The Insane Biology of: The Harpy Eagle

In the dense forests of Central and South America lives one of the most formidable predators of the sky. Swooping between the branches of the canopy, snatching its prey from above and below, the harpy eagle is an incredibly agile predator. It hunts by sight and hearing, carefully scanning the canopy for prey, which it can spot from great distances. With a grip strong enough to crush bones instantly, it can lift prey that weighs as much as its own body weight. Among the largest of eagles, a harpy eagle’s wingspan can exceed 2 meters, perfectly suited for maneuvering through dense forest vegetation. They can weigh up to 10 kilograms (over 20 pounds) and can fly at speeds up to 80 kilometers per hour, delivering a powerful impact to their unfortunate victims.

In one incident, a harpy eagle nearly knocked a BBC cameraman unconscious, and could have caused serious harm if it weren’t for the man’s protective gear. These eagles are often considered the most powerful of all raptors, and being a sloth or monkey in harpy eagle territory would be a precarious situation.

However, it’s not just raw power that makes the harpy eagle one of the deadliest predators of the rainforest. These eagles are also incredibly intelligent, with cognitive abilities that rival those of their primate prey. The relationship between the eagles and the monkeys they hunt has evolved into an extraordinary arms race of cleverness, with each pushing the other to adapt new communication, defenses, and attack strategies over time.

With legendary strength and fearsome hunting abilities, the harpy eagle is unmatched in the sky. The first written record of the harpy eagle is in Carl Linnaeus’ 1758 “Systema Naturae,” where he describes the bird as Vultur harpyja, named after the Greek mythological spirit, the harpy, which had the body of an eagle and the face of a human. This comparison is fitting, largely due to the harpy eagle’s facial disk, composed of feathers that form a circle around the bird’s face, which they can lift or lower at will. When raised, these feathers help direct sounds to the birds’ ears, allowing them to pinpoint prey in the dark, dense forest where they hunt.

Harpies often perch completely still and silently as they wait for prey to wander near. To assist in spotting their prey, harpy eagles have some of the best eyesight of any animal in the world, able to spot an item as small as 3 centimeters from 200 meters away. This level of acuity far exceeds that of humans.

Researchers have conducted experiments to test eagle vision, training them to fly down a long tunnel toward two TV screens. One screen displayed a striped pattern, rewarding the birds with food when they landed on it, while the other displayed a solid grey color. The eagles were found to be able to discriminate detail that is four times finer than what humans can see.

Several features of an eagle’s eyes contribute to their sharp vision. Eagles have large eyes for their size, 1.4 times greater than the average for other birds of the same weight, and their tube-shaped eyes produce a larger retinal image. This image is transformed into a neural response by the light-sensitive elements of the eye, known as photoreceptors. The more photoreceptors there are, and the less space between them, the better the visual acuity.

Eagles have a special central region called the fovea, which contains numerous tightly packed photoreceptors, providing the clearest vision. Harpy eagles have a much deeper fovea than most animals, giving them especially high acuity vision. For harpy eagles, the highest acuity occurs in their peripheral vision, allowing them to detect prey from a large distance.

Pursuit raptors like the harpy eagle possess a second, shallower fovea that allows for sharp vision in the frontal field of view. This unique adaptation enables them to see two sharp images simultaneously, which is why they often turn their heads repeatedly while focusing on an object.

Having both regions of acute vision is beneficial for these predatory birds, but it can create a conflict, especially for those that dive at prey at high speeds. To mitigate this, raptors dive along a logarithmic spiral path with their head straight, keeping one eye focused sideways on the prey.

From wingtip to wingtip, harpy eagles are longer than many full-grown humans are tall, with a wingspan of about 2 meters. Female harpies are almost twice the size of males. While their wings are immense, they are relatively short compared to other large flying birds, such as the Andean condor, which has a wingspan over 3 meters. This is because harpy eagles are not built for soaring; instead, they are designed for agile movement through dense forests. Their short, broad wings allow them to fly almost straight up, enabling them to attack prey from below as well as above.

Like all flying birds, harpy eagles can fly due to the structure of their flight feathers, or remiges. Unlike other feathers, remiges are anchored to the bone, aiding in thrust and lift generation. The remiges are divided into primary and secondary feathers based on their position along the wing. The primary feathers are the longest and asymmetric, preventing midair twisting. They occupy the outer half of the wing and can be controlled and rotated individually, much like human fingers. This rotation allows for precise control of flight direction and adjustments to lift and air resistance.

