Where are all the dinosaur-sized birds?

Added: 2026-05-12

[00:00:01]But, our dinosaurs, birds are dinosaurs.

[00:00:03]Yes, I know.

[00:00:05]But listen, that's the whole point of this video.

[00:00:08]Everyone knows that birds are descended from and can technically still be classified as dinosaurs.

[00:00:14]But just how far would Hammond get with that argument if those Jurassic Park gates swung open and guests were greeted by nothing more than a couple of pigeons?

[00:00:23]One of the main things that fascinates us about dinosaurs is just how bloody big they were.

[00:00:27]Theropods like Tyrannosaurus Rex and Spinosaurus are the heaviest predators to ever roam the earth.

[00:00:33]The massive hadrosaur Shantungosaurus was the length of three elephants.

[00:00:38]And sauropods like Argentinosaurus were so gigantic that it's genuinely hard to imagine them existing on the same planet as us.

[00:00:46]As the dinosaurs' last living representatives, birds haven't done too badly for themselves, it has to be said.

[00:00:52]The Cenozoic is often called the age of mammals, but birds actually outnumber us by about 4,000 species.

[00:00:59]They inhabit every continent [music] and come in a wide range of shapes, lifestyles, colors, and sizes.

[00:01:06]Yet despite their diversity, birds have never gotten close to the titanic sizes of their ancient ancestors.

[00:01:12]And at least in some ways, that is quite surprising.

[00:01:16]The thing is, there are some logical reasons why even the mammals we might call megafauna never got as large as the largest of dinosaurs, with one pretty notable aquatic exception.

[00:01:26]Since mammals give birth to live young, they have to carry their offspring around inside their bodies while they develop.

[00:01:33]And larger offspring need more time to grow from a tiny embryo to a fully developed infant that can survive outside the womb, leading to more costly, more time-consuming [music] pregnancies.

[00:01:44]Dinosaurs obviously didn't have this problem, as their young could develop outside their bodies, hatching from relatively small and inexpensive eggs.

[00:01:52]In fact, the sauropod eggs that scientists have found are only about 15 cm [music] in diameter, which sounds pretty big. You wouldn't like to lay one, but actually this is about the same size as a modern ostrich egg.

[00:02:05]The theropod and sauropod dinosaurs also benefited from the pneumatized skeletons. Their bones are honeycombed with cavities, holes which could fill with air, and were directly linked to their specialized respiratory systems.

[00:02:17]These systems included not just the throat and the lungs, but also several large air sacs spaced throughout their bodies.

[00:02:24]Dinosaurs didn't just have hollow bones, they were basically inflatable.

[00:02:29]This system initially evolved to help dinosaurs extract more oxygen from the atmosphere, but it would have also reduced the density of their bodies, allowing them to be a lot lighter than they looked, and helping them grow to such gigantic sizes.

[00:02:42]Lacking these features, a land mammal when scaled up to the size of a colossal sauropod would likely be crushed under the weight of its own skeleton.

[00:02:51]On the other hand, birds have pneumatized bones and air sacs, just like dinosaurs.

[00:02:56]Unlike dinosaurs, they lay eggs.

[00:02:58]In other words, they've retained these adaptations that allowed the terrible lizards to get so terribly large.

[00:03:05]So, why is the biggest bird in the world, the common ostrich, dwarfed not just by the dinosaurs, but also the largest mammals alive today?

[00:03:14]In this video, we'll take a look at the most hulking bird species to have ever existed, and try and work out why they stopped short of becoming true titans.

[00:03:23]A significant factor that might limit birds between proportions is the same one that's helped them see such stunning success, their ability to fly.

[00:03:31]This might seem pretty obvious. The bigger a bird gets, the bigger its wings and flight muscles need to be, the harder it needs to flap, and the more energy it requires.

[00:03:41]That could set the upper bounds for bird sizes, preventing monstrous airplane-sized avians from evolving.

[00:03:48]But actually, it's a bit more complicated than that.

