Why Have Birds Never Gotten as Big as T. Rex?
Even the most massive birds have never reached the sizes of their dinosaur relatives
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Evolution has a fondness for big birds. During the past 66 million years, repeated on different continents and islands all over the world, avian dinosaurs have reached prodigious sizes and even become apex predators in their ancient habitats. Ten-foot-tall elephant birds strutted across Madagascar until a thousand years ago. The sharp-beaked “terror birds” of prehistoric South America were formidable carnivores for tens of millions of years. And the nearly seven-foot-tall, nut-cracking Diatryma strutted through ancient forests of western North America in search of ripe fruit and nutritious seeds 45 million years ago. Such enormous birds almost seem like a return to the Mesozoic days of giant, feathery dinosaurs, which raises the question of whether such avians could ever reach Tyrannosaurus rex sizes.
The repeated evolution of huge birds is part of the dinosaurian legacy. Beaked birds were the only dinosaurs to have survived the asteroid-triggered mass extinction at the end of the Cretaceous. Avians like the six-foot-tall Palaeeudyptes that waddled across ancient Antarctica about 30 million years ago and Titanis, a towering carnivore that was the only terror bird to live in North America between 1.8 million and 5 million years ago, underscore that prodigious dinosaurs were not only relegated to the times of Stegosaurus and Triceratops. The conditions that allowed birds to evolve to large size over and over again have varied from case to case, however, and the process has left a lingering question. If birds possess the traits that opened the possibility of truly giant, multi-ton statures for non-avian dinosaurs, why have we not seen a bird the size of a T. rex?
Often framed as laboratories of evolution, islands can provide some insight into the aviary of big birds our planet continues to host. In a 2023 study, ornithologist Raquel Ponti of Portugal’s University of Porto and colleagues looked at the influence of the “island rule” in birds. The hypothesis contends that on relatively isolated island environments, large species often become smaller and small species become larger. The phenomenon is often attributed to a lack of large, hungry predators like big cats or dogs, allowing island species to evolve in different ways than if ever-hungry predators were influencing island ecology. Birds can get big, in other words, in the absence of predators eating eggs, chicks and even flightless adult birds that can’t simply fly off to escape. The varied moa species of ancient New Zealand, the enormous elephant birds of Madagascar and even the recently extinct dodo of Mauritius are all examples of what can happen when birds settle in on islands.
“When a species colonizes a new environment where there are no predators or competitors, as well as new resources and food availability, the species is free to evolve toward optimal sizes and shapes,” Ponti says. One common trend she and her colleagues noticed is that island-dwelling birds not only tend to become larger than their mainland counterparts, but also evolve longer legs and rounder wings. Both traits are connected to spending more time on the ground and a shift away from migrating by flying, only possible because of the absence of predators eager to pick off birds that spend much of their time foraging, nesting and moving around on the ground.
A landmass does not have to be entirely devoid of predatory animals to foster the evolution of big birds, however. “We know that in the past, large birds evolved alongside large avian predators,” says ornithologist Hanneke Meijer of the University of Bergen in Norway. The large, flightless moas of New Zealand evolved about 17 million years ago, but between 600 to a million years ago they lived alongside the huge Haast’s eagle, the abundance of the large birds perhaps even allowing such a predator to evolve. The important thing, Meijer notes, is that big birds tend to evolve in habitats with plenty of ecological possibility. Isolation on an island without large carnivores can open plenty of possibilities for birds, but so can living on larger landmasses where resources are abundant and predators are few. Big fossil birds such as Diatryma, for example, lived among vast forests around 50 million years ago when mammal carnivores were small and didn’t have as strong an influence on the prehistoric landscape.
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The fearsome “terror birds” of South America certainly benefited from such opportunity. The continent was a giant island between 2.7 million and 66 million years ago, where the biggest mammalian predators ranged from weasel-size to about as big as a leopard, around a hundred pounds. “Isolated in South America without competition from large mammalian carnivores, terror birds were able to evolve into a variety of predatory niches, including apex predators,” says Indiana University Bloomington paleontologist Thomas LaBarge. The carnivorous birds, like the mammals, evolved into a broad array of sizes among coexisting species, undoubtedly with different prey preferences and niches.
Whether on islands today, or larger continents during prehistory, big birds evolved wherever such flightless giants could peck out a niche for themselves. Tall birds called mihirungs —nicknamed “demon ducks” by some—lived on the island continent of Australia between 30,000 and 25 million years ago. And even on ancient North America, which was more connected to Eurasia during the past 66 million years, large birds like Diatryma lived between 45 million and 55 million years ago. The pattern, Ponti notes, suggests that in habitats vacated by large, hungry predators, the easiest thing for some birds to do was get big and not fly. In each of these cases, it appears that birds grew to large sizes relatively quickly, during times that predatory mammals and birds of prey were either rare or absent. “We often look to islands where we see all this crazy experimentation happening due to islands being smaller and isolated, like an evolutionary pressure cooker, but it can happen on the mainland, too,” Meijer notes, “just less often and probably more slowly.”
Despite how often big birds have evolved, however, none have reached the heights or masses of the non-avian dinosaurs that thrived between 66 million and 232 million years ago. For flying birds, the lack of airplane-sized birds like the monster in the 1957 creature feature The Giant Claw can be explained by constraints around flight. Flying is an energy-intensive behavior that requires more and more muscle power the bigger an organism gets, to the point that a flying creature can only become so massive before it’s grounded.
Flightless birds, however, are another story. At least two factors have allowed dinosaurs, including birds, to evolve to a wide range of sizes. Paleontologists have proposed that air sacs emanating from the respiratory systems of many dinosaurs surrounded and invaded their bones, which allowed their skeletons to be lighter without sacrificing strength, with the bonus of helping the animals breathe more efficiently. Egg-laying, too, has been cited as a critical factor in gigantism. Laying eggs, rather than gestating offspring inside, freed dinosaurs from having to carry bigger and bigger babies for longer terms, as elephants and other large mammals do. Birds retain both these traits from their dinosaur ancestors, yet they have never equaled their largest extinct relatives in size.
Even though air sacs and eggs are important contributing factors to giant dinosaur size, these biological quirks removed barriers rather than required bodies to get bigger. In a post-Cretaceous world, birds have likely never experienced sufficient evolutionary pressure to be as tall or as massive as T. rex. Perhaps predatory birds like terror birds could have grown larger, Meijer notes, as being bigger would have allowed them to catch, kill and consume a broader array of prey, but apparently no such evolutionary interaction took place to boost their size further. In fact, a 2023 study suggests that terror birds were stamping on and kicking prey smaller than themselves rather than going after large animals like giant sloths and armadillos of prehistoric South America, perhaps limiting how large they eventually became.
Nevertheless, some impressively big birds roamed our planet. Based on recent finds in South America, LaBarge notes, paleontologists know that terror birds that were at least 10 percent larger than the giant, ten-foot-tall Kelenken lived on the continent. It’s unlikely that terror birds got much larger given that available prey were mostly small, LaBarge says, “but I will say that the largest terror birds absolutely ventured into the size and weight range of some non-avian theropods.” The largest terror birds may have even been the biggest birds of all time.
If there is any possibility for tyrannosaur-size birds, the avians will have to evolve during some future time. With more than 11,000 species of living birds on Earth now, feathery creatures alive today will almost certainly give rise to future giants as our planet continues to change. The only question is how large they may become, a potential outcome in what birds remind us is a persistent and still-unfolding Age of Dinosaurs.
