fossils

The biggest bird of all time once patrolled the skies of Patagonia

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Vasika
Udurawane

Writer
The heaviest flying bird of all time dominated the skies of Patagonia with iron wings. It strode overland while on the hunt, swallowing its prey in one mouthful. It was certainly pushing the limit of heavy, feathered flying creatures but was still able to patrol immense distances of grassland bordered by the young Andes. How did a bird with such a huge wingspan even manage to get off the ground?

Argentavis magnificens, the "magnificent Argentine bird" deserves its evocative name. In life it was among the most massive of all flying birds. Today the heaviest flying birds are swans and a family of birds called bustards. None of them match the prehistoric giant in terms of weight. Bustards for one, are still short-winged ground birds that forage for plants and small animals. Argentavis was a bird of prey and its wingspan was incredible. On the ground this aerial colossus was two meters tall and its wings were at least six meters across. This makes its span one of the greatest of all birds, exceeded by a few bony-toothed seabirds and the biggest members of a group of flying reptiles called pterosaurs from the Mesozoic.


It was a teratorn, part of a family of normally big carnivorous birds. These creatures ranged from condor-sized to twice that in the case of Argentavis. The majority of these animals existed during the Pleistocene in North America and would have encountered the first Americans towards the end of the period. Argentavis’ fossils date back to the final days of the Miocene. It is the biggest teratorn to show up in the fossil record but not the earliest. This goes to Taubatornis, a much smaller bird whose fossils are about 25 million years old.


All in all the teratorns are a rather poorly-known family. For a long time they were depicted as being like vultures and certainly they are now thought to be related to the New World vultures. For a while the position of even these was disputed, bouncing from the traditional raptors to being stork relatives. Currently teratorns are depicted much more like eagles in their habits, lacking the bald heads associated with a vulture. This is due to having long beaks and a wide gape like other actively predaceous birds. But Argentavis and its relatives did not swoop on or carry off their prey with their talons. Today these huge birds are displayed striding across open grassy plains.


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.

Image Credit: Julio Lacerda

3986b46bb4f35aa1ff4d42167a12e0fc

Vasika
Udurawane

Writer


http://sulc.us/le88g
http://www.eartharchives.org/articles/the-biggest-bird-of-all-time-once-patrolled-the-skies-of-patagonia/