With a wingspan estimated at seven meters across, Argentavis was roughly twice the size of the largest flying bird today (Wandering Albatross), and only the long extinct pterosaurs could have rivaled and exceeded it for size. How such a large bird like Argentavis could fly has been the key area of study associated with this bird, something that has resulted in some interesting conclusions. The first is that the keel of the breastbone is quite small which suggests the main flight muscles were reduced when compared to other flying birds. This means that even though the wings were huge, Argentavis did not have the stamina to continuously flap them.
It’s most likely that as a result of these under developed muscles Argentavis relied upon prevailing wind currents to keep itself aloft with flapping only occurring during the take-off and landing phases. This would see Argentavis using its large wings to exploit a combination of thermal up draughts as well as dynamic soaring. Dynamic soaring is essentially where a flying creature uses the boundary between two air masses to pick up speed by cartwheeling into oncoming wind and using the wind speed to accelerate itself forward. Repeating this process further increases the speed of the bird and resulting effect of the next manoeuvre resulting in an extremely energy efficient form of flight, one that is now even used by human glider pilots to stay airborne longer.
Argentavis also seems to have relied more upon air currents for taking off as the immense size of its wings means that it could not flap them when outstretched without the tips hitting the ground. Instead Argentavis would have had an easier time just stretching out its wings and facing into the oncoming wind.
From this position Argentavis could run into the prevailing wind to get air moving across its wing surfaces and then use its legs to jump up into the air. This would be the most critical time for Argentavis as getting airborne is not the same as staying airborne (ask any pilot). However, if Argentavis had positioned itself to run down a slope it could have gotten itself airborne while increasing the distance between itself and the ground just by flying horizontally level. Argentavis could then flap its wings while it adjusted its course to take better advantage of the air currents.