Female bald eagles are slightly larger than males. Males range in body length from 30 to 34 inches (76.2 to 86.4 cm), while females measure 35 to 37 inches (89 to 94 cm). The wingspan of a male stretches from 72 to 85 inches (182.9 to 215.9 cm), while a female’s wingspan ranges from 79 to 90 inches (200.7 to 228.6 cm). Bald eagles weigh between 8 and 14 lbs. (3.6 to 6.4 kgs.).
The skeleton of a bald eagle weighs 0.5 lbs (250 to 350 grams), which is only 5 to 6% of the total weight of the bird. The bones are extremely light, because they are hollow, and the feathers weigh twice as much as the bones.
The bald eagle’s average body temperature is 106°F (41°C). They don’t sweat, so they cool themselves in other ways, such as panting, holding their wings away from their bodies, and perching in the shade. In cold weather, an eagle’s skin is protected by feathers which are lined with down. Their feet consist mainly of tendon and are cold-resistant, and little blood flows to the bill, which is mostly nonliving material.
The beak, talons, and feathers of an eagle are made of keratin, the same material as in our fingernails and hair. Because of this, the beak and talons continuously grow and are worn down through usage. An eagle’s beak can be used as a weapon and is sharp enough to slice skin, but is also delicate enough to groom a mate’s feathers and feed a chick. The talons are important for defense and hunting.
An eagle’s call is a high-pitched, whining scream that is broken into a series of notes. They don’t have vocal cords, so sound is produced in a bony chamber called the syrinx, located where the trachea divides before the lungs. Scientists have differentiated four different calls. Eagles are most vocal when they are threatened, annoyed, or mating.
When eagles first fledge, they are mostly brown, except under the wings, which are mostly white. As immature eagles grow, their body coloration changes and they molt and replace feathers each summer. As juveniles mature, their feathers become a mottled brown and white. By three-and-one-half years, the head and tail are nearly all white, and by four-and-one-half, immature eagles are nearly indistinguishable from adults.
Bald eagles have 7000 feathers. Feathers protect them from both heat and cold and offer a barrier to snow and rain. They have several layers of feathers that tightly overlap and provide a solid covering. It is because of this coat of feathers that eagles can spend winters in extremely cold climates. Depending on the ambient temperature, an eagle can rotate its feathers to reduce or increase their insulating effect. They puff up their feathers for a variety of reasons, including preening, to insulate themselves from cold temperatures, to make themselves appear larger when threatened, and when they are sick.
Feathers are of course also necessary for flight and for gliding and soaring. Like the bones, the feathers are hollow and lightweight, but they are structurally very strong. The primaries, the large feathers along the tips of the wings, provide lift and are the main controls for flight. An eagle twists these feathers to brake, turn, and maneuver. The tail feathers are also important for flying, maneuvering, and landing and for stabilizing an eagle when it dives toward prey.
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