In the right circumstances birds can evolve into giants. In the vast majority of cases they have done this on oceanic islands in the absence of any large land predators and most of these extinct giant birds are decidedly lacking when it comes to predatory ferocity, as birds like the moa and elephant bird attest; big, but decidedly vegetarian animals. However, a long-legged bird living in South America several million years ago, probably very similar to the living seriemas (Cariama cristata and Chunga burmeisteri), gave rise to a group of birds collectively known as terror birds (, technically known as phorusrhacids) and as their name suggests they were not the sort of feathered critters you would find nibbling nuts at a bird-table. They were big birds; the smallest of the 17 known species were at least 1m tall, while Brontornis burmeisteri stood as high as 3m and may have weighed as much as 350–400 kg, but even B. burmeisteri may have looked a bit pathetic next to an even bigger species, the skull of which was discovered by a high school student in Patagonia in 2003. There’s every possibility these animals were the largest birds ever to have lived and all of them were undoubtedly fierce predators. Why these nightmarish birds came to evolve in South America is not fully understood, as no other place on Earth has ever produced a group of predatory giant birds. Gigantism in birds is normally associated with herbivory, yet whatever conditions prevailed in South America many millions of years ago allowed the evolution of a varied group of feathered carnivores that were around for a huge stretch of time; from around 60 million years ago to about 1.8 million years ago, which goes to show how successful these birds were.
Following the extinction of the dinosaurs, many niches in the Earth’s ecosystems were left wide open for the vertebrate survivors – the mammals, birds, and remaining reptiles to evolve into, and for a while, apparently, the terror birds had a power struggle with the mammals for the dominance of the terrestrial ecosystems in South America. Many of them were big and powerful enough to have been the top predators at the time, and many mammals were definitely their prey.
All but one of the terror birds paleontologists know of today have been unearthed in South America. One species (Titanis walleri) managed to reach North America, and it appears to have been quite a success, surviving for more than three million years until it disappeared around 1.8 million years ago – the last of its kind to become extinct. Even though this American species was not the biggest terror bird it must have still been a terrifying animal. Its vital statistics are impressive: 1.4 – 2.5m tall and 150kg in weight. It also had an immense, hooked bill and with such an impressive beak it could have probably swallowed a lamb-sized animal in one gulp.
Although we can piece together the skeletons of the terror birds it’s impossible to know what their plumage was like. However, we can look at living birds for clues, and if the other flightless birds are anything to go by, the terror bird’s feathers may have been rather hair-like. Like the vast majority of flightless birds, terror birds had stubby little wings, but what they lacked in the wing department they more than made up for with their long, powerful legs that ended in large feet and long claws. These legs gave these animals a good turn of speed and it has been estimated that some species of terror bird could reach speeds of 100kmh – comparable to a cheetah. The combination of running, big talons and a monstrous beak made the terror birds very effective predators. It is possible to imagine one of these birds snapping at the hooves of ancient mammals as it pursued them across the grasslands of the Americas. Smaller animals were probably immobilized with the sharp talons before being torn apart by the fearsome hooked bill or even swallowed whole after having their skull crushed in the bird’s vice-like grip. Larger prey animals may have been disemboweled with Kung-Fu styles kicks and it is even possible that crushing kicks may have been used to crack the larger bones of big prey to get at the nutritious marrow within.
Even if the last terror bird became extinct around 1.8 million years ago, these were successful animals that, as a group, survived for more than 50 million years, some of them even taking on the mantle of top land predator in the ecosystems in which they lived. However, around 2.5 million years ago (during the Pliocene epoch) something happened that completely changed the course of life for South America’s unique animals – the Great American Interchange. The land bridge that eventually appeared between North and South America, what is now known as the Isthmus of Panama, allowed animals from the north to migrate into South America. Among them were lots of predatory cats and it has been proposed these animals were so effective as predators that they outcompeted the terror birds. The talons and beaks of the terror birds were no match for the teeth, claws, and hunting prowess of the invaders from the north. This is a very neat answer for the cause of the extinction of the terror birds, but, the extinction of successful animals is very rarely due to one factor, but a combination of events. Perhaps climate change directly affected the terror birds by changing their habitats and the populations of their prey. Although there is a great deal we don’t know about the life and times of the terror birds we do know that one of their number somehow managed to cross into North America and spread through the southern states. For a long time it was assumed that the American terror bird spread north via the land bridge, but analysis of its ancient bones paints an alternative picture as they appear to have reached the southern states of America before the land bridge formed. Perhaps falling sea levels, due to the growth of the polar ice sheets, revealed a path of ‘stepping stones’ across islands in the gap of open ocean in what would become the Isthmus of Panama, allowing the giant birds to colonize the prehistoric North America. Maybe other species of terror bird, the remains of which are as yet undiscovered, also reached North America before following the rest of their amazing kind into the pages of earth history.
Read more about the terror birds and other beasts that have long since ceased being extant in the book Extinct Animals
Further reading: Marshall, L.G. The terror birds of South America. Scientific American 270 (1994) 90–5; Alvarenga, H.M.F. and Höfling, E. a systematic revision of the phorusrhacidae (aves: ralliformes). Papéis Avulsos De Zoologia 43 (2003) 55–91; MacFadden, B.J., Labs-Hochstein, J., Hulbert, R.C., and Baskin, J.A. Revised age of the late Neogene terror bird (Titanis) in North America during the Great American Interchange. Geology 35 (2007) 123–126.