The biggest bird recognized to have taken to the skies would have been a extraordinary 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.
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 Sankar Chatterjee, a professor of geology 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.