The Eagle
He clasps the crag with crooked hands;
Close to the sun in lonely lands,
Ringed with the azure world, he stands.
The wrinkled sea beneath him crawls;
He watches from his mountain walls,
And like a thunderbolt he falls.
-- Alfred, Lord Tennyson
Eagles and their relatives, the hawks and
falcons, have long been a source of wonder. The golden eagle's mastery of the
wind, the piercing eyes of a Cooper's hawk on its kill, the baleful glare of
a hunting red-tailed hawk or the tremendous speed of a peregrine in full stoop
have intrigued falconers and inspired poets. Their regal appearance and haughty
manner have made them the emblems of kings and empires. Now, as their numbers
decline and as some species seem destined toward extinction, man's regard for
the magnificent birds of prey is tinged with concern for their survival.
Those raptors which are active during daylight belong to the Order Falconiformes, a group that originated about 75 million years ago. At one time they were considered closely related to the nocturnal raptors, the owls. Better understanding of differences in anatomy, physiology and behavior, however, show that the two orders are not closely related. Their similarities are the result of parallel evolution whereby similar structures evolved in unrelated animals adapting to similar life styles.
The Order Falconiformes comprises 290 species worldwide. Of the 33 in North America, 19 can occur in Missouri. Our birds of prey vary enormously in size, from the bald eagle--which can weigh as much as 15 pounds and have a six- to eight-foot wingspan--to the tiny American kestrel, which weighs only three or four ounces.
Raptors have evolved to perform certain basic functions from their ecological
positions at the top of food chains. As Aldo Leopold noted, with typical poignancy:
"The hawk, as a lethal agent, is the perfect flower of that still utterly
mysterious alchemy--evolution. No man-made machine can, or ever will, synthesize
that perfect coordination of eye, muscle and pinion as he stoops to his kill."
In general, birds of prey remove the old, young, sick or surplus individuals
of small- or medium-sized animals with high reproductive capabilities. By removing
the inferior individuals from a population of prey species, raptors help control
the numbers of these animals, maintaining them at levels that best can be supported
by an area of land, and ensuring that only the strong, healthy animals pass
genes on to their progeny.
Because birds of prey are at the top of food chains, generally large and easily observable, they are excellent indicators of ecological diversity and environmental health. Abundance and variety of raptors in a given area demonstrate the presence of a healthy assortment of plants and animals on which they ultimately depend for survival. Thus, habitats managed for raptors benefit countless other wildlife species as well. These majestic birds live in every type of habitat, from mountains to plains and from dense forest to farmland. Because the various kinds of eagles, hawks, falcons and vultures have different feeding habits, they have developed different styles of flight--flapping, gliding, hovering, soaring and diving. They are among the most spectacular fliers of the entire bird world.
Raptors share several physical characteristics, including exceptionally keen eyesight, a sharp, hooked beak for tearing flesh, and strong, grasping feet with large, sharp talons for killing and holding prey. Only the vultures have weak feet.
The size and curvature of the talons, as well as the length of the toes, varies according to the type of prey. Bird-hunters such as sharp-shinned hawks have long, thin toes, while mammal-eaters like red-tailed hawks possess shorter, thick, powerful toes. The legs of most species are protected by rough scales. Bald eagles and ospreys have sharp, needlelike scales on their feet, which help them get a firmer grip on their fish prey.
Many raptors, like this red-tailed hawk, attack from a long, shallow dive so as not to alert their prey.
The bills also vary in relation to food preferences, but to a lesser degree than the feet. The hooked beak typically is only used to pull apart prey already killed by the feet and is rarely used for defense. Anyone who has handled live hawks knows that it is the feet that are dangerous. In falcons and some insect-eating kites, the upper mandible has a notch in the edge which is used to sever the spinal cord of prey.The raptors' vision may well be the keenest of any living creatures. The unparalleled resolving power of eagles, hawks and falcons comes partly from their large, rounded eyeballs. A red-tailed hawk's eyes are nearly as large as a human's, though the bird may weigh only three pounds. Such size results in a relatively large image cast upon the retina. The retinas have a dense concentration of cones and a great many optic nerve fibers. The area of critical focus, known as the fovea, in the back of a hawk's eye has 1,000,000 visual cells or cones per square millimeter, compared to 200,000 in a human eye. Even more impressive, the hawk's eye has two foveas in the retina where the highest concentration of visual cones occur, while we have but one. Thus raptors can see eight times better than man and have an exceptional ability to judge distance.
Bird Identification
View from the ground - Field observation of hawks is often just impressions of light and dark markings on the birds as they pass overhead. The birds shown below are relative in size, with wingspread and length of adults in inches.
