The last giant flyers
Contrary to popular misconception, birds are about the worse flyers of the three volant tetrapod groups. Their feathered wings are made of dead tissue, making them numb to relevant data captured by pterosaur and bat wings; while resistent to damage, they are more easily grounded than pterosaurs or bats, as bats can still fly with 30% of their wing missing, while birds can be rendered flightless by simply shortening their wing feathers slightly. Birds have heavy hindlimbs, and waste a lot of energy taking off, characteristics pterosaurs and batswere free from altogether. Finally, their aberrant wing muscle structure distorts their flight profile.
Perhaps it is not surprising, then, that birds accomplished more on the ground than on the air. Bats still hold the reccord for flight manouverability, pterosaurs still hold the reccord for the most terrestrial lifestyles while still retaining flight (the closest avian analogues to dsungaripteroids and tapejarids are flightless, after all).
Size, therefore, is one of those reccords one would expect birds to fail at. While bats have unfortunately been kept small due to the necessities of avoiding predatory birds, pterosaurs reached the largest sizes of any flying animal ever known, and on average most pterosaurs were bigger than birds. The smallest known pterosaurs have wingspans of 50 centimeters, but the average small size for pterosaurs are wingspans of 1 meters, and most pterosaurs known range from 1.5 meters to 3 meters. By contrast, seeing birds with a wingspan of over 1 meter is considered a lucky rare event on most parts of the world.
However, two clades of birds managed the impossible, and produced flying animals comparable in size to the largest pterosaurs known. These were the marine albatross-like pelagornithids and the terrestrial vulture-like teratorns, the latter producing the biggest flying bird ever known, the titanic Argentavis at a whopping wingspan of 8 meters, rendering it the second largest flying animal to have ever existed. How these birds ever flew is far greater a mystery than how pterosaurs did it; while the giant pterodactyloids could launch themselves using their forelimbs, teratorns and pelagornithids still had the detrimental heavy hindlimbs and were forced to run. More amazing yet, teratorns were very terrestrial birds, hunting on the ground like azhdarchids; they had to waste a lot of energy every day to launch into the sky.
Suggestions of unique atmospheric conditions have been suggested, and this might be partially true for Argentavis – who lived in an epoch where the Andes were less tall, and Pacific winds blew across South America -, but such bizarre conditions are unlikely to have been the constant.
This rings especially true for the last of the great avian flyers: North America’s Giant Teratorn, Aiolornis incredibilis.
Amidst the giant condors
Aiolornis incredibilis was once considered part of the iconic north american teratorn genus, Teratornis itself. However, in 1999 a morphological examination of teratorn fossils clearly placed it in another genus. Aiolornis predates Teratornis considerably, having been around since the Pliocene, about the time Argentavis‘ youngest remains date from (direct ancestry can probably be excluded), and lasted until the latest Pleistocene, having died out slightly before the smaller Teratornis. Due to the large temporal span, it has been suggested that Aiolornis incredibilis might actually represent several different species, though equally old species are not new or rare (Panthera tigris, for example).
Teratornithidae is wooefully understudied. With the exception of Argentavis, all described teratorns are Pliocene/Pleistocene north american taxa, with several undescribed south american and possible caribbean fossils. Teratornithidae is almost certainly south american in origin, and likely dating from as far back as the Eocene. They are part of the New World vulture assemblage, though having diverged very early, and almost certainly outside of Cathartidae. Where New World vultures fit in the avian tree of life is a mystery, with studies ranging from Accipitriformes to Ciconiformes. The most recent genetic studies classify them as basal falcons, thus being part of the seriema + falcon + parrot + passerine assemblage.
The largest modern flying bird
With the extinction of the last pelagornithids in the mid-Pleistocene, and having only died out a few millenary decades ago by anthropogenic influence (see below), Aiolornis was the largest modern flying bird. On average, it had a wingspan of 5 meters, with exceptional specimens reaching 5.5 meters, and with an estimated weight of 23 kilos. Size wise, it was comparable to the average azhdarchid and thalassodromedid pterosaur, which it probably resembled ecologically (again, see below). It was considerably larger than the most common teratorn, Teratornis merriami, which it exceeded in size by 40%, but closely matched by the other member of the genus, Teratornis woodburnensis, which reached a wingspan of 4 meters. Another teratorn, Cathartornis gracilis, is known from the Pleistocene of La Brea, but it was slightly smaller than Teratornis merriami. Teratornis merriami was appearently the most common of these birds, with all other taxa, Aiolornis included, being mostly known from far fewer specimens; size appearently mattered relatively little to the success of the individual species, as the small Cathartornis is rarer than Teratornis merriami, and Aiolornis is more common than Teratornis woodburnensis.
