In the dense forests of central and south America
lives the most formidable predator of the sky. Swooping in between the branches of the canopy,
snatching its prey from above - and below - the harpy eagle is a terrifyingly agile predator. It
hunts by sight and hearing, carefully scanning the canopy for prey, which it can spot from
huge distances. It has a grip strong enough to crush its prey’s bones in an instant and can lift
prey that weighs as much as its own body weight. Among the largest of the eagles, a harpy
eagle’s wingspan can stretch to up over 2 meters - wings perfectly suited to
maneuvering in the dense forest vegetation. They can weigh up to 10 kilograms, or over 20
pounds, yet can still fly at speeds up to 80 kilometers per hour, packing a huge punch
on impact for their unfortunate victims. In one incident, a harpy eagle
even nearly knocked a BBC cameraman unconscious and could have killed him if it
were not for the man’s stab-proof kevlar vest. These eagles are arguably the most
powerful of all of the raptors. Being a sloth or a monkey in harpy eagle
territory would be an unfortunate fate. But 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 - so intelligent that their smarts rival that of their primate
prey. The relationship between the eagles and the monkeys they hunt has evolved into an
extraordinary arms race of cleverness - 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 a force unmatched in the sky.
How has nature crafted the harpy eagle to be such a monstrous predator? What is it that makes
the harpy eagle the scariest killer of the sky? The first written record of the harpy eagle
is in Carl Linnaeus' 1758 Systema Naturae. He describes the bird as the Vultur harpyja, named
after the Greek mythological spirit the harpy, that 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. The facial disk is composed of feathers
that form a circle around the bird’s face, which they can lift or lower as they please.
When the feathers of the facial disk are raised, they help direct sounds to the birds’ ears,
which are located on the sides of its head. It’s a lot like when you cup your hands
behind your ears to hear something better. This allows the eagles to pinpoint prey
in the dark, dense forest where it hunts. Harpies will often perch completely
still, and completely silently, as it waits for prey to wander near
them. And to help in spotting their prey, harpy eagles have some of the best
eyesight of any animal in the world. They can spot an item as small as
3 centimeters from 200 meters away. This level of acuity - the ability to resolve
fine detail - in their eyes far exceeds our own. To test just how well eagles can see,
researchers set up a clever experiment. They trained eagles to fly down a
long tunnel toward two TV screens. One TV would display a striped
pattern, and when the birds land on it, they get rewarded with food. The
other TV displays a solid grey color. The TVs were then separated so the eagle
had to make a choice at the start of the trial and stick to it. Over the course of the
trials, researchers would determine from what distance the eagles can discern the
stripes and choose the correct TV. The eagles were found to be able
to discriminate detail that is four times as fine as the
finest detail that we can see. A few features of an eagle’s
eyes make them so sharp. For one thing, eagles have
large eyes for their size, 1.4 times greater than the average
for other birds of the same weight, and the eye is tube-shaped. The tube
shape produces a larger retinal image. And this retinal image is transformed
into a neural response by the light sensitive elements of the eye, the photoreceptors. The more photoreceptors there are, and the less
space between them, the better the visual acuity. As the spacing between photoreceptors decreases, so does the minimum size of the detail
that can be discriminated by an eye. [4] However, simply having more photoreceptors spread
all over the retina would be energetically costly. Because of this, animal eyes have a special,
central region with extremely numerous and tightly packed photoreceptors. This is
called the fovea and it provides the brain with the clearest vision of any part of the eye. 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, at around 45 degrees to the right and left of the head axis, and it is
used for detecting prey from a large distance. But pursuit raptors like the
harpy eagle have something else almost no other animal has - a second,
shallower fovea that allows for sharp vision in the frontal field of view - at 15
degrees to the right or left of the head axis. These two foveae allow eagles to see two sharp
images in their vision simultaneously. This is why you can often see raptors like this turning
their head repeatedly while looking at an object. They can’t rotate their eyes in
the sockets, so these movements are helping to focus the object back and forth
between the deep and shallow foveae. [6] Having both regions of acute vision is of
course a huge benefit to these predatory birds. But having their most acute vision in the side of
their vision causes a conflict for many raptors - especially ones that dive at prey at high speeds.
