CHICAGO - It cruised the skies above the Argentine pampas about six million years ago, a soaring behemoth of a bird, the size of a modern light aircraft, dragging about 140 pounds in ballast.

This handout illustration recieved courtesy of Proceedings of the National Academy of Sciences (PNAS) shows Argentavis magnificens, the world's largest known flying bird with a wingspan of 7 meters, (7.6 yds) about the size of a Cessna 152 aircraft, soaring across the Miocene skies of the Argentinean Pampas six million years ago. [Agencies]

But with little in the way of muscle to flap its wings and propel itself through the air, just how did the largest bird to ever take wing stay aloft?

That question has puzzled paleontologists for decades, but in a study released Monday, US researchers suggest that the now extinct Argentavis magnificens was essentially an expert glider, hitching a lift on thermals and updrafts.

"Once it was airborne, there was no problem. It could travel 200 miles in a day," said Sankar Chatterjee, a distinguished professor of geology at the Museum of Texas Tech University in Lubbock, and lead author of the paper.

Chatterjee and a team of researchers analysed the aerodynamics of the ancient bird of prey by plugging information about its flight parameters into flight simulation software.

The analysis showed that the prehistoric aviator, like most large soaring landbirds, was too large to sustain powered flight, but could soar efficiently, reaching speeds of up to 67 mph in the right conditions.

Like modern-day condors, the Argentavis would have relied on updrafts in the foothills of the Andes, or columns or pockets of rising air known as thermals over the grassy pampas where it hunted its prey, for lifting power.

In all likelihood, the bird would have circled upwards on a thermal and glided from thermal to thermal sometimes over long distances between its roost site and feeding areas.

Although it had a 21-foot wingspan, its 100 foot turning radius was short enough that it could keep circling within a thermal as it rose high to search the plains for its prey.

"The hardest part would be taking off from the ground," said Chatterjee. "It would have been impossible to take off from a standing start.

"It probably used some of the techniques used by hang-glider pilots such as running on sloping ground to get thrust or energy, or running with a headwind behind it."

The study appears in the Proceedings of the National Academy of Sciences.