Why Aren't Birds Dinosaur-Sized?

In case you haven't been paying attention over the last 20 or 30 years, the evidence is now overwhelming that modern birds evolved from dinosaurs to the extent that some biologists maintain that modern birds *are* dinosaurs (cladistically speaking, that is). But while dinosaurs were the largest terrestrial creatures ever to roam the earth, birds are much, much smaller, rarely exceeding a few pounds in weight. Which raises the question: if birds are descended from dinosaurs, why aren't any birds the size of dinosaurs?

Actually, the issue is a bit more complicated than that. During the Mesozoic Era, the closest analogues to birds were the winged reptiles known as pterosaurs, which weren't technically dinosaurs but evolved from the same family of ancestors. It's a striking fact that the largest flying pterosaurs, like Quetzalcoatlus, weighed a few hundred pounds, an order of magnitude larger than the largest flying birds alive today. So even if we can explain why birds aren't the size of dinosaurs, the question remains: why aren't birds even the size of long-extinct pterosaurs?

Some Dinosaurs Were Bigger than Others

Let's address the dinosaur question first. The important thing to realize here is that not only aren't birds the size of dinosaurs, but not all dinosaurs were the size of dinosaurs, either — assuming we're talking about huge standard-bearers like Apatosaurus, Triceratops and Tyrannosaurus Rex. During their nearly 200 million years on earth, dinosaurs came in all shapes and sizes, and a surprising number of them were no bigger than modern dogs or cats. The smallest dinosaurs, like Microraptor, weighed about as much as a two-month-old kitten!

Modern birds evolved from a specific type of dinosaur: the small, feathered theropods of the late Cretaceous period, which weighed five or ten pounds, soaking wet. (Yes, you can point to older, pigeon-sized "dino-birds" like Archaeopteryx and Anchiornis, but it's not clear if these left any living descendants). The prevailing theory is that small Cretaceous theropods evolved feathers for insulation purposes, then benefited from these feathers' enhanced "lift" and lack of air resistance while chasing prey (or running away from predators).

By the time of the K/T Extinction Event, 65 million years ago, many of these theropods had completed the transition into true birds; in fact, there's even evidence that some of these birds had enough time to become "secondarily flightless" like modern penguins and chickens. While the frigid, sunless conditions following the Yucatan meteor impact spelled doom for dinosaurs large and small, at least some birds managed to survive — possibly because they were a) more mobile and b) better insulated against the cold.

Some Birds Were, in Fact, the Size of Dinosaurs

Here's where things take a left turn. Immediately after the K/T Extinction, the majority of terrestrial animals — including birds, mammals, and reptiles — were fairly small, given the drastically reduced food supply. But 20 or 30 million years into the Cenozoic Era, conditions had recovered sufficiently to encourage evolutionary gigantism once again--with the result that some South American and Pacific Rim birds did, in fact, attain dinosaur-like sizes.

These (flightless) species were much, much bigger than any birds alive today, and some of them managed to survive right up to the cusp of the modern era (about 50,000 years ago) and even beyond. The predatory Dromornis, also known as the Thunder Bird, which roamed the plains of South America ten million years ago, may have weighed as much as 1,000 pounds. Aepyornis, the Elephant Bird, was a hundred pounds lighter, but this 10-foot-tall plant-eater only disappeared from the island of Madagascar in the 17th century!

Giant birds like Dromornis and Aepyornis succumbed to the same evolutionary pressures as the rest of the megafauna of the Cenozoic Era: predation by early humans, climate change, and the disappearance of their accustomed sources of food. Today, the largest flightless bird is the ostrich, some individuals of which tip the scales at 500 pounds. That's not quite the size of a full-grown Spinosaurus, but it's still pretty impressive!

Why Aren't Birds as Big as Pterosaurs?

Now that we've looked at the dinosaur side of the equation, let's consider the evidence vis-a-vis pterosaurs. The mystery here is why winged reptiles like Quetzalcoatlus and Ornithocheirus attained 20- or 30-foot wingspans and weights in the neighborhood of 200 to 300 pounds, while the largest flying bird alive today, the Kori Bustard, only weighs about 40 pounds. Is there something about avian anatomy that prevents birds from attaining pterosaur-like sizes?

The answer, you may be surprised to learn, is no. Argentavis, the largest flying bird that ever lived, had a wingspan of 25 feet and weighed as much as a full-grown human being. Naturalists are still figuring out the details, but it seems that Argentavis flew more like a pterosaur than a bird, holding out its massive wings and gliding on air currents (rather than actively flapping its huge wings, which would have made excruciating demands on its metabolic resources).

So now we face the same question as before: why aren't there any Argentavis-sized flying birds alive today? Probably for the same reason that we no longer encounter two-ton wombats like Diprotodon or 200-pound beavers like Castoroides: the evolutionary moment for avian gigantism has passed. There is another theory, though, that the size of modern flying birds is limited by their feather growth: a giant bird simply wouldn't be able to replace its worn-out feathers fast enough to remain aerodynamic for any length of time.