If you look up on a windy day on Kodiak Island, you will likely see several eagles soaring high in the sky. Bald eagles are built for flight, particularly for soaring and gliding. An eagle expends a great deal of energy flapping its large wings, so to conserve energy when gaining and maintaining altitude, it utilizes thermal convection currents or “thermals,” which are columns of warm air generated by terrain such as mountain slopes. It has been estimated a bald eagle can reach flying speeds of 35-43 mph (56-70 kph) when gliding and flapping and 30 mph (48 kph) while carrying a fish. While not known as particularly fast fliers, eagles can soar and glide for hours at a time.
The construction of an eagle’s wings and tail make soaring and gliding possible. The wings are long and broad and are covered by a layer of lightweight feathers arranged to streamline the wing. The primary feathers, or primaries, provide lift and control an eagle’s flight during turning, diving, and braking. An eagle can tilt and rotate individual feathers to maneuver and brake. The tail also assists in braking and stabilizes the eagle when it dives toward prey. While soaring, tail feathers spread wide to maximize surface area and increase the effect of updrafts and thermals.
When an eagle finds an air current or a thermal, it can gain altitude without flapping its wings. If it is dead calm with no air currents moving up or down, eagles cannot soar, and that is why you see more eagles soaring on windy days or sunny afternoons and sitting on their perches on calm, cool mornings.
When a young eagle first leaves the nest, its wing and tail feathers are longer than those of an adult. As an eagle matures, its wing and tail feathers become shorter and narrower with each successive molt. The larger wings of a juvenile make it easier for the bird to catch an updraft or weak thermal and to fly slower and in tighter circles than an adult. The downside of the larger wings and tail is the juvenile rises slower, sinks faster, and cannot soar as far as the adult. Adult bald eagles can flap their wings faster and fly at a greater speed than immature eagles, making them more efficient at chasing down live prey.
Female bald eagles are larger than males, and while their wings are also slightly larger, the larger wing size does not make up for the increased weight of the female. Therefore, females require more wind or stronger thermals than males to be able to gain altitude and soar. Since thermals are weaker during the morning and evening hours, females are more likely to remain on their perches during these times and soar when it’s windy or in the afternoon when thermals are stronger.
An eagle’s large wings make landings and takeoffs tricky, and landing on a perch is something eagles manage to do gracefully only after much practice. A newly-fledged juvenile looks very awkward when it tries to land on a perch and may even crash land or swing upside down if it grabs the perch while it still has too much forward momentum.
An eagle’s acute vision allows it to see prey while soaring high in the air. The eyes of an eagle are larger than those of an adult human, and an eagle’s eyesight is at least four times sharper than that of a human with perfect vision. An eagle flying at an altitude of several hundred feet can spot a fish under water. The eyes are protected by a nictating membrane, and each eye has two fovae or centers of focus, letting the bird see both forward and to the side at the same time. Eagles have binocular vision, so they can perceive depth, allowing them to judge how far away their prey is when they begin a dive.
Next week, I’ll write about what bald eagles eat and how they hunt. Once again, I want to remind you to sign up for my free monthly Mystery Newsletter and read about true crime in Alaska.