Biology - Structure and Anatomy

The expression "eagle eye" describes quite accurately one of the unique physical adaptations of the bald eagle. The bald eagle's eyesight is three to four times stronger than that of humans. The superior performance of the eagle's eyesight is due to several anatomical features. First, the eagle's eyes are so large that they occupy most of the space of its head. The large eye allows for a large retinal area where the visual image appears. Second, the eagle's eyes contain a high number of features which allow for visual accuracy and color perception. These features are called cones. Human eyes also contain cones, but proportionately not as many. Because of the high number of cones, as opposed to rods, the eagle's vision is not particularly good at night. A third feature which contributes to the eagle's keen eyesight is that the eagle eye contains two foveae. Humans have only one fovea - one for binocular or forward vision. When we look at an object, both eyes are directed toward the object. This type of vision allows us to distinguish depth. The eagle contains a binocular fovea like ours, but also has a fovea for monocular vision which allows it to look sideways. Finally, because they can only move their eyes slightly within the sockets, they are able to rotate their heads almost 270 degrees. One more unique feature of the eagle eye is that the eagle has a second, transparent eyelid. This membrane protects the eagle's eyes from accidents when feeding young or catching prey.

The eagle's other senses are less impressive. The eagle's sense of hearing is comparable to that of humans. Its outer ear openings are located just behind its eyes. The eagle seems to have a less-developed sense of taste, and its sense of smell is poorly developed. For example, a bald eagle cannot detect even strong food odors under snow. On the other hand, its sense of touch is acute on the beak and feet, a feature that aids the eagle in capturing and killing prey. The bald eagle is well-adapted for hunting. Its feet and talons are large even for such a huge bird. The talons are impressively long and are backed by tremendous muscular force from the legs. The talons can easily penetrate to the bone and even crush some bones. .When the muscles in the legs contract, the tendons in the lower legs tighten and the talons close together. This locking system allows the bird to secure its toes around a catch and keep its body steady during rest and sleep. On the undersides of its fleshy toes are small, rough projections called spicules which help the eagle to grasp and hold slippery prey such as fish.

In addition to its legs, feet, and talons, the bald eagle's beak is massive. Its main purpose seems to be for killing and tearing. The beak may be used as a weapon if the talons have not finished off the prey. However, generally the beak is used for tearing prey into small pieces since eagles do not chew their food. The eagle's mouth allows it to swallow sizeable objects, but the food must be torn into reasonably-sized pieces. After ingestion the food is temporarily stored in a huge sack, called the crop, in the throat. The bones, fur, and other undigestible parts of small animals like squirrels or mice are kept there. Eventually these waste parts congeal and form into a pellet that the eagle spits or casts out of its mouth.

Although poets sing praises of the apparently effortless soaring and gliding of bald eagles, in fact their flight may be awkward and exhausting. Because of their size, eagles need optimal flying conditions to display true elegance. Optimal conditions include enough wide-open space (unhampered by trees or branches), a high take-off point, and good wind conditions. Eagles depend on large masses of warm air called thermals to soar and glide gracefully. The rising air lifts the birds to great heights. When winds pass over hills and mountains, an updraft of air is created along the windward sides. This upward sweep of wind works with the eagle's wing size and span to keep the eagle aloft.

The shape and size of the eagle's wings work with the air currents to keep the eagle flying high. Eagles have wing slots on the ends of their wings - finger-like gaps between the large and primary feathers. These can be adjusted to help the eagle maintain a steady flight pattern, increase lift, and permit precise maneuvering.

Feathers are the eagle's principal adaptation for flight, though scientists believe they may originally have developed to help the eagle maintain body warmth. Eagles have about 7,000 feathers. 30 feathers weigh about as much as a penny.

Another feature which contributes to the eagle's flight is its structure or skeleton. The skeleton of an eagle weighs less than half as much as its feathers. Many people are puzzled to learn that the bald eagle typically weighs only between 8 and 16 pounds. With their massive size and wingspans, they appear monstrous. In fact, the bones of the eagle are hollow.

To learn more about the bald eagle's anatomy and flight, consult the Bibliography section of this web site for materials which describe the eagle's flight and structure in more detail.