Argentavis magnificens soared, not flapped

From PhysOrg:

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.

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.