Secondary flight feathers provide most of the lift by overlapping to form an efficient airfoil. Tail feathers, or rectrices, are also essential for flight, needed for steering. The harpy eagle compensates for its shorter wingspan with a long tail, which it uses like a rudder to navigate through dense vegetation.

The chest muscles of birds, which attach to the sternum, are highly developed, especially in harpy eagles, giving them some of the strongest flight muscles of any bird. Their legs can be as thick as a human wrist, and their curved, black talons are larger than grizzly bear claws, measuring 13 centimeters long. These are the largest talons of any bird of prey and can exert over 50 kilograms of pressure, capable of breaking bones or snatching prey from trees with a force stronger than a large dog’s jaws.

This fierce hunting ability alone would be enough to instill fear in the hearts of the primates and sloths of South America. However, harpy eagles are also clever, outsmarting their prey in one of the most fascinating predator-prey communication arms races ever observed.

In the animal kingdom, predators need to capture prey, and prey needs to avoid being captured, leading to a spectacular phenomenon of evolutionary arms races. One famous example is the arms race between bats and moths, where bats evolved echolocation to detect moths in darkness, prompting moths to develop defenses against detection.

For a long time, little was known about how predators hunt primates and how primates develop adaptive responses to counteract such predation. Researchers began an observational experiment on Barro Colorado Island, a biological reserve in the Panama Canal, home to various primate species, including howler monkeys.

These howler monkeys lived in approximately 65 troops, each with an average of 19 members. Typically, primates develop predator-specific calls to warn their fellow monkeys of specific dangers and coordinate a defensive response. However, for the last 100 years, the howler monkeys on Barro Colorado Island faced no major predators—no jaguars, few snakes, and no eagles—until 1999, when two radio-collared harpy eagles were introduced.

Researchers wanted to know if the monkeys would have an organized reaction to these new predators and how long it would take for such a response to develop. As expected, once the eagles were released, they wreaked havoc on the monkeys, picking off a howler monkey every 3 to 4 days. Initially, the monkeys had one of two responses: they either acted in a panicked and agitated manner with no coordination or remained completely calm and unaware of the danger.

The howler monkeys had lost their ability to recognize the danger posed by the harpy eagle, making them extremely vulnerable to predation upon the eagles’ introduction. Surprisingly, the eagles did not rely on rapid stealth attacks. Instead, they perched in a nearby tree, in plain view of the monkey troop, and observed the animals. To announce their presence, they would utter a series of calls, which was the opposite of stealth.

These calls had a consistent acoustic structure, were only used before pursuing prey, and were specific to hunting primate prey. Researchers noted that if the monkeys’ response to the eagle’s call was minimal or chaotic, the eagles would attack. Conversely, if the monkeys displayed a coordinated defense response, the eagles would leave and either find different monkeys to prey upon or return later to attack stealthily.

The harpy eagle’s call served as a means to gain critical information about its prey. By observing the monkeys’ reactions, the eagles learned about their alertness, escape abilities, and defense mechanisms. This allowed the eagles to assess the risk of confrontation and decide whether or not to engage.

Over time, more monkey troops established coordinated defenses, such as moving young closer to the trunk of trees, relocating to denser areas of the canopy, organizing themselves to mob the eagle, or producing specific alarm calls. After just one year, nearly all monkeys on the island had regained such defense mechanisms, making hunting more challenging for the harpies.

Researchers had the unique opportunity to observe this predator-prey arms race in real-time. Unfortunately, the experiment ended before the eagles could adapt to the monkeys’ new defense strategies, as the harpies were removed from the island after just one year. After their removal, the howler monkeys maintained their defensive responses to recordings of harpy eagle calls for at least seven months.

Field biology of this nature rarely continues indefinitely, but this study provided incredible insights into harpy eagle behavior, further proving their status as remarkable predators.

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If you’re interested, visit hellofresh.com and use code REALSCIENCE16 for 16 free meals and three surprise gifts. Thank you for watching, and if you’d like to explore more, check out our previous video about the effects of long-term space flight on the human body or Real Engineering’s latest video about the future of the International Space Station.