What Were the Terror Birds? Three-Metre Predators That Ran Down South America
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After the dinosaurs died, South America was an island for most of sixty million years, and the top predators that evolved there were not mammals but birds: flightless, up to three metres tall, with skulls the size of a horse's head and beaks like an axe. The phorusrhacids, called terror birds by the people who dug them up, ran down prey on the open plains from about 53 million years ago until the land bridge to North America let in the cats and dogs that replaced them. They are the nearest thing the age of mammals produced to a return of the dinosaurs, which, being birds, they were.
The birds
The terror birds were a family of about twenty known species, ranging from a metre tall to nearly three, with the largest, Kelenken, described in 2007 from a skull 71 centimetres long, the largest bird skull known. They had long, powerful legs, small wings that were useless for flight and may have carried claws, and a deep, narrow skull ending in a hooked beak that was reinforced from top to bottom like a hatchet. Their closest living relatives are the seriemas of South America, two species of long-legged birds about 90 centimetres tall that hunt snakes and lizards on the grasslands of Brazil and Argentina and kill them by picking them up and beating them against the ground, which is probably what their ancestors did to larger things.
How they killed
Engineering analysis of the skull of Andalgalornis, a 1.4-metre species, in 2010 found that the beak was strong against forces from above and below and weak against sideways forces, which rules out the shaking, tearing bite of a lion and points to a different method:
- •Running the prey down: leg bones and reconstructions suggest speeds of 50 kilometres an hour or more in the mid-sized species
- •Striking with the beak: repeated downward blows with the hooked tip, like a hatchet, to injure and stun, retreating between strikes
- •Swallowing small prey whole, as the seriema does, and taking larger prey apart with the beak once it was down
- •Not grappling: the small wings and the weak lateral skull mean a terror bird did not wrestle, and the prey was the size of a deer or smaller
The island continent
South America broke from Africa about 100 million years ago and from Antarctica about 35, and for most of the Cenozoic it had no connection to any other continent, so that its animals evolved in isolation: marsupial predators such as the sabre-toothed Thylacosmilus, hoofed mammals with no living relatives, giant armadillos, ground sloths and the terror birds, which took the role that wolves and cats had elsewhere. The birds shared the top of the food web with the marsupials for tens of millions of years and, as the plains opened and the climate dried after 20 million years ago, the fast runners among them flourished; the largest species are from the Miocene, between about 15 and 5 million years ago.
The end
About three million years ago the Isthmus of Panama closed and the Great American Biotic Interchange began, with animals crossing in both directions; sloths, armadillos and opossums went north, and horses, deer, bears, dogs and cats came south. The placental carnivores of the north, the jaguar's ancestors and the wolves, outcompeted the marsupial predators and, on most accounts, the terror birds, which declined and were gone from South America by about 1.8 million years ago. One species went the other way: Titanis, two and a half metres tall, crossed into Florida and Texas, the only terror bird ever to live in North America, and survived there until about two million years ago. The story is not quite as tidy as competition, since the birds' decline began before the cats arrived and the climate was changing too, but an animal that had ruled an island for fifty million years met the mainland and lost.
Why they matter
The terror birds are a natural experiment in what evolution does with the same job in a different place: given a continent without large predatory mammals, the birds, which had inherited the two-legged running body of their theropod ancestors, took the role back. They are also the largest carnivorous birds that have ever lived, and the seriema in a Brazilian field, beating a snake against a stone, is what they have come to. The interchange that ended them is the reason a jaguar and a llama share a continent, and the terror bird, in a museum in La Plata or Buenos Aires, is what the continent had before.
The takeaway
The terror birds were a family of flightless predatory birds, up to three metres tall with hatchet-like hooked beaks, that were the top predators of the isolated continent of South America from about 53 to 1.8 million years ago, running prey down and killing it with downward strikes rather than a grappling bite. They declined when the Isthmus of Panama joined the Americas about three million years ago and the cats and dogs of the north arrived, and their nearest living relatives are the seriemas of the Brazilian grasslands.