Fossils from the Pleistocene of Eurasia show that Old World vultures did reach sizes comparable to Teratornis merriami, but giants on the level of Aiolornis were nowhere to be found.
With the exception of Teratornis woodburnensis, all north american teratorns occur south of La Brea. This likely in part due to the relative poor fossil reccord, though habitat restrictions imposed by the glaciations are also a possibility. These birds appearently occured in scrub and grassland biomes, as typical of large flying terrestrial birds.
Historically, teratorns have been considered scavengers, and not unreasonably so, given their cathartid relations, their powerful beaks and their occured in the La Brea pits alongside other scavengers. However, a predatory interpretation is omnipresent in literature: the powerful, terror bird-like beaks are quite overbuilt for a scavenger, and combined with their long and poweful legs, a terrestrial predatorial lifestyle not unlike that of the azhdarchid pterosaurs has been suggested long before azhdarchids were found to be terrestrial. The exact predatorial behaviour of these birds was definitely varied; while Cathartornis, with it’s long and slender legs, was like quite analogous to modern secretary birds, Teratornis merriami was probably closer to modern condors in lifestyle, and the truly spectacular forms like Aiolornis and Argentavis were probably like aerial phorhusrhacids, being capable of dispatching considerably large prey. Coincidently, the arrival of giant teratorns seems to coincide with the decline of the flightless terror birds.
In response, there has been some suggestions of depicting teratorns with feather-covered, eagle-like heads rather than naked vulture-like ones. It’s worth to note, though, that the reason vulture heads are naked is frequently more due to thermal concerns rather than hygienic ones, and several scavenging and predatory birds have fully feathered and naked heads, respectively.
Aiolornis co-existed with the famous massive variety of Pleistocene megafauna, from titanic mammoths and giant ground sloths to the saber-toothed cats and american lions. Aiolornis likely fed on the lesser members of this assemblage, such as the calves or the weak adults of the several camelid, bovid and horse species. It’s sheer size and bulk, as well as the typical teratorn beak, was sufficient to overwhelm proportionally large prey – studies on Argentavis suggest that animals as big as a llama could be killed -, and being volant allowed these birds great endurance, simply soaring above the chosen target until it fell from exhaustion. Nonetheless,the bulk of it’s diet was probably composed of smaller prey.
Like other large flying birds, Aiolornis was probably a KT strategist, reaching sexual maturity very late and providing small clucthes. Little is known about the teratorn lifecycle, but based on modern vultures they probably spent several months raising a single chick. No fossils of teratorns occur in highlands, and they likely were content to build their nests on trees or large rocks on lowlands, in spite of the presence of many predatory mammals.
How they flew
The most interesting thing about giant teratorns is, of course, how they managed to reach their massive sizes. At 23 kilos, Aiolornis was much heavier than any modern flying bird, and pretty much beyond the limit theorised for the heaviest possible flying birds.
As said before, the main constraint in avian size is their method of taking off. Pterosaurs, with their quadrupedal launching mechanisms, saved a lot of energy and did not had to produce detrimentally massive hindlimbs, allowing them to reach giant sizes on average. Birds, being forced to run for lift and having heavy hindlimbs, waste a lot of energy pulling themselves into the air, and as such large forms need special conditions to ensure such a high amount of energy is not lost.
There is one group of birds that has solved this problem, however. Galliformes, despiste common claims of being poor flyers, are actually specialised vault launchers: with extremely powerful flight muscles, even the largest forms, like wild turleys, can launch quickly like bullets into the air, forsaking having to run to gain lift.
Here’s an example:
It’s not known if teratorns could vault like that, but modern condors do take off equally as quick despiste their bulk, so it is not unlikely, especially in an environment with several terrestrial predators.
Teratorns share with condors several specialisations to stactic soaring, having their index finger producing a wide platform to support longer and more rigid flight feathers. Modern condors, like other large flying birds, also have an extensive forelimb pneumacy, with airsacs on the propatagium; the same likely applies to teratorns.
Aiolornis died out in the late Pleistocene, slightly before it’s relative Teratornis. Historically, their extinction was thought to be connected to the death of the mammalian megafauna, but since it is thought that teratorns were not dedicated scavengers, the extinction of smaller herbivorous prey mammals like horses and camelids seem to have been a larger factor. Most likely, however, was direct competition and possible predation from human colonisers. Much like New Zealand’s Haast’s Eagle, there is no evidence that teratorns hunted humans, but the larger forms like Aiolornis could had been a potential threat.