Key
- Merlin: W-25" L-12"
- Peregrine Falcon: W-40" L-18"
- Cooper's Hawk: W-32" L-17"
- Northern Harrier (Marsh Hawk): W-42" L-20"
- Red-tailed Hawk: W-50" L-18"
- Swainson's Hawk: W-52" L-20"
- Bald Eagle: W-80" L-32"
- Northern Goshawk: W-42" L-24'
- Osprey: W-65" L-22"
- Red-shouldered Hawk: W-40" L-19"
- Northern Rough-legged Hawk: W-56" L-22"
- Black Vulture: W-60" L-22"
- American Kestrel: W-22" L-10"
- Prairie Falcon: W-40" L-16"
- Sharp-shinned Hawk: W-24" L-13"
- Mississippi Kite: W-36" L-14"
- Broad-winged Hawk: W-33" L-16"
- Golden Eagle: W-85" L-35"
- Turkey Vulture: W-72" L-27"
The fierce expression characteristic of raptors results from a projecting, bony shield above each eye. This shield helps protect the eye from tree limbs and brush when the birds are in pursuit of dodging, feinting prey. It also shades the eye from the sun's glare and from damage due to struggling prey or clumsy feeding attempts by overzealous nestlings.
In many of the Falconiformes, the female is larger than the male. This difference is greatest in the more aggressive species -- the larger falcons, accipiters and eagles--and is less pronounced among the kites, kestrels, vultures and those hawks that feed on insects, rodents, reptiles or carrion. The term tiercel denotes a male falcon. It derives from the Latin tertius meaning "third." Males often are that much smaller than their mates. Indeed, in some species the female can be nearly twice as large.
Most Falconiformes nest in relatively high, inaccessible locations in trees or on cliffs, though some, like the northern harrier, nest on the ground. Hawks and eagles build stick nests, while falcons dig a shallow scrape for their eggs on a cliff ledge, in a tree cavity or in a nest of another bird. Vultures lay their eggs on the ground inside a cave, log or tree.
The larger species are potentially long-lived. There are many examples of large vultures and eagles living 30, 40, even 50 years. Probably the oldest bird in Missouri is a 47-year-old male golden eagle residing at the Raptor Research and Rehabilitation Project at Tyson Research Park near St. Louis. In the wild, the life span of these great birds usually is much shorter, frequently attributable to man.
Birds of prey have been seriously threatened by man's activity for much of this century. Major factors contributing to the decline of raptors include indirect and direct persecution, pesticide poisoning and habitat loss.
Indirect persecution results from the unwitting violation of a raptor's need for solitude, especially during the nesting season. Direct persecution involves shooting, poisoning and trapping--widespread in the 1950s and '60s. Though all birds of prey now are protected by state and federal laws, shooting remains a significant problem in many states. It appears to have declined in Missouri due to a growing appreciation for conservation, although as late as 1977, 17 bald eagles were shot in our state within a four-month period. Open, baited traps intended for other animals also pose a problem.
The unrestricted and indiscriminate use of pesticides which began soon after World War II has been the single most important cause of the near-disappearance of several species of raptors. With continued use, increasingly large quantities of long-lived pesticides like DDT entered ecological food chains world-wide. These poisons become increasingly more concentrated as they pass link by link, animal to animal, up these food chains until the highest concentrations accumulate at the top--in fatty tissues of birds of prey. Bird-eating raptors such as falcons and accipiters, and fish-eaters like the osprey and bald eagle are especially affected by sublethal doses of DDT. The results are decreased productivity due to altered nesting behavior, addled or thin-shelled eggs, and even eggs with no shell at all. DDT and some other organochlorine pesticides now are banned in the United States, and some birds, such as the bald eagle, appear to be making a comeback.
The most widespread and constant cause of decline in birds of prey is habitat loss. In the face of marshes, swamps, prairies and forests lost to farming, logging and urban development, many no longer nest in Missouri or nearby states.
Raptor conservation can be achieved in two ways--preventive and constructive. We can protect existing birds by creating better public understanding, by stronger law enforcement and harsher penalties, by discontinuing the use of environmentally unsafe pesticides, and by conserving adequate habitats.
Vultures are normally shy of people, but this immature turkey vulture was camera tolerant
The second method involves management techniques to stimulate population recovery. These include habitat improvement projects in breeding or wintering areas and rehabilitation programs for sick and injured birds. For some critically endangered species like the peregrine falcon, captive breeding projects produce birds to be reintroduced to former haunts. In recent years, it has been found that artificial nest structures may help stimulate significant recoveries in certain raptors. Artificial nest structures are readily used by ospreys in Florida and New England and by prairie falcons in Canada. Barn owls utilize boxes in silos and barns, while American kestrels and screech owls are attracted to nest boxes in many areas.
Recently, the Department of Conservation has been involved in several studies and restoration projects involving birds of prey, including studies of American kestrels and of the wintering habits of bald eagles. Artificial nests have been provided for bald eagles, barn owls and kestrels; young eagles and ospreys are being released in an attempt to restore nesting populations.
© 1985 Missouri Conservation Commission
Content revision: 20030728
