What has feathers, T-rex-like feet, and a hooked beak that could sever the spinal cord of a horse with one blow? It might sound like a beast from a fantasy novel or horror film, but in fact it’s a creature that actually existed - the scariest bird you can imagine, scaled up to nightmarish proportions. Say hello to the terror bird.

After a meteor wiped out Velociraptor and Tyrannosaurus rex, the terror bird family rose to occupy the niche of terrifying top predator in South America – a supremacy that lasted for almost 60 million years. During that time, 17 species in this family of lethal-beaked meatheads came and went, all the while gorging on a smorgasbord of plant-eating mammals until these scary birds disappeared about 2.5 million years ago.

With few fossils documenting their existence, the behaviour of these birds is still somewhat of a mystery. What exactly did these feathered aggressors get up to, and why did they disappear? It’s a story that paleontologists have been painstakingly unearthing for over 100 years. What they’ve learned so far suggests the birds were absolutely dominant as predators, voraciously gobbling up prey across the continent.

The second coming

Sixty million years ago the continents had largely assumed the geographic position they occupy now, though what we now know as Central America had not yet been created. South America was an island, and while saber-toothed cats and wolves took over the job of top predators elsewhere, most mammals in South America were happy herbivores. This buffet of lumbering vegetarians provided the fresh flesh that terror birds feasted on.

All birds are considered dinosaurs – most being closely related to meat-eating dinosaurs such as the velociraptor that disappeared 65 million years ago. In a way, the reign of the dinosaur never truly ended in South America, it merely changed form.

Its beak was designed to make powerful up and down movements

Dr Luis Chiappe, Director of the Dinosaur Institute at the Los Angelos Museum of Natural History, agrees. “Nature continues to fill the gaps as animals evolve, and the world evolves,” he says. “It’s fascinating to think of the terror birds as the dinosaurs of the Cenozoic era in South America.” On that continent, he says, there were no animals that could rival these birds at the top of the food chain.

So how did they do it? What exactly made these terror birds so terrifying?

Death by terror bird

Terror comes in many forms, something one learns quickly when trying to understand how terror birds, hunted, killed and eviscerated their prey. It’s really not the kind of research to be done before bedtime - there are apparently several options available to an animal equipped with a pickax for a beak.

It’s generally agreed that terror birds were seriously carnivorous, but there’s long been debate over how they killed prey, says Dr Stephen Wroe, Director of the Function, Evolution & Anatomy Research (FEAR) Lab at the University of New England, Australia. Based on CT scans of fossils from the terror bird known as Andalgalornis - an agile, swift-moving bird that lived between 23 and 5 million years ago - Wroe and colleagues were able to narrow down the type of movements this animal was capable of.

Their work suggests that its beak was designed to make powerful up and down movements (think stabbing prey in the head or back from a height), but was relatively weak when moving side to side. The ability to give a rapid side-to-side shaking is what you would expect of an animal that is getting up close and personal with relatively large, dangerous prey, says Wroe, adding that wrestling with large prey is risky. So it’s unlikely Andalgalornis killed this way.

As if rock hard skulls, deadly beaks, and the use of creepy low sounds to ferret out prey weren’t scary enough, we still haven’t talked about their feet

“If it was using its beak as the primary killing mechanism, then it would have been restricted to relatively small prey,” says Wroe. However, he’s quick to add that the beak was certainly strong enough to have enabled the bird to bring down larger prey in a sort of strike, retreat and repeat type of hunting, and hasn’t ruled out big animal take-downs just yet.

This year, a new medium-sized species of terror bird, Llallawavis scagliai, was identified, providing more clues about the behaviour of this group of birds, and the diversity of the terror bird family tree. Like other terror birds, the joints between skull bones are fused, unlike other birds in which they are more mobile. This rigidity would have been helpful with pummeling prey to death or using their head as a giant meat tenderiser. It would also provide the stable structure needed to rip flesh from bone.

Another intriguing revelation of the study was the analysis of the inner ear canals, reconstructed through 3D imaging. Imaging results suggest the bird may have had good low frequency hearing, possibly to communicate vocally with other terror birds, or in prey detection.

As if rock hard skulls, deadly beaks, and the use of creepy low sounds to ferret out prey weren’t scary enough, we still haven’t talked about their feet.

Terror birds were the pinnacle of South America’s food chain for tens of millions of years

Wroe points out that many living birds of prey use their feet to kill. Take for example the secretary bird, an avian predator that he says “basically kicks the crap” out of snakes and reptiles with devastating force. “It’s certainly possible certain terror birds may have used that form of attack as well,” he says.

