Raptor derives from a Latin word meaning "plunderer." Etymologists studying the origins of language might contend that the word raptor illustrates a fusion of fear and awe that birds of prey have always inspired in the human mind. Historically, three massive raptors are acclaimed as fiercest of the fierce (we list them in descending size, but not necessarily in decreasing ferocity): The harpy eagle of the Neotropics, the Philippine eagle found only on the island of Mindanao, and the crowned hawk-eagle of Central, East, and Southern Africa. The harpy eagle has been described as a "flying wolf." (Webster's New Collegiate Dictionary's definition of the word harpy, however, sounds unremittingly misogynistic; the nicest part reads: "A foul, malign creature of Greek mythology that is part woman and part bird.") Crowned hawk-eagles were labeled "leopards of the air" by the European explorers of Africa, and the Usambara people of the former Belgian Congo called the crowned hawk-eagle kumbakima, "the monkey beater."18 In all three of these species the female is the larger of the sexes, and each species sports a signature crest of feathers on their heads that rises and collapses with the fire of their temperament. We began calling them the "fearsome threesome," but then decided "viragos"—defined by Webster's as overbearing women (and in the case of these eagles, the males qualify also) of great stature, strength, and courage—was by far the best collective label.
Heavy eagles that prey on primates within the forest canopy, such as the harpy, Philippine, and crowned hawk-eagles, have short, broad wings and relatively long, graduated tails. This silhouette is not the common one we visualize when thinking of a soaring eagle; we tend to envision a powerful bird of prey with a huge wingspan and a tail that is relatively short. That combination of long wing and short tail works well for soaring eagles hunting in open country. Unlike the soaring eagles, though, our viragos hunt within dense rain forests and they have, accordingly, evolved the apparatus to give them maximum proficiency in their ecological niche. A long tail is a crucial trait; the ratio of tail length to wingspan can predict the maneuverability of eagles. The tail of a raptor, then, is quite as important as its wings when the bird lives in a rain forest. The length of a virago tail is up to 85% of its wingspan.19 When you see a raptor with this ratio in the flesh, it registers as a notably robust bird.
Wingloading—the ratio of body weight to surface area of wings—has great effect on the flight performance of birds of prey. It is also a subject best left to experts in aerodynamics to discuss. We can only make the simple statement that the weight, wingspan, and tail length of a harpy eagle, a crowned hawk-eagle, or a Philippine eagle occur in serendipitous combination. Their wing—tail adaptations allow the eagles to maneuver
better in the air and dodge dexterously around trees and other obstacles in thick forest as they target their prey. Crowned hawk-eagles, in some cases, are able to do the seemingly impossible and lift off almost vertically from the forest floor while clutching their dead prey.20
Scientific research has upheld and, for the harpy and crowned hawk-eagle, even expanded upon their reputations as rapacious non-human primate predators. The harpy eagle—nearly the largest and undoubtedly the most powerful eagle in the world—is the premier predator of many Neotropical monkeys. Harpy eagles weigh 15—20 pounds and have a wingspan of approximately 8 feet. The harpy eagle exhibits the same classic characteristics as the crowned hawk-eagle. A harpy eagle possesses a tarsus as thick as a child's wrist; its feet span 9—10 inches; and the massive dagger-like talons are nearly 3.5 inches long.21
People who have worked with captive harpies report that a fully grown man must brace himself securely to prepare for the landing of one of these eagles on his arm. Not a surprising observation since harpy eagles attain speeds of 40—50 miles per hour and can exert 13,500 footpounds upon impact with their prey: that's nearly three times the muzzle energy of a bullet from a heavy rifle. The harpy soars low and missile-like over the rain-forest canopy and hits its victim from the back with powerful talons, catching the monkey totally unawares. After the harpy eagle's initial impact with its prey, momentum allows it to continue in flight and carry its victim to a nesting or feeding tree. Either the talons are driven through the body of a monkey to kill it instantly, or the monkey may be struck with such force that it dies by falling to the ground.22
Those lucky enough to have beheld the elusive and endangered harpy in action wax poetic in their descriptions of its magnificence as a predator: "King of the raptorine birds . . . [f]rom the topmost branch of some dead forest giant it surveys the forest below for signs of movement. Then, like a bolt from the blue, it swoops with unerring accuracy upon a sleeping sloth or a leaping monkey and bears off its prey in triumph."23
Another species of colossal, primate-eating raptor, the Philippine eagle, has a huge, narrow bill (which may be an adaptation to enhance its binocular field of vision) and tarsi almost as heavy as the harpy eagle. The Philippine eagle's scientific nomenclature, Pithecophaga jefferyi, emphasizes predation on monkeys—Pithecophaga is derived from the Greek words pithekos, meaning monkey, and phagein, eater. Its common name was officially changed from "monkey-eating eagle" to "Philippine eagle" in 1978 by President Ferdinand Marcos as a public relations move, since the monkey-eating appellation was seen as a denigration of this noble bird and the nation it represents.24
Of course, names may not always be entirely accurate. While data on the frequency of primates in the diet of Philippine eagles are sparse, three studies have estimated only 3—6% of their diet consists of macaque monkeys. This range is considerably lower than the level of primates found in the diets of harpy or crowned hawk-eagles. Pairs of Philippine eagles have been observed hunting together, and estimates for successful capture of monkeys were significantly higher when two mated birds combined their efforts. But actually, it appears that the Philippine eagle relies more on a diet of colugos—squirrel-sized, gliding mammals sometimes called flying lemurs—as the staple of its diet, rather than primates.25
Of the three viragos, the crowned hawk-eagle is the smallest in size. Female crowned hawk-eagles—the larger of the sexes—have wingspans of just under 6 feet and weigh in at 8—9 pounds, about 50% lighter in mass than the harpy eagle.26 Yet a combination of stout tarsi with excep tionally strong talons has given the crowned hawk-eagle a reputation as a metaphorical "leopard with wings." (Not a bad metaphor at all, since the leopard is a relatively lightweight member of the big-cat family, yet has the strength and boldness to assail a gorilla. And, like the leopard, the crowned hawk-eagle is a primate specialist; studies of this raptor in the Kibale Forest of Uganda found that approximately 85% of its prey were monkeys of three types—colobus, mangabeys, and guenons. At another research site in the Kiwengoma Forest Reserve, Tanzania, a species of guenon, the blue monkey, composed nearly 90% of the prey remains at a crowned hawk-eagle roost.)27
The crowned hawk-eagle is the second largest of all African eagles (second in size only to the martial eagle), but it is without a doubt the most powerful raptor on the continent and is able to kill the largest prey. While its average weight is 8—9 pounds, it routinely kills antelope weighing 40—44 pounds—nearly five times its own weight. The largest recorded kill by a crowned hawk-eagle was a 66-pound subadult bush-buck ram.28 It's a feeble understatement, but these are incredible birds.
