These are the modern dinosaurs, and the most biomechanically efficient still-living vertebrates, able to fly at 120 mph and to go around the world with amazing ease. In short, the worthy dinosaur descendants. Here we'll talk about those which range from being slightly older than Archaeopteryx to nearly as young as modern day. On the other hand, we'll not talk about historically extinct birds such as the Dodo or the Elephant-Bird: they have nothing to do with only-prehistoric beasts, and they'll deserve a Useful Notes Page on their own.
Every day I'm shuffling: Xiaotingia & Aurornis There are many dinosaur groups whose interrelationships are uncertain, but none are as confused as the early birds and their relatives. For a long time, things seemed pretty simple: Archaeopteryx was a primitive bird (or, more properly, avialian). Dromaeosaurids and troodontids formed Deinonychosauria, which, along with the birds, formed Eumaniraptora ("true maniraptorans"); together with some more primitive forms, eumaniraptorans formed Paraves ("close to birds"). However, this would soon change. In 2011, a new paravian from Late Jurassic China was named: Xiaotingia ("of Zheng Xiaoting"). The study that introduced this new theropod found a new arrangement of the paravian family tree that was quite different from the previous consensus: not only did Xiaotingia and the supposed troodontid Anchiornis end up closely related to Archaeopteryx, but these forms were found to be primitive deinonychosaurs rather than birds! Predictably, popular media balleyhooed these new findings a lot. However, the authors of this study noted that support for this position was not terribly strong and was subject to change. And change it did. For the next year or so, these three generally bounced around between being primitive birds, primitive deinonychosaurs and primitive paravians. However, in early 2013, a new phylogeny arose. This new analysis found that Deinonychosauria was not a natural group; instead, troodontids were primitive paravians, while the narrow-snouted unenlagiines and four-winged microraptorines — previously regarded as primitive dromaeosaurids — were found to be birds even more primitive than Archaeopteryx, Anchiornis & Xiaotingia. Unfortunately, this analysis was plagued with multiple problems (most notably, the bird group was renamed "Averaptora" ["bird thieves"], which is redundant and best ignored), so its conclusions have not received much support. Fortunately, however, a new paravian from Late Jurassic China was announced a few short months later. Called Aurornis ("dawn bird"), the paper introducing this new form featured yet another new analysis — this one being bigger and better than any before. This one found that the bizarre scansoriopterygids, the supposed troodontid Eosinopteryx, dromaeosaurids (as in most analyses, unenlagiines and microraptorines were placed in the group), troodontids, Aurornis, Anchiornis, Archaeopteryx, Xiaotingia, a new group including Jeholornis (see below) and the Madagascan Rahonavis & the supposed dromaeosaurid Balaur were progressively closer to modern birds. This analysis is tentatively accepted here, but it is unlikely that paravian relationships will be resolved anytime soon.
Birds from ancient China: Confuciusornis & Jeholornis Confuciusornis lived in Early Cretaceous and was from the same famous Chinese Liaoning site in which the popular feathered dinosaur fossils come from. This animal had some evolved traits, for example had already lost its teeth (convergently from modern birds) and shortened its tail, but still retained an old legacy: three-clawed wings. As is easy to think from a Liaoning animal, the Confucius-bird has also preserved prints of feathers, which show two very elongated tail-feathers rather peacock-like. Another basal bird, Jeholornis from the same age and habitat, is also known as "Shenzhouraptor". Another bird from Early Cretaceous China, Sapeornis, has an interesting Science Marches On story. It was once thought to form a group (traditionally called Sapeornithidae, but properly Omnivoropterygidae) with three other birds: Omnivoropteryx, Didactylornis & Shenshiornis. However, further work suggests that these four birds were really just one all along; Sapeornis, as the oldest of these names, is the proper one. Yet another prehistoric Chinese bird, Gansus, was described in 1984, well before the other birds mentioned in this section. This form shows similarities to various groups of living birds (and it does indeed appear to be related to them), but it is most likely a diving bird also capable of flight. It is known from much of a skeleton, but the skull is currently unknown. As with several other feathered creatures from prehistoric China, the color of its feathers has been determined; it had dark-colored plumage.
The Mirror Universe birds: Enantiornithines The most successful Late Cretaceous birds were the Enantiornithes, whose name means "mirror birds". Why? Because of some skeletal features which appear specular compared to modern fliers. They were a sort of middle-ways between the aforementioned Early Cretaceous birds and modern feathered guys, and were very diversified among each other. The Zerg Rush birds in the Walking with Dinosaurs episode about pterosaurs were enantiorns, as well as, arguably, those mentioned in the last episode which made the "omnipresent chorus" from the trees. The Mirror Universe birds went extinct along with non-avian dinos only after the comet/asteroid.
