On a grassy plain, even if the grass is long, there is little cover. Large predatory animals cannot hunt by stealth, so instead they must hunt by speed. Back in the Quaternary, the most famous of the swift-moving grassland predators was the cheetah (the fastest land animal on Earth at that time).
The swiftest hunter of the Amazon Grasslands is a flightless bird called the carakiller. It can run 39.75 miles per hour (64 kilometers per hour).
One of the most common falconiforms of South America during the Quaternary was the caracara. The caracara was a versatile, predatory bird, and easily adapted to life on the plains. Its descendants became large ground-dwelling birds and eventually evolved into a bird that was completely flightless: the carakiller.
The wings of the carakiller no longer have large flying feathers, but they are still muscular and aerodynamic. The bird's flightless wings now act as stabilizers, balancing the carakiller when it runs at full tilt, and helping it to turn corners quickly. In addition, each wing has a long curved claw at the tip.
The carakiller's body is covered in feathers for insulation, shaggy on the back and legs, and fine on the chest. Its neck and wings are bare, presenting smooth surface when the bird is eating (feathers would soon become sticky with the flesh and blood of prey).
Carakillers live and hunt in loose groups, stalking across the grassland in open formation, looking for the telltale movements of prey, like a troop of babookaris. At about 6.875 feet (2.1 meters) tall, the carakiller can easily see its prey over long distances. When prey, such as a troop of babookaris, is spotted, the carakillers signal silently to one another, raising and lowering colorful, peafowl-like plumes on the backs of their heads.
The carakillers begin to close in on their targets. If the selected prey is a babookari troop, the monkeys will scatter. The carakillers can single out one of the babookaris and swiftly run it down, with some carakillers splitting off from the main group to catch it. A babookari troop can outmaneuver a single carakiller, so carakillers have to hunt in groups. Some stalk around the prey, unseen in the long grass, and wait. Using their colorful head plumes as signals to each other, the rest of the pack drive the monkeys into the trap. Most of the troop will escape, but one or two will be caught.
At times, carakillers employ a different hunting strategy, using the frequent bushfires to their advantage. As a fire races across the savannah, the animals of the grasslands run for their lives. Carakillers can do this easily, but other animals are not so swift. The birds run ahead of the flames, snapping up small mammals, snakes and lizards as they are flushed from their hiding places. Other birds walk behind the line of fire, picking at the charred corpses left there. In this respect, carakillers are very similar to the marabou stork.
Carakillers lay their eggs on the ground communally during the wet season, when fires are less frequent. Parents take turns looking after the eggs while the other hunts. They will sometimes find that her nest is being raided by something like a grassland rattleback. Normally, most egg thieves are killed right on the spot by a carakiller mother, but the bird of prey's powerful beak is useless against a grassland rattleback's armor and no amount of scratching and clawing will pry the rattleback loose from its "wedged into the ground" defense position.