Again, this mode of hunting is not unusual. Secretary birds in Africa, similar in height to most teratorns also hunt in this manner. Fossils show that they walked on a pair of long, stout and strong legs. From its height, Argentavis could spot little animals scurrying across the ground, track them down and swallow them whole. Of course this does not exclude a scavenging lifestyle. Even the biggest eagles have been known to seek out carrion and an Argentavis would have no problem with scaring away carnivorous mammals simply by its sheer size. At the time the main mammalian predators were predatory relatives of marsupials. At best these creatures could reach puma-size and were probably heavy, flat-footed ambush hunters with not much agility or speed. At this size they are also incredibly small in comparison to the large birds.
Estimates show that this bird had to patrol a massive territory. It would have covered roughly 542 square kilometers on a hunt. This is definitely a wide area, and an adult Argentavis would have to consume at least ten kilos of meat per day. Luckily it was gifted with jaws wide enough to eat a hare-sized prey item. It must have been an intimidating sight on the hunt and downright magnificent in the air. But the burning question is, how could such a massive bird even get off the ground?
Today’s heavy modern birds need to perform a run-up in order to get off the ground. Birds like bustards, turkeys and swans are seen doing this in order to gain lift and momentum. They need to flap to take off and after the initial launch the heavy bird is in the air and can either flap or just soar, conserving energy. Argentavis even exceeds the heaviest flying birds today, like the Kori bustard in terms of weight. Today's bustards may be a little under 20 kilograms. Argentavis was three to four times heavier. The highest weight ascribed to this monster bird is roughly 80 kilograms.
The leg bones of Argentavis show that it certainly had to take a run-up to launch itself into the air even though studies of leg bones show that it was better at slow stalking than a sustained sprint. Today’s larger ground birds run against the wind to make up for any deficiencies so Argentavis must have done the same. Also it was once thought that this bird’s wings were too long and its breast muscles were too weak for a long, sustained flapping flight. In general though, the larger the bird, the lower the frequency of wingbeats. It would have to already be aloft if it were to apply true powered flight.
Of course it could always use the wind to gain lift and momentum. It would have to, in this case, live around mountainous areas and places where there were a good many thermals from which it could gain the necessary support to gain altitude. At that time, around six million years ago in the Miocene, the major orogenic forces in South America had just begun. The Andes had not yet risen to their fullest height. Their youthful foothills still dominated much of the Patagonian landscape, and the birds probably made use of the thermals rising off these mountains.
There is also the eternal question of wing loading. This is denoted as the loaded weight of the winged object divided by the area of the wing. A very high wing loading may decrease the maneuverability of the animal in question, while it is vice versa for a lower figure. For bird flight, the uppermost wing loading is 25 kilograms per square meter. From a small sample of bird species it was determined that Argentavis’ wing loading was higher than that of a California condor but lower than a wandering albatross. Its relatively high wing loading thus was poorly designed for aerial acrobatics or any kind of swift motion. It could still fly on a moderately strong wind beneath its wings, relying only on this external power. Flaps could be used in quick bursts of energy, preferably on a shorter trip.
Being as huge as it was, it was probably not a common bird. It certainly nested on mountainsides. We may imagine it as building a nest similar to an eagle’s even though this is entirely speculation and no such constructs have been found. It would be quite impressive, a massive, wonderfully built mass of sticks and small branches, lined with downy feathers. Inside would lie either one or two eggs. Argentavis’ rate of reproduction was about as low as you could get with birds of its size. It would lay an egg once every two years. This also meant that the population sizes for Argentavis were pretty small for such a large bird.
Then there was also the other factor of dietary preferences. In a normal ecosystem, larger meat-eaters are not as common as smaller ones and herbivores often outnumber carnivores by ten to one. Being a carnivore and a huge one at that meant that a very small percent of the ecosystem was made up of these avian giants. This meant a tiny population scouring a huge territory by air, hunting small animals and taking carrion by force.
Argentavis was the supreme ruler of the southern skies between eight and six million years ago, after which it disappeared in an instant. We still have no idea why it died out so suddenly. Until the ice ages of North America there were no teratorns to be found anywhere in the fossil record. All of the succeeding birds were much smaller than this gargantuan beast, with the best-known like Teratornis being just half the size of Argentavis. These northern teratorns encountered humans at the end of the Pleistocene, roughly 11,000 years ago. They probably influenced the various Thunderbird myths of the Native Americans, stories which are still told today. We do not know what the first Americans would have thought had they seen Argentavis blotting out the sun with its titanic wings. Neither do we know if another bird or family of birds will evolve to such proportions in the future.