|Reconstruction of A. magnificens|
Campbell & Tonni 1980
Campbell & Tonni, 1980
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 Andalhuala Formations in central and northwestern Argentina dating to the Late Miocene (Huayquerian), where a good sample of fossils has been obtained.
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's 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.
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 7.5 to 8 m (24 ft 7 in to 26 ft 3 in) but more recent estimates put the wingspan more likely in the range of 5.09 to 6.5 m (16 ft 8 in to 21 ft 4 in). Whether this bird could've span up to 7 m (23 ft 0 in) 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, was described in 2014 as having a typical wingspan of 7 to 7.4 m (23 ft 0 in to 24 ft 3 in). Its height when standing on the ground was roughly equivalent to that of a person, at 1.5 to 1.8 m (4 ft 11 in to 5 ft 11 in), furthermore its total length (from bill tip to tail tip) was approximately 3.5 m (11 ft 6 in). Prior published weights gave Argentavis a body mass of 80 kg (180 lb) but more refined techniques show a more typical mass would've likely been 70 to 72 kg (154 to 159 lb), although weights could've 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 22 to 40 kg (49 to 88 lb). For comparison, the living bird with the largest wingspan is the wandering albatross, averaging 3 m (9 ft 10 in) and spanning up to 3.7 m (12 ft 2 in). 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 3.3 m (10 ft 10 in). This condor averages roughly 11.3 kg (25 lb) in mass and can weigh up to 15 kg (33 lb). 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 8.5 kg (19 lb) and 11.3 kg (25 lb), 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 21 kg (46 lb) (there are several contenders, among which are the European great bustard and the African kori bustard). A mute swan, which may have personally lost the power of flight due to extreme weight, was found to have weighed 23 kg (51 lb). Meanwhile, the sarus crane is the tallest flying bird alive, at up to 1.8 m (5.9 ft) tall, standing about as high as Argentavis due to its long legs and neck.
The largest known flying creatures overall are not birds, but instead the azhdarchid pterosaurs of the Cretaceous. The wingspans of larger azhdarchids, such as Quetzalcoatlus and Hatzegopteryx, have been estimated to exceed 10 m (33 ft), with less conservative estimates being 12 m (39 ft) or more. Mass estimates for these azhdarchids are on the order of 200–250 kg (440–550 lb).
Currently accepted estimates for the size of Argentavis are:
- Wingspan: 5.09–6.5 m (16 ft 8 in–21 ft 4 in)}}
- Wing area: 8.11 m2 (87.3 sq ft)
- Wing loading: 84.6 N/m2 (1.77 lbs/ft2)
- Body length: 3.5 m (11 ft 6 in)
- Height: 1.5–1.8 m (4 ft 11 in–5 ft 11 in)
- Mass: 70–72 kg (154–159 lb)
Comparison with extant birds suggests it laid one or two eggs with a mass of somewhat over 1 kg (2.2 lb) (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.
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 11 m/s or 40 kilometres per hour (25 mph). 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.
Argentavis' territories measured probably more than 500 square kilometres (190 sq mi), 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 be have 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. largish rodents, smallish armadillos and the young of large animals such as camels. The species would've required about 2.5 to 5 kg (5.5 to 11.0 lb) 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.
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