Other clues may be found in the grand-slam feeding style of the terror birds’ closest living relatives, the seriemas. Seriemas pick up snakes, frogs and other prey, then smash them on the ground, or throw them against hard surfaces repeatedly. Though they look like an animal that needs a course in anger management, the technique is actually very effective in breaking the bones and tendersing the meat of their prey.

While sorting all of this out, it’s important to consider that terror bird species came in a variety of shapes and sizes. They ranged in height from one to three metres, with body types that ranged from sleek and light to heavier and stalky. It’s likely the feeding strategies varied between species depending on their size and build, and how fast the animal was able to move.

It’s big, as fast as an ostrich, with feet that could snap the femur of a cow

“When looking at the foot bones, these animals clearly had very different styles of locomotion,” says Chiappe. Some of the lower leg bones were very long, suggesting the species was very fast, versus others with shorter lower leg bones that indicate a “heavier, slower animal – more of a walker,” he says.

For example, the 350–400 kg terror bird Brontornis, may have been a slower moving ambush predator. Conversely, the lighter and more agile Phosphoracus may have been able to run prey down.

However they did it, one thing is certain: terror birds were the pinnacle of South America’s food chain for tens of millions of years… until everything changed.

Twelve to 15 million years ago, what we now know as Central America was created through a combination of tectonic up-thrusting and volcanic activity. This new land bridge connected North and South America, and animals slowly began to migrate across it in both directions - a phenomenon known as the Great American Interchange. This exchange of species between hemispheres changed the game for terror birds forever.

Terror comes to North America

Imagine you are a khaki-clad researcher huddled in a blind on the savannah of what is now Panama. Time means nothing to you - millions of years are only mere moments, as you sit quietly and record the biggest continental exchange of animals the Americas has ever seen.

Titanis was getting larger and larger so that it could not only compete, but open up some of these carcasses

From the north come ancient camels, massive elephants, tapirs, deer and horses, followed by ravenous cougars, saber-toothed cats, wolves and bears. From the south lumber the glyptodons, giant ground sloths, and bizarre-looking hoofed mammals that looked like a cross between a horse and an anteater. You’re writing this down on a pad of paper when suddenly, the shadow of a three metre high, 150-kilogram bird falls over you.

Meet Titanis, evolution’s latest, and last, of the terror birds. It’s big, as fast as an ostrich, with feet that could snap the femur of a cow. A swift peck from this bird will knock your head off. So don’t move, and good luck.

“Titanis is one of the species that over about seven million years makes it from South America all the way up into North America,” says Dr Robert Chandler of Georgia College in Milledgeville, US. Chandler found fossils from Titanis while scuba diving in the Santa Fe River in Florida. It’s likely this monstrous bird was following the northern migration of the prey it was eating, he says.

Changing habitats and habitat loss that may have sealed the terror birds’ fate

For Titanis, life in North America may have been more complicated than back home in the south. Once relatively unchallenged by competitors, now encounters with fanged cats and packs of wolves were cramping its style. It’s all well and good to kill a sloth, but what if a pack of wolves want a piece of the meat?

Chandler believes the big birds found ways to adapt. “It seems like Titanis was getting larger and larger so that it could not only compete, but open up some of these carcasses,” he says, pointing to similarities in beak structure with modern day vultures. It may even have taken carcasses from other predators, he suggests.

Not everyone agrees. Some of Chandler’s colleagues maintain that the beak was primarily an offensive weapon for taking prey down. Regardless of whether Titanis was thriving or just surviving, its reign of terror came to an end about 2.5 million years ago. Then, the terror bird disappears altogether from the fossil record.

And then there were none

“There is almost always more than one thing going wrong for a species to go extinct,” says Wroe. “It’s kinda like a plane crash. When a plane goes down, there’s almost always a series of events that add up to take it out.”

Paleontological detectives remain in awe of these enigmatic birds, calling them serious, kickass, terrestrial predators

Early theories suggested that competition with predatory mammals led to the demise of the terror birds. But most scientists think there’s more to it.

Terror birds arrived in North America and were successfully competing, says Chandler. “They lived for almost seven million years along with all these placental mammals, and then what happens? Well, we have climate change.”

With a changing climate came changing habitats, and habitat loss that may have sealed the terror birds’ fate, explains Chandler. Along with them came the extinction of a massive lion, large horse, and some species of elephants and mastadons.

“In past extinctions, almost always some sort of climate change plays a role,” Wroe says. But he still feels the problems climate change created were likely exacerbated by the pressures on terror birds created by the mixing of animals from North and South.

As scientists piece together the mystery of this ferocious feathery beast, it’s hoped that further clues about terror bird lifestyle, as well as more fossil bits and pieces, will continue to emerge. In the meantime, paleontological detectives such as Wroe remain in awe of these enigmatic birds, calling them “serious, kickass, terrestrial predators.”

Now that’s something everyone can agree on.