When going fast, turning the head sideways to view the prey that is in front of them would cause a
ton of aerodynamic drag and slow the bird down. So to solve this, raptors don’t necessarily
follow a straight path to their prey. Instead, they dive along a logarithmic
spiral path with their head straight, and one eye looking sideways at the prey.
[7] This is especially true for birds like the diving peregrine falcons. Harpy
eagles don’t fly or dive nearly as fast, but at 80kph, they still very much need
to keep aerodynamics in mind as they hunt. From wing tip to wing tip, harpy eagles are longer
than many full-grown humans are tall, with a wingspan of about 2 meters across - with female
harpies being almost twice the size as males. These wings are immense, but for
the size of the rest of their body, and compared to other large flying
birds, their wings are relatively short. Andean condors, for example, weigh about
the same as harpy eagles but have a wingspan over 3 meters across.
This is because harpy eagles aren’t built for soaring. Instead, they are
built to move more nimbly from tree to tree through the dense forest. Their short, broad
wings help them be highly agile, and even give them the ability to fly almost straight up, so
they can attack prey from below as well as above. Like with all flying birds, harpy eagles are
able to fly due to the structure of their flight feathers, or remiges. Unlike other
feathers, remiges are anchored to the bone, and their primary function is to aid in
the generation of both thrust and lift. The remiges are divided into primary and secondary
feathers based on their position along the wing. The primaries are longest of the flight feathers.
These feathers are asymmetric with a shorter, less flexible leading edge that prevents midair
twisting. They occupy the outer half of the wing, and can be controlled and rotated
individually much like our own fingers. This rotation of the feathers controls flight
directions and adjusts lift and air resistance as needed. For example, if the primary feathers are
lifted and splayed, the bird's flight will slow and the altitude will drop. If the primary
feathers are held tightly together, the bird's flight will be faster and more
precise, and is often done when chasing prey. This ability to morph the wing
in different flying situations gives flying birds a maneuverability that
aircraft engineers could only dream of. Secondary flight feathers cannot
be controlled as extensively, but they provide most of the lift by
overlapping to form an efficient airfoil. Tail feathers, or rectrices,
are also essential for flight. They are needed for steering, and only the
two most central feathers attach to bone. Where the harpy eagle lacks length in
wingspan, it makes up for in its long tail, which it uses like a rudder on a
boat to steer through dense vegetation. Tail feathers are more symmetrical than flight feathers, but have increasing levels
of asymmetry toward the outer pairs. And controlling most of this acrobatic flight are
the chest muscles, which attach to the sternum. The chest muscles and sternum are highly developed
in birds, much more so than in other vertebrates like mammals. But for the harpy eagle, it is even
more extreme. Soaring birds like the Andean condor have a comparatively small sternum and less
developed chest muscles. Once birds like them are airborne, they hardly need to flap. But harpy
eagles’ with their short wings and heavy weight and agile lifestyle have a huge need for some
of the strongest flight muscles of any bird. And on top of this strength, the harpy
eagle's legs can be as thick as a human wrist. And its curved, black talons are larger than
grizzly bear claws at 13 centimeters long. These are the largest talons of any bird of prey,
and can exert over 50 kilograms of pressure. These claws are known to
break bones in an instant, or snatch prey from trees with a force
stronger than a large dog’s jaws. This fierce hunting ability alone
would be enough to strike terror in the hearts of the primates and sloths
of south america. But on top of this, harpy eagles are clever - so clever, that they
don’t merely ambush their prey, but outsmart them, in one of the most interesting predator-prey
communication arms races ever observed. In the battle of survival in the animal kingdom,
predators need to capture prey, and prey needs to avoid being captured. These simple truths have
lead to a spectacular phenomenon - predators and prey throughout the world are locked
in all sorts of evolutionary arms races. One famous example of this is the
arms race between bats and moths. This arms race began when bats
evolved the ability to echolocate, in order to detect their moth
prey in complete darkness. In response, moths evolved ears that allow them
to hear the echolocation and evade capture. Some bats then improved their attack strategy by
evolving stealthier echolocation – that is, echolocation at frequencies that can’t be heard
by moths. Some moths, in turn, evolved ultrasonic clicks of their own that they can use defensively
during an attack. And so it goes, on and on. These types of dynamics can be seen throughout