[00:03:51]Large birds fly by soaring, riding the air currents and extracting their energy from the environment. And this makes flying a lot less energy intensive.

[00:03:59]One of the heaviest flying birds, the Andean condor, has been observed gliding for 5 hours, covering more than 100 miles without flapping its wings a single time.

[00:04:10]Adaptations for soaring have allowed birds to get bigger than you might expect and still remain capable of flight.

[00:04:17]The largest example is the elegasin pelagornis, a pseudo-toothed seabird, whose ridiculously long wings stretch 24 ft, double the wingspan of the wandering albatross, which holds the record today.

[00:04:29]There's also the terrifying argentavis, which had smaller wings than pelagornis, but was significantly heavier, weighing roughly as much as an adult man.

[00:04:39]It may have lived a lot like the modern-day condor, soaring through the sky for hours, only landing when it found carrion to eat.

[00:04:46]The extent to which argentavis was also an active predator is still hotly debated, but its flying method is certain. It was most definitely a glider.

[00:04:55]In fact, its flight muscles weren't strong enough to continuously flap its own wings.

[00:05:01]This is as big as airborne avians ever got, and perhaps as big as they could get.

[00:05:06]However, the limiting factor is likely to be not flying itself, but just how tricky it is to start flying.

[00:05:13]Taking off from the ground is where size and weight becomes a more serious problem, with large birds often requiring an awkward taxiing period or a lot of exhausting flapping to actually get airborne.

[00:05:24]There must come a point where the immense energy cost of takeoff or the risk of not being able to actually launch without the right conditions outweighs any advantages gained by sizing up.

[00:05:35]This might help to explain how pterosaurs, in particular the azhdarchid family, which includes the famous quetzalcoatlus, were able to get so big they make these prehistoric birds look puny, while never losing their flight.

[00:05:48]Since they relied on their forelimbs for walking and for flight itself, they had a natural advantage when it came to getting large as most of their musculature could be focused in the one set of limbs.

[00:05:58]Large birds, meanwhile, must have powerful legs for walking and launching and powerful wings for flying, giving them twice as many strong muscles to fuel.

[00:06:07]It's also theorized that pterosaurs used all four limbs for takeoff, which might have put the same strain on their metabolisms, but would have also provided more lift than birds' measly two legs, allowing for flight to be retained at larger sizes.

[00:06:21]As obligate bipeds, birds can't use their wings to support themselves and walk like pterosaurs and bats can, so this was never really an option for them.

[00:06:30]Actually, there is a slight exception, the hoatzin.

[00:06:34]This Amazon-dwelling bird looks like it fell out of the Cretaceous, not least because its chicks have two functional claws on each wing tip, which they use for climbing before they're able to fly.

[00:06:44]While it's an intriguing reminder of birds' theropod ancestry, the forelimb-powered movement of the hoatzin chicks is pretty rudimentary and awkward, and their claws are lost by adulthood.

[00:06:55]So, that may explain why flying birds haven't got bigger, but as those of you who watched my bats video may recall, the separation of their wings and legs make birds uniquely suited to becoming flightless.

[00:07:06]This is something that's happened time and time again, and it's often associated with developing a large body size.

[00:07:13]For instance, the ratites, that's the ostrich, emu, cassowary, kiwi, and rhea, share many similarities, but they're not actually all that closely related.

[00:07:24]Instead of an ancestral bird losing flight and then splitting off in all these different directions, the ratites actually descended from a flying common ancestor as much as 80 million years ago, long before the end Cretaceous extinction.

[00:07:37]They lost their flight and gained their larger sizes independently, which gives you some idea of just what a common path this is for birds to take.

[00:07:45]Once again, we can find even bigger examples of flightless birds by turning back the clock.

[00:07:50]And this time, we don't need to go far to find the record breakers.

[00:07:54]The elephant birds of Madagascar and the giant moa of New Zealand were larger than any bird alive today.

[00:08:00]And both went extinct in the last 1,000 years as a result of human activity.

[00:08:06]The giant moa, [music] or Dinornis, is the tallest bird we have evidence for.

