Size and Weight:
Just how big was Argentavis? To put things in perspective, one of the largest flying birds alive today is the Andean Condor, which has a wingspan of about 9 feet and weighs about 25 pounds. By comparison, Argentavis' wingspan was comparable to that of a small plane--about 25 feet--and it weighed somewhere between 150 and 250 pounds. By that token, Argentavis is best compared not to other birds, but to the huge, reptilian pterosaurs that preceded it by 60 million years, most notably the giant Quetzalcoatlus.
A prehistoric bird the size of Argentavis presents some difficult issues, chief of which is how it managed to launch itself off the ground and keep itself in the air. It's now believed that this bird flew much like a pterosaur, unfurling its wings (but only rarely flapping them) in order to catch the high-altitude air currents. It's still unknown if Argentavis was an active predator of the huge mammals of late Miocene South America, or if, like a vulture, it contented itself with scavenging already-dead corpses.