Featherweights and Talon Tips

We knew from our data collection and quantitative analysis of predation on non-human primates that eagles and hawks are the major and most competent predators on primates worldwide.3 Our research found that 46% of published eyewitness accounts of primate predation relate to raptor kills. And a good portion of these primate kills were the work of crowned hawk-eagles. In what seems, from our study at least, to be a fairly "typical" attack by a crowned hawk-eagle, the bird initially launched itself into flight approximately 500 feet away from feeding vervet monkeys. Using tree cover to make an undetected approach, it suddenly burst skyward from the canopy with a monkey grasped in its talons. In another instance, a crowned hawk-eagle snatched a colobus monkey from a tree less than 5 feet above an observer's head, driving its talons straight through the monkey's cranium in the process.4

We found that 81 species of raptors in all are known or alleged to prey on primates. The raptor category, we should add, includes diurnal species of hawks, eagles, and falcons, plus owls (the nocturnal equivalent of the daylight-hunting birds of prey) and other miscellaneous carnivorous birds (crows, toucans, and shrikes).

The most singular feature of birds is their feathers. Many members of the animal kingdom fly—bats, butterflies, mosquitos—but only birds have feathers; parenthetically, many birds do not fly—penguins, ostriches, kiwis—but they all possess feathers. Many birds of prey are to a large degree "all feathers"; efficient killing machines that weigh little and, with very few exceptions, take prey that is half or less than the bird's own weight.5 The crowned hawk-eagle (and certain other large raptors) represents an exception to this rule—it kills extremely large prey for a raptor, even forest antelopes and adult male colobus monkeys. Eagles support the large loads they must carry back to a nest through the unequaled structure of their feathers. Feathers—an aerodynamic marvel—prevent any "stalling" or sudden breakdown in the lifting power of the wings by securing smooth airflow onto the wing's top surface.6 Most raptors have ten primary feathers (located farthest from the body on each wing) and twelve secondary feathers (those closest to the body). The responsibility of the primaries is to propel the bird through the air with each beat of the wing. The secondary feathers create lift, allowing a bird to maintain altitude during flight as the wings beat.7

Nocturnal raptors (the owls) usually hunt from perches and at fairly close quarters, without the impressive variety of adaptations found in diurnal raptors.8 Nevertheless, one hallmark adaptation of the owl is as impressive as it is understated. Wing feathers of diurnal birds are sharp and well defined, but owl feathers have soft, fringed edges to cut down on noise. The result is silent flight that surprises prey as the owl swoops through the night air.9 Herein may lie the perception of owls as bad omens found in many African cultures. As one famous ornithologist noted: "Man fears the night for, like a baboon, he is then at a disadvantage; and as a result the creatures of the night tend to be disliked and feared."10

For the diurnal birds of prey, sight is the most important of all the senses. Great speed, high visual acuity, and deadly accuracy make them astonishing predators. They possess full-color vision and the ability to rapidly adjust the focus of their eyes while moving at tremendous speeds. What is truly remarkable is the "resolving power" of the raptor eye— what can only be described, in essence, as a hyper-resolution of an image that the lens casts on the retinal surface of the eye.11 The raptor eye possesses a resolving power four to eight times greater than the human eye. Using the lower of these estimates means that a soaring eagle can see a rabbit at a distance of 2 miles. The late Leslie Brown, a Kenyan ornithologist who literally wrote the book on large raptors, gave a real-life example of raptor optics in one of his numerous volumes: If a grasshopper is placed on a contrasting background, the human who placed it (and therefore, knows its location) cannot see the insect at greater than 35 yards; a bird of prey, on the other hand, can detect a green grasshopper in green grass at greater than 110 yards when it has no knowledge that there may be an insect in that spot.12

Strong legs and powerful feet equipped with sharp curved talons, in conjunction with a hooked bill, define all birds of prey. The upper bill, especially of an eagle, can be 4 inches in length, with a menacing pointed hook overlapping the flattened lower bill. Hooked, curved beaks are not tailored for obtaining prey but, instead, are universally used by hawks, eagles, and even vultures to tear apart the flesh of prey already killed or scavenged.13 Again, the crowned hawk-eagle exceeds the norm. When scientists saw a dramatic crowned hawk-eagle attack on a subadult mandrill, they were amazed that the eagle had the baboon on the ground, holding onto its prey with its talons, and striking repeated blows at the mandrill's head with its beak.14

Raptor feet, however, are the actual killing apparatus. A raptor's feet are dangerous weapons, and their power is exceptional. Upon impact, the talons are performing three functions simultaneously: (1) they drive into the body of the prey, (2) they contract and close, and (3) they exert a crushing pressure. All of these actions work in concert; but the blow, or force of impact, delivered by the feet in a strike is at least as important as the piercing, gripping, or crushing force of the talons. And it is the relative size, curvature, and thickness of talons—along with the length of the toes—that will vary among birds of prey with the type of food consumed.15

Raptors that prey on good-sized mammals have thick toes, well-curved talons, and large tarsi (a raptor tarsus is the anatomical equivalent to a human ankle). It is this combination that gives the legs of a raptor such strength. Raptor talons probably serve as much to provide anchorage for the powerful grip as to be used as daggers for incapacitating prey. The inner and hind toes are the most powerful and work in opposition to each other, applying the main grip to prey. Crowned hawk-eagles have exceptionally thick, powerful legs and short thick toes that end in very strong rigid talons. Their talons can wrap around the top of a branch that is as thick as a man's thigh, and their "killer" hind claw is the diameter of a man's little finger.16

Tom Struhsaker, a primatologist who has consistently incorporated predation into his ecological studies of African forest monkeys, commented about the extreme precision of a crowned hawk-eagle attack. Picture this: The crowned hawk-eagle not only has the power and the momentum, the surprise and the speed, but those great talons are used with such exactitude that the heart of the prey is the target. When freshly killed monkeys were examined by Struhsaker and his colleagues, the heart was pierced in three out of three examinations. In one juvenile monkey the heart was triple-pierced from a single deathblow; the talon went in one side of the heart, came out the other side, and—achievable only because of the camber of the eagle's talon—curved back and reen-tered the heart once again.17

African Fish Eagle Talon

Feet and talons are the raptor's killing devices. This illustrates the massive legs, feet, and talons of harpy eagles and crowned hawk-eagles in comparison to other large eagles: (a) harpy eagle; (b) Bonelli's eagle; (c) African fish eagle; (d) crowned hawk-eagle; (e) Asian black eagle. (Redrawn from Brown 1977)

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