Argentavis

Argentavis magnificens was among the largest flying birds ever to exist, quite possibly surpassed in wingspan only by "Pelagornis sandersi", which was described in 2014. "A. magnificens", sometimes called the Giant Teratorn, is an extinct species known from three sites in the Epecuén and Andalhualá Formations in central and northwestern Argentina dating to the Late Miocene (Huayquerian), where a good sample of fossils has been obtained.
Description.
The single known humerus (upper arm bone) specimen of "Argentavis" is somewhat damaged. Even so, it allows a fairly accurate estimate of its length in life. "Argentavis" humerus was only slightly shorter than an entire human arm. The species apparently had stout, strong legs and large feet which enabled it to walk with ease. The bill was large, rather slender, and had a hooked tip with a wide gape.
Size.
"Argentavis" wingspan estimates varied widely depending on the method used for scaling, i.e. regression analyses or comparisons with the California condor. At one time, wingspans have been published for the species up to but more recent estimates put the wingspan more likely in the range of . Whether this span could have reached appears uncertain per modern authorities. At the time of description, "Argentavis" was the largest winged bird known to exist but is now known to have been exceeded by another extinct species, "Pelagornis sandersi", described in 2014 as having a typical wingspan of . "Argentavis" had an estimated height when standing on the ground that was roughly equivalent to that of a person, at , furthermore its total length (from bill tip to tail tip) was approximately .
Prior published weights gave "Argentavis" a body mass of , but more refined techniques show a more typical mass would likely have been , although weights could have varied depending on conditions. "Argentavis" retains the title of the heaviest flying bird known still by a considerable margin, for example "Pelagornis" weighed no more than . For comparison, the living bird with the largest wingspan is the wandering albatross, averaging and spanning up to . Since "A. magnificens" is known to have been a land bird, another good point of comparison is the Andean condor, the largest extant land bird going on average wing spread and weight, with a wingspan of up to . This condor can weigh up to . New World vultures such as the condor are thought to be the closest living relations to "Argentavis" and other teratorns. Average weights are of course much less in both the albatross and condor than this teratorn, at approximately and , respectively.
The ability to fly is not a simple question of weight ratios, except in extreme cases; size and structure of the wing must also be taken into account. As a rule of thumb, a wing loading of 25 kg/m2 is considered the limit for avian flight. The "heaviest" extant flying birds are known to weigh up to (there are several contenders, among which are the European great bustard and the African kori bustard). An individual mute swan, which may have lost the power of flight due to extreme weight, was found to have weighed . Meanwhile, the sarus crane is the tallest flying bird alive, at up to tall, standing about as high as "Argentavis" due to its long legs and neck.
The largest flying creatures overall that are known to have existed are not birds, but instead distantly-related archosaurs, namely the azhdarchid pterosaurs of the Cretaceous. The wingspans of larger azhdarchids, such as "Quetzalcoatlus" and "Hatzegopteryx", have been estimated to exceed , with less conservative estimates being or more. Mass estimates for these azhdarchids are on the order of and their estimated height on the ground was roughly analogous to an elephant or small giraffe.
Currently accepted estimates for the size of "Argentavis" are:
Paleobiology.
Life history.
Comparison with extant birds suggests it laid one or two eggs with a mass of somewhat over (smaller than an ostrich egg) every two years. Climate considerations make it likely that the birds incubated over the winter, mates exchanging duties of incubating and procuring food every few days, and that the young were independent after some 16 months, but not fully mature until aged about a dozen years. Mortality must have been very low; to maintain a viable population less than about 2% of birds may have died each year. Of course, "Argentavis" suffered hardly any predation, and mortality was mainly from old age, accidents and disease.
Flight.
From the size and structure of its wings, it is inferred that "A. magnificens" flew mainly by soaring, using flapping flight only during short periods. It is probable that it used thermal currents as well. It has been estimated that the minimal velocity for the wing of "A. magnificens" is about or . Especially for takeoff, it would have depended on the wind. Although its legs were strong enough to provide it with a running or jumping start, the wings were simply too long to flap effectively until the bird was some height off the ground. However, skeletal evidence suggests that its breast muscles were not powerful enough for wing flapping for extended periods. "Argentavis" may have used mountain slopes and headwinds to take off, and probably could manage to do so from even gently sloping terrain with little effort. It may have flown and lived much like the modern Andean condor, scanning large areas of land from aloft for carrion. The climate of the Andean foothills in Argentina during the late Miocene was warmer and drier than today, which would have further aided the bird in staying aloft atop thermal updrafts.
Studies on condor flight indicate that "Argentavis" was fully capable of flight in normal conditions as modern large soaring birds spend very little time flapping their wings regardless of environment.
Feeding.
"Argentavis" territories measured probably more than , which the birds screened for food, possibly utilizing a generally north–south direction to avoid being slowed by adverse winds. This species seems less aerodynamically suited for predation than its relatives. It probably preferred to scavenge for carrion, and it is possible that it habitually chased metatherian carnivores such as Thylacosmilidae from their kills. Another apex predator on land at the time and place were the giant, ground-dwelling "terror birds", phorusrhacids, which too may have been displaced from kills. Unlike extant condors and vultures, teratorns generally had long, eagle-like beaks and are believed to have been active predators. This is seemingly true as well of "Argentavis" but other teratorns were likely far less ponderous considering the substantial size differences. "Argentavis" may have used its wings and size to intimidate lone land predators of their kills. "Argentavis" may have also ambushed some small live prey, i.e. large rodents, small armadillos and the young of large animals such as ground sloths. The species would've required about of meat each day. When hunting actively, "A. magnificens" would probably have swooped from high above onto their prey, which they usually would have been able to grab prey by its bill, kill, and swallow without landing. However, they may too have lied in wait from a ground position, which would render them likely grounded until heavy winds allowed them to fly. Skull structure suggests that it ate most of its prey whole rather than tearing off pieces of flesh.