the animal kingdom. But for a long time, relatively little was known about how
predators hunt primates in particular, and how primates might acquire adaptive
responses to counteract such predation. So, researchers began a - somewhat
brutal - observational experiment. Barro Colorado Island is a biological reserve
island in the middle of the Panama Canal. It is home to many species of primate,
including marmosets, spider monkeys, and most importantly to
this study - howler monkeys. These howler monkeys lived in approximately
65 troops, each with an average of 19 members. Often, primates develop predator-specific calls -
specific calls that discriminate between big cats, snakes, and eagles, to warn their fellow monkeys
of the specific type of danger that is near, and coordinate a defensive response. But, for the last
100 years, the howler monkeys on Barro Colorado Island have faced no major predators - no jaguars,
few snakes, and most importantly - no eagles. That is, until 1999, when two
radio-collared harpy eagles were introduced. Researchers wanted to know - would
the Barro Colorado monkeys have an organized reaction to the
never-before-seen predators? And if they didn’t, how long would it
take for such a response to develop? As expected, once the eagles were
set loose, they wreaked havoc on the monkeys - both eagles were picking
a howler monkey off every 3 to 4 days. And at first, the monkeys had one of two
responses to a harpy eagle attack - either they would act in a random panicked and agitated
manner with no coordination between the monkeys, or, they would remain totally calm,
completely unaware of the danger before them. The howler monkeys had indeed lost their ability
to recognize the danger from the harpy eagle, thus becoming extremely vulnerable to
predation upon their re-introduction. This is not all together that surprising. The surprising finding, however, was the strategy
the eagles used to hunt their primate prey. Instead of rapid stealth attacks, like you might
expect, the eagles would perch in a tree nearby, in plain view of the monkey troop, and observe the
animals. Then, as if to make their presence known they would utter a series of calls
- literally the opposite of stealth. These calls always had the
same acoustic structure, were only ever used before prey pursuit,
and only when hunting for primate prey. Why would a predator let its prey
know that it is about to attack them? What happened after the harpies’
call gave researchers a hint. If after the harpy made its
presence known with its call, and the howler monkeys' response was either
minimal or chaotic, the eagles would attack. If instead, the monkeys displayed a coordinated
defense response, the harpies would leave, and either find different monkeys to prey upon,
or come back later and attack using stealth. The harpy eagle’s call was being used to gain
critical information about its prey. By observing what happens after the call, the eagles are
learning about the level of prey alertness, their escape ability, and their defense
mechanisms. By probing the potential defense strategies before initiating an attack, the
eagle can assess the risk of the confrontation and make a snap decision whether or not
it is a good use of their time and energy. Over time, more and more of the monkey
troops established coordinated defenses, such as picking up the young and moving them
closer to the trunk of the tree, moving to more dense areas of the canopy, organizing themselves
to literally mob the eagle if it came close; or producing an eagle specific alarm call. After
just a year, nearly all of the monkeys on the island had regained such defense mechanisms,
making hunting for the harpies more difficult. Researchers got to observe this
predator-prey arms race play out in real time - a unique and valuable opportunity. But unfortunately for our curiosity, the
experiment ended before the eagles could adapt once more to the monkeys new defense strategies.
The harpies were removed from the island after just one year. After they were gone, the howler
monkeys maintained the defensive responses to recordings of the harpy eagle calls for at least
seven months, which is how long they were observed for. No one knows how long this defense response
will last in the howler monkey populations there. Field biology of this sort unfortunately
can rarely continue on forever. But luckily, an incredible new insight
into harpy eagle behavior was uncovered, proving in one more way, that harpy
eagles are an incredible predator. If hunting primates in the
rainforest isn’t your thing, but neither is meal planning
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even like cooking food. I don’t want to eat just ‘egg’. I just don’t have time to go to the grocery
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for up to 16 free meals AND 3 surprise gifts! As always thanks for watching, and if you’d
like to watch something else right now, you can watch our last video about the effects
of long-term space flight on the human body, or watch Real Engineering’s latest video about the
imminent death of the International space station.
