Terror Birds: South American Predators

Terror birds, the common name for giant terrestrial birds known as Phorusrhacids, were giant predatory birds of South America during the Cenozoic period. They lived throughout the Cenozoic period following the demise of the dinosaurs and only died out a mere two million years ago.

Terror birds were the largest birds to ever exist on Earth, dwarfing even the giant gastornis of the Eocene forests as well as present-day ostriches. Standing nearly ten feet tall, they must have been an intimidating presence indeed. Being so large, the terror bird could not hope to get off the ground, so instead devoted its evolutionary energy to getting larger and faster, making it the top predator in South America during much of the Cenozoic. It sported an enormous head with an equally large, hooked beak that could have made quick work of most mammals it preyed on. The long, powerful legs of the slender bird could have made it a very fast runner, capable of taking down large grazing animals on the South American prairies. Unlike the European gastornis, the terror bird was lightweight considering its size, weighing in at only around 300 pounds. Its success was unmatched by many other predators of the Cenozoic that died out much sooner due to overspecialization or vulnerability to shifts in climate or food sources.

Oddly enough, the terror birds exhibit a kind of reverse evolution. Whereas birds originally evolved from small dinosaurs and other archosaurs, the terror birds reverted back to a number of dinosaur-like qualities. As small birds evolved in the Cretaceous period, the forelimbs of small dinosaurs began to elongate and develop feathers for flight. However, when an animal adapts for life in the air, it must sacrifice body size because of the low weight needed and the enormous food requirements to propel flight. The terror birds probably started out as smaller, flying birds that began to adapt increasingly to life on the ground in the grasslands, much like present-day caracaras. Once the terror birds got bigger, they sacrificed the need for flight altogether, which allowed their bulky wings to shrink and their legs to grow longer, more powerful, and more adapted for running. In the end, they began to resemble the large therapod dinosaurs like tyrannosaurus, with their large head, tiny forelimbs, and long legs.

Terror birds died out shortly after South America collided with North America at the Isthmus of Panama. Some species were able to spread north and be successful in North America, but the land bridge spelled trouble for many South American species of phorusrhacids. Recent discoveries in Texas reveal that some species of terror bird actually made it to North America before the land bridge formed, nearly five million years ago instead of two. It is likely that a combination of a change in climate as well as an influx of large mammalian predators into their once-undisputed domain drove them from existence.