Lead study author Michael Lee, an associate professor of ecology and evolutionary biology at The University of Adelaide, said that birds are the remnants of the fastest-evolving group of dinosaurs. The family tree that his team created tracked more than 1,500 skeletal characteristics over 50 million years, showing that the theropods—the carnivorous dinosaurs, including Tyrannosaurus, that would eventually become birds—shrank markedly at least 12 times. Starting from an average mass of 163 kilograms, the theropod suborder eventually produced the .8 kilogram Archaeopteryx, which is considered the earliest bird.
The theropods, Lee said, were the only group to continually push the envelope when it came to skeletal size. It's possible that herbivores simply couldn't shrink, since a plant-based diet requires a larger gut for digestion. Meanwhile, theropods could explore alternate resources, habitats, and even prey. "It would have permitted them to chase insects, climb trees, leap and glide, and eventually develop powered flight," Lee said. "All of these activities would have led to novel new anatomical adaptations." So as the dinosaurs shrank, their other features evolved more quickly (which led to faster shrinking to take advantage of these new abilities, and so on).
It's possible that the small size of bird ancestors provided more than one entry into flight: Their small size would make it physically possible for wings to lift their bodies into the air, while larger relatives relied on gliding instead. The awkward gait of a bird—a symptom of its forward center of gravity, which makes flight possible—would have been impossible on a larger frame as well.
Even flight-capable feathers might have miniaturization to thank for their evolution. We now know that simple feathers existed as insulation and decoration on dinosaurs that would never fly, and only later developed into aerodynamic aids. But they might have started to become more elaborate and cover more of the body, Lee said, because smaller dinosaurs needed more powerful insulation to keep from shivering. "Ultimately, those proved to be incredibly useful for flight," Lee said.
Thanks to this and other recent studies, we now know more about the origin of the bird than almost any other evolutionary transformation of the same magnitude. In fact, our fossil record is so complete (with nary a missing link left to be found) that it's hard to say where theropods ended and birds began. "We now know beyond a doubt that birds are a subgroup of dinosaurs," Lee said, "In the same way as humans are a subgroup of mammals."
So the origin of birds is far from a mystery. Not all flying vertebrates have been as forthcoming, however. Pterosaurs—the flying reptiles that were contemporaries of many dinosaurs—went extinct, and we don't know much about their evolution. And bats, whose fragile skeletons hardly ever survive in the fossil record, pose questions as well. "We don't have any truly transitional pterosaur or bat fossils yet," Lee said, "But they must be out there, somewhere."