[00:08:10]Females could grow close to 12 ft tall.

[00:08:12]And astonishingly, were twice the size of the males.

[00:08:16]An adaptation that may have arisen from interspecies competition.

[00:08:19]>> [music] >> And an arms race to produce larger, more competitive offspring.

[00:08:23]Meanwhile, the elephant bird is the heaviest bird that we know of.

[00:08:27]The largest species, Aepyornis maximus, was 10 ft tall. And thanks to its stocky build, it had an incredible maximum weight of around 850 kg. Comparable to a small bull moose. Or a large polar bear.

[00:08:42]Despite looking like they could happily swallow you up, both the elephant bird and giant moa were plant, fruit, and seed-eating browsers. Filling the large herbivore niches in their respective ecosystems.

[00:08:54]Still, that doesn't mean they would have posed no danger.

[00:08:56]Like modern ratites, they were likely pretty territorial.

[00:08:59]And capable of kicking you to pieces if you troubled their eggs.

[00:09:04]These two genus are classic examples of island gigantism.

[00:09:08]Where animals adapt to an isolated environment with reduced competition by growing in size.

[00:09:13]But evolving on islands may also have moderated their magnitude.

[00:09:19]With many examples both modern and prehistoric, island dwarfism is another common phenomenon.

[00:09:24]That is often explained by a reduction of resources. Favoring a reduction in size.

[00:09:30]It seems to sometimes take place even on very large islands like Madagascar.

[00:09:34]Where for instance, the extinct Malagasy hippo evolved to be significantly smaller than those on mainland Africa.

[00:09:40]So, perhaps there was a size limit inherent to these island habitats.

[00:09:45]But, there's no hard rule that says it must be islands where birds become big and go flightless.

[00:09:51]All that's really required are that predators are few, resources plentiful, and large animal niches unoccupied.

[00:09:58]That was the case for the ancestors of the Dromornithids, Australia's demon ducks of doom.

[00:10:04]This family emerged in Australia during the Oligocene, where it was able to take advantage of a relative lack of big mammals to again fill a large herbivore niche with a heavy beak designed to crush tough seed pods and hard-shelled fruits.

[00:10:18]The Dromornithids slowly grew from an early species about the size of a cassowary to 10-ft tall giants.

[00:10:24]Their success can be chalked up to the ditchiness of contemporary mammalian predators.

[00:10:29]For instance, the early Wakaleo was no larger than a medium-sized dog, and it would be many millions of years before its lineage gave rise to the more fearsome marsupial lion.

[00:10:41]Also evolving on a larger landmass, the forest ratite family responded to the lack of large predators in South America directly after dinosaurs died by becoming those large predators.

[00:10:52]The terror birds reigned for tens of millions of years in their homeland, and even made it into North America to compete with Smilodons as the continents collided.

[00:11:01]Terror bird is the right name for them.

[00:11:03]These creatures had absolutely massive beaks and powerful legs that could sustain speeds of around 35 to 40 mph, comparable to a sprinting wolf.

[00:11:13]Some models suggest that the fastest species could have even given a cheetah a run for its money.

[00:11:20]With their sickle claws and sharp beaks for tearing flesh, the terror birds feel like an echo of the carnivorous dinosaurs that came before them, which again raises the question, why do the largest species cap out at only 9-ft tall?

[00:11:33]It could be to do with the available prey.

[00:11:35]The most massive theropod, T-Rex, only managed to reach its colossal size because it was regularly able to kill and eat large herbivores, like the duck-billed Edmontosaurus and the famous Triceratops.

[00:11:47]But, there's some evidence to suggest that terror birds were adapted for hunting animals as fair bit smaller than themselves.

[00:11:52]For one thing, the terror bird's closest living relative, the seriema, has an unusual method for killing prey by picking it up and throwing it down on hard surfaces, which obviously only works if you're significantly bigger than whatever you're hunting.

