With a wingspan of nearly 7 metres, Argentavis magnificens is the largest known bird to ever fly. It lived 6 million years ago in the open plains of Argentina and the Andes mountains, and it is related to modern-day vultures and storks—but with feathers the size of Samurai swords. It rivals some light aeroplanes in size, but it is believed to have flown on the wind more like a glider, soaring to speeds of 240 km/h. But with its massive flight muscles and enormous wings, the behemoth bird weighed 70 kilograms, so flapping its wings was not enough to achieve lift-off. “Birds are commonly compared with aircraft, but in reality helicopters are a better analogy,” says Sankar Chatterjee of Texas Tech University. Chatterjee and his team came to understand the bird’s flight by collaborating with a retired aeronautical engineer, inputting measurements from fossils into a computer program designed to study flight performance in helicopters. They determined that Argentavis must have run downhill into a headwind in order to become airborne, just like hang gliders, then gained elevation by circling inside columns of air known as “thermal elevators.” It would have easily hitched a ride a few kilometres up without even flapping its wings—then by just gliding to adjoining thermals, it would have been able to travel hundreds of kilometres per day.