In the final day of the Cretaceous period, some 66 million years ago, Earth was teeming with a dazzling variety of dinosaurs. In North America, the superpredator Tyrannosaurus rex stalked its favorite prey, the three-horned Triceratops. In Asia, agile raptors eyed herds of duck-billed and armored herbivores, and a menagerie of miniature carnivores and plant eaters roamed the European islands. South of the equator long-necked behemoths heavier than jet airplanes shook the ground as they walked. And all over the world feather-covered dinosaurs flaunted their plumage, some flapping and flying through the air.

Then, suddenly, the Age of the Dinosaurs was over. A massive asteroid slammed into the Gulf of Mexico, triggering a chain reaction of carnage: earthquakes, tsunamis and wildfires followed by years of darkness and cold. It was probably the worst moment in Earth history, and before long three out of every four species were extinct. The asteroid was so catastrophic that it spawned one of the greatest myths in science, one so pervasive and repeated so constantly that most of us think it is true. It is the myth that dinosaurs are gone, felled one and all during the end-Cretaceous extinction.

In fact, some dinosaurs survived the asteroid apocalypse. Although canonical species such as T. rex and Triceratops perished, members of one dinosaur group managed to endure: birds. Why did birds persevere when every single other type of dinosaur died? Scientists have puzzled over this question for decades. The mystery has deepened in the past 30 years as paleontologists have uncovered scores of feathery and winged dinosaurs that were closely related to birds and similar in many aspects of biology and behavior, though not actually part of the avian lineage. Some of these dinosaurs could even fly. What, then, allowed birds alone to escape the fate of their family?

Recently an answer has emerged, based on new research into fossils, genetics and ecology. Many birds were flying over the heads of T. rex and Triceratops when the asteroid hit, and most died alongside their dinosaur cousins. The only birds to make it out of the Cretaceous were modern-style species. Their survival came down to circumstance: where they happened to live and the features they happened to possess served them in good stead when the world went to hell.