Toothy seabird 1: Hesperornis Hesperornis and Ichthyornis are two of the most famous Dinosaur Age-related birds (not counting Archaeopteryx), both from Late Cretaceous North America. Since hespero is far cooler, here we'll mention it first. Hesperornis lived in the same habitat in which Pteranodonts, Mosasaurs, Elasmosaurs and Archelons roamed: the shallow inland sea which used to cover US Midwest at that time, dividing North America in two parallel stripes of land running from Arctic down to the south. Despite its earliness, Hesperornis was already a very derived bird. 6 ft long (the size of a human), it was flightless, with vestigial wings, short splayed legs for swimming, a long neck, and a long beak with small true teeth. It spent most of its life in water, but returned on land to lay its eggs. Once, the hesperorn was shown as a sort of proto-penguin with an erect pose; we know now its legs were too weak to well-substain its body, and the animal is now portrayed more similar to modern grebes and loons. It was once thought it had palmated feet like a loon, but it's more likely its feet were lobed like a grebe.
Toothy seabird 2: Ichthyornis The much smaller, far less striking Ichthyornis lived in the same age and habitat of the former, but this time we're coping with a sorta toothed, long-billed proto-seagull. The ichthyorn's lifestyle was arguably similar to modern flying sea birds, catching fishes in flight or maybe diving under the sea to pursue them like modern boobies. You can almost be certain that if pterosaurs are involved, these little guys will be depicted as pests who like to steal food from them, a behavior that while possible, has never been proven. Both birds sometimes fell preys of large marine reptiles, as shown by stomach-remains. Ichthyornis and Hesperornis were (almost) full-birdies at that point, and if alive today, they'll be taken for components of modern avifauna. In some artistic works, both Hesperornis and Ichthyornis are depicted black and white like modern gulls or penguins, but their real coloration is totally unknown.
The first full-birdies: Prehistoric neornithines Neornithes (meaning new birds) or colloquially "Neorns", is the name indicating the last common ancestor of all modern birds and all its descendents. Neornithes were the only Cretaceous birds which managed to overcome the mass-extinction and to make their way in the Cenozoic, the Mammal Age. It's worth noting that their descendants, our modern birdies, have much, much more species today than mammals. Most Cenozoic "new birds" were very similar to their descendants: some were rather generic-looking, while others were more specialized, but still not too different to modern avians. Furthermore, their fossil record is extremely scant, maybe the scantiest of all Vertebrates; thus, evolution of the single modern-bird lineages is mostly unknown even today, and their phylogenetic tree is full of question marks. But don't worry...there were also many exceptions to this rule: we're going to talk about these.
When birds ruled the world: Gastornis (once called "Diatryma") Long-standing paleo-fans will remember for sure the name "Diatryma": that large, flightless, large-headed predatory bird who used to hunt the small "horse" Eohippus in so many paleo-artistic depictions. Well, now poor Diatryma seems having definitively disappeared... but luckily, it's not such: it has simply changed identity. Now we have to call it Gastornis (a far less awesome name, we've to admit, but...never mind.) Whatever name should be used, this is actually one of the most enigmatic extinct birds. It might not even be carnivorous at all: its strong beak wasn't hooked like an eagle's, and its body frame was stocky, seemingly slow-moving. Maybe it only was an omnivore who used its bill to crack nuts, cut vegetation, and sometimes, tear flesh from its prey (but it was more probable it swallowed its eohipps whole, like most modern non-raptorian birds). Anyway, it was a real giant in its forestal world, 40 million years ago: while mammals were still small, some birds grew to large size, creating a sort of Bizarro Universe in which mammals could be lower-ranking in the food pyramid.
Running eagles: Phorusrhacos & Titanis With Phorusrhacids (grassland-dwelling non-fliers), we have no doubts this time: thanks to their light weight and slender running legs, they were active hunter of small mammals. Not only that, with their strongly hooked, very eagle-like bill, they did not swallow their prey whole. It has recently been discovered they had even one clawed finger protruding from each of their tiny wings note , for uncertain purpose. Perhaps the most amazing-looking among all prehistoric birds, they have recently nicknamed terror-birds in pop- documentaries (for example, Prehistoric Park.) Originary from South America, they have left a legacy in our modern world as well: the closely-related Seriema is a medium-sized South American bird whose shape and habits resemble a miniaturized "terrorbird". Even though is also nicknamed "terrorbird" sometimes, Gastornis/Diatryma was not related to Phorusrhacids: it has left any descendant since 40 million years. The prototypical South American Phorusrhacos (often misspelled "Phororhacos") and the recently-discovered North American Titanis (first-originated in South America as well) are the two stock species of the family.