[00:12:05]But, more convincingly, an analysis of terror bird's skulls has shown that while their beaks were capable of delivering powerful vertical strikes, they would struggle to cope with the sort of lateral forces which could be thrown up by a large struggling creature, such as a giant ground sloth.

[00:12:21]The matter is far from settled, but if it's true that terror birds were mostly hunting smaller mammals, this would have imposed a clear limit on their maximum size.

[00:12:30]Taken as a whole, flightless birds have come very close to T-Rex in height with Dinornis, but never achieved anything like their bulk.

[00:12:38]And of course, they're totally dwarfed by the largest herbivorous dinosaurs.

[00:12:43]Is it just down to chance to mammals taking the large-bodied niches in most ecosystems before avians could move into them, or are birds lacking in some way that prevents them from getting so bulky?

[00:12:54]Well, for starters, one thing they're obviously lacking is legs.

[00:12:57]And now that I say that, it seems silly that I didn't bring this up sooner.

[00:13:01]Four-legged vertebrates not only have a natural advantage when it comes to balance, spreading their weight across four limbs also means those limbs don't have to be as immensely muscular to provide sufficient support and mobility.

[00:13:12]It can be no coincidence that sauropods and numerous ornithischian dinosaurs started out as small bipedal creatures and became quadrupeds as their body sizes increased.

[00:13:22]But, even compared to large theropods like the Tyrannosaurus and Spinosaurids, birds have a missing bit of anatomy.

[00:13:28]They've abandoned the tail.

[00:13:31]Instead of a long mobile tail, birds' caudal vertebrae are fused together into a shortened pygostyle, which supports the tail feathers. It's one of their many adaptations for reducing weight and improving flight control.

[00:13:43]Tails were important for balance, and they were also a vital muscle attachment site for archosaurs, especially theropods, which had particularly large caudofemoralis muscles between the tail and the thigh bone that powered their locomotion.

[00:13:56]They were particularly impressive in T-Rex to cope with its massive size.

[00:14:00]But as avian dinosaurs developed a more recognizably bird-like body plan, and the tail shrank down, this muscle was considerably reduced, and it has disappeared altogether in a variety of modern species from owls to flamingos.

[00:14:13]With these essential retractor muscles atrophied, how are birds able to manage anything more than ineffectually hobbling around?

[00:14:20]Well, with this tail-driven style of locomotion lost to them, birds have found a new way of walking.

[00:14:26]Their femurs are positioned almost horizontally and remain still, providing balance, while the animals effectively walk from the knee down.

[00:14:34]The upper parts of the leg are usually hidden away by the feathers, and what we often think of as a bird's knees are actually its ankles, which is why they appear to bend backwards.

[00:14:44]This means birds' legs are constantly held in a crouched position, which is good for stability, staying perched on a skinny branch when you don't have a counterbalancing tail, for instance, but bad for growing larger.

[00:14:56]Studies have shown that most tetrapods undergo a straightening of the limbs as they evolved to larger sizes, which reduces muscle strain and increases their efficiency.

[00:15:06]Meanwhile, birds are stuck with these confined bent femurs, which have to stay relatively short as well to keep their knees under their center of mass.

[00:15:14]I'd have to run it past a zoologist to make sure, but I reckon this long-femured bird here would not be an effective design.

[00:15:22]The unusual positioning puts the bones under a lot of extra stress, as well.

[00:15:26]They have to cope with torsion as a bird moves, a twisting force, which long bones like the thigh aren't able to cope with well, the way they can withstand a bit of regular bending.

[00:15:35]The force obviously increases with the size of the animal.

[00:15:38]And so, avoiding spiral fractures seems like as good a reason as any not to become too heavy.

[00:15:44]So, why didn't birds get as big as dinosaurs?

[00:15:46]Well, for both flying and flightless species, it seems the limits are set not by the wings, but the legs.

[00:15:52]Launching into the air off two limbs becomes prohibitively costly at a certain scale.

[00:15:56]And birds' unique body plans, adapted for the skies, place unexpected restrictions on their evolution, even when a bird family has been grounded for millions of years.