that it clearly had the right physiology for flying. But unpicking how, exactly, it took to the skies has been tricky.

Sankar Chatterjee of the Museum of Texas Tech University in Lubbock and his colleagues modified computer programs normally used to model aircraft and used them to calculate the energy needed for Argentavis to sustain flapping flight. They found that this was a minimum of 600 watts — about 3.5 times the amount of energy the bird could provide, according to estimates of the bird's musculature as scaled up from living birds. Instead, the team concludes, Argentavis probably coasted on currents of air rather than flapping its wings for lift.

The results are published in Proceedings of the National Academy of Sciences¹.

The conclusion fits with general ideas about bird size and flight, says biologist Steven Vogel, a professor emeritus at Duke University in Durham, North Carolina. "The bigger you are, the more wind dependent you are," he says.

Lift and glide

In the Andes, Argentavis could have flown on the winds that are created by air deflecting over ridges and cliffs. But in the plains of the Argentine pampas, the bird would have had a tougher time. There it would have had to rely on 'thermals' — pockets of hot, rising air created by the Sun heating up the ground.

Modern birds such as eagles can use the trick of turning in tight circles within small thermals in order to keep gaining height. But because Argentavis was so large and couldn't turn in very tight circles, Chatterjee's team estimates that the bird would have been limited to thermals that were more than 100 metres in diameter.

The climate of Argentina during the upper Miocene, when Argentavis patrolled its skies, was dryer and as much as ten degrees warmer than it is today, says Chatterjee, possibly fuelling large thermals across the pampas.

Runway

The big question, says Chatterjee, is how the bird managed to lift off.

The largest living flying bird — the Kori Bustard, an 18-kilogram bird found in Africa — has to run in order to take off, like an aircraft taxiing down the runway. Argentavis was approximately 3.5 times heavier, and probably would have needed a 10-degree downward slope and a brisk run to get lift from the pampas. Chatterjee says the bird's powerful legs could have managed short bursts of 30 kilometres per hour. "During take-off, that would be the sort of speed it needed," he says.

And then comes the landing. Birds have a number of techniques for putting on the brakes, including lowering their legs, flaring their wings upward and raising their tail. But landing is particularly thorny for heavy birds. It's possible, says Vogel, that a big bird such as Argentavis would have taken a tumble as it hit the ground, much as albatrosses sometimes roll to a stop. "I suppose these guys probably did a tuck and roll," he says.

If it's so hard for extremely large birds to fly, how did Argentavis get so big in the first place? Based on the size and shape of its beak, palaeontologists predict that Argentavis was a predator rather than a scavenger. Being big may have opened up the possibility of feeding on larger prey. And as long as the wind above Argentina was sufficient, Argentavis could surmount the limitations of its girth. "In funny environments you get funny sizes," says Vogel.