The Magnificent Mihirungs: Dromornis & Genyornis Dromornithids were among the largest birds that ever lived (they varied in size from about as big as a cassowary to the largest and Trope Namer of the group, Dromornis stirtoni, 3 meters tall and half a ton in weight); and yet, they've not gained much consideration in popular media, unlike their American contemporary counterparts, the phorusrhacids. It's probably because they likely weren't, fast, vicious killers. Instead, the 'thunderbirds', with their vast bulk, thick, robust bones, hoof-like toes and strong, crushing beaks were browsing and grazing herbivores, slowly plodding across a wetter, more wooded ancient Australian outback. Typical of Australian things, they've been given many nicknames: "thunderbirds" obviously refers to their huge bulk and robust bones; "demon ducks of doom" refers to their closest living relative being the Australian magpie goose, and other waterfowl, and an old debate as to whether they were carnivores. A recent addition to the list is "Mihirung", from an Aboriginal story that might mention them as the "mihirung paringmal" or Giant Emu: it is a certainty that the first people to arrive in Australia encountered them, and possibly drove them extinct (though the jury's still out on that score).
Deadly feast: Teratornis We leave (almost) definitively the flighless bird's world and start to watch more traditional fliers. Among prehistoric flying birds, the most depicted (and most striking) are the Teratorns. They were very vulture-like animals, but were actually more related to falcons than to "true vultures": just like modern condors and North-american vultures. The namesake Teratornis is one of the most abundant birds in fossil record, and has been found in huge numbers in the famous Californian tar-pits in which mammalian sabretooths, giant wolves, mastodons and ground sloths have also been found. Arguably, they went to feed on the carcasses of these mammals, and remained stuck in tar just the same.
A feathered airplane: Argentavis The aforementioned Teratornis had an earlier relative, which lived in South America 8 million years before: Argentavis (its name means "argentinian bird"). Why should we mention it separately? Well... simply because, along with giant pterosaurs, it deserves the Giant Flyer title more than every other prehistoric creature. Its wingspan was 25 ft, as much as Pteranodon; its weight 80 kg, as much as the two-times-wider-winged Quetzalcoatlus. Imagine a giant condor with a ostrich-sized body, huge roc-like wings, a sharp uncinated beak, and a love for carrion (and maybe even an occasional hunting attitude). With no doubt, the largest flying bird ever discovered. And yet, Argentavis has yet to appear in fiction. And **heck**, it has actually had one single apparition in documentaries to date: "Paleoworld", as a side-note of Phorusrhacids! Since Walking With Beasts producers did recreate Argentavis world (the Sabretooth episode)... they wasted a perfectly good opportunity.
Toothy seabird 3: Osteodontornis However, Argentavis wasn't the only Giant Flyer in the Cenozoic: we have to add the Pelagorns. These were rather albatross-like or pelican-like marine birds, but they had two cool traits: their wingspan reached 20 ft (a bit less than Argentavis) and their beak was toothed, seemingly revealing the trope Toothy Bird being a Real Life thing in the past. Sadly, this is not true: these "teeth" weren't real teeth, but their bill had an ondulating, pseudo-toothed edge, just like one modern bird, the duck-like Merganser. The only Real Life toothy-birds were those living alongside non-avian dinosaurs, such as Archaeopteryx, Hesperornis, and Ichthyornis (which weren't even typical "birds"). Pelagorns were the new feathered version of Pteranodon, almost as large as it, and went extinct only 1 million years ago. Osteodontornis is the typical member of the group.
Everything's even better with giant penguins: Anthropornis When hesperornithines went eventually extinct at the end of the Mesozoic, a new kind of birds took soon its niche: but this time we're talking about much, much familiar-looking creatures: penguins. Giant penguins. The largest of them, Anthropornis, was nearly as tall as a fully-grown human and weighed 200 kg, more than a modern ostrich; but it probably was as nice-looking as modern penguins are. Giant penguins swum in the southern seas for million years, until they were outcompeted 20 million years ago by a new group of large marine animals, their mammalian equivalents: seals and sea-lions.
The never-never birds: Pterosaurs mistaken for early birds In paleontology, it is very common for isolated bones to be misinterpreted as something very different from what they actually are. One of the best examples of this is the alleged bird Samrukia, from the Late Cretaceous of Kazakhstan. In 2011, Darren Naish of Tetrapod Zoology fame and his colleagues described a large jawbone, which they thought came from a large bird that they named Samrukia ("phoenix"). Later the same year, another paleontologist showed that this jaw did not come from a bird at all, but instead from a pterosaur. History repeated itself about a year later with the reexamination of three other Cretaceous birds. In 1986, two birds were named from the Early Cretaceous of Romania. The first of these, Palaeocursornis ("ancient running bird"), was known only from a leg that showed it to be a flightless runner; the second, Eurolimnornis ("European water bird"), was a conventional flyer. In 2002, another bird was named from Late Cretaceous rocks of the United States. Called Piksi ("big bird"), it appeared to be a fowl-like ground bird. In the latest part of 2012, these birds were shown to be pterosaurs instead, and the leg of Palaeocursornis was shown to actually be an arm. Ironically, all of these forms, originally used to support the theory that Mesozoic birds were surprisingly diverse, now show that pterosaurs truly were the dominant flyers of the time.