The crowned hawk-eagle's geographic range extends across the width of sub-Saharan Africa in dense tropical rain forest. Their hunting strategy consists of long periods of silent watchfulness from a perch within the forest, culminating in a swift and deadly accurate drop onto the unfortunate prey. Most kills are made on the ground where the carcass is dismembered and partially consumed. Such is the strength of the crowned hawk-eagle that, if a monkey is killed on the forest floor, the eagle will fly almost vertically upward with the whole carcass.29 Here's another firsthand description regarding their power and maneuverability:
It was not long before there were some astonishing events. The male [crowned hawk-] eagle suddenly, and in a shower of green leaves, burst through the canopy where the monkeys were feeding, upside down with wings closed, and with a monkey firmly grasped in both sets of talons. In other words he must have approached and taken his prey at high speed from below and his momentum was such that it carried him a full 15 feet upwards above the tree tops before he opened his wings, rolled over, and flew heavily back carrying his prey to his cliffside perch.30
The crowned hawk-eagle's unremitting reliance on intelligent primates has been credited with the evolution of several predatory behaviors, such as coordinated hunting of monkeys by breeding eagle pairs. Team hunting by a mated pair seems to be a tactic often employed by crowned hawk-eagles. One of the pair may swoop low over the trees; as it flies away a monkey may climb out of the foliage to watch the departure of its feared enemy. At that point a second eagle will take off from its perch and dive at the monkey from behind. Other observers have detected soft whistling on the part of the hunting eagle to attract naturally curious monkeys.31
A protracted 2-year breeding cycle on the part of the eagles has been documented. This adaptation—biennial rather than annual breeding— is also found in the harpy eagle and may indicate specific coevolution with their primate prey. The pressure on prey species (in the eagles' case, slow breeding primates) is doubled when a pair of harpy or crowned hawk-eagles are feeding a fledgling in the nest. (Nests, by the way, are massive—6 feet deep and 6 feet wide.) Economical utilization of prey has very wisely evolved; spacing the extra food requirements of a nestling out to every other year gives prey a respite to recover their numbers before the next breeding cycle of the eagles.32
Coevolution between raptors and their primate prey is patently visible in primate polyspecific associations, when two or more different monkey species feed, travel, or rest together. What may have provided a strong incentive for these multi-species aggregations? The evidence points to birds of prey since polyspecific associations of many New World and African monkeys are limited to geographic regions inhabited by harpy eagles of the Neotropics and crowned hawk-eagles of Central Africa.33
Harpy eagles probably have exerted such strong selective pressure on many Neotropical primate species that predation is manifested not only in group living and polyspecific associations but also in the evolution of increased size. Neotropical primatologist John Terborgh discussed the relation between size and escape from predation for the primates at Cocha Cashu, Peru. Escape through an increase in size was identified as a distinct evolutionary strategy of Neotropical primates to thwart predation. The smallest primates, tamarins and marmosets (weighing 1—3 pounds), spend many hours per day in safe hiding places. Slightly larger species, such as capuchin and squirrel monkeys, seek protection in groups. The strategy of escape through size increase applies to adults of the largest species (spider, woolly, and howler monkeys). These primates are often found at rest in conspicuous exposed perches in the canopy. From such vantage points, the larger primates scan for harpy and other big eagles, species in which a few birds are highly dispersed over huge territories. Increased size, combined with group-living and agility, may render the larger Neotropical primates very difficult prey even for the harpy eagle. Howler monkeys were observed to evade and chase a harpy eagle that attacked one of their group, but the successful capture of a fully grown adult male howler by a harpy eagle was witnessed, even though smaller individuals were in the same group and available as prey. Medium-sized monkeys in the range of 4—8 pounds, such as capuchin monkeys, sakis, or bearded sakis, constitute the most regular prey for harpy eagles.34
While the medium-sized monkeys of the rain forest are the most commonly recorded primate prey for crowned hawk-eagles also, a wide range of larger species are within the potential of this bird. The geographic range of the crowned hawk-eagle extends into huge areas of the eastern and southern African savanna, where it depends heavily on various species of antelope and the occasional baboon.
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