Identifying birds of prey in flight

Bald Eagle or Turkey Vulture? Is that a Peregrine Falcon or something else? Understanding body plans can be helpful in identifying soaring, stooping, and flying birds. Coming to our After the Fledge party in Decorah? Get ready for Turkey Vulture or Not with this article! 

Bald eagles are soaring generalist hunters that eat almost anything they can catch. Peregrine falcons are energetic, acrobatic flyers that specialize in catching birds in the air. Both are birds of prey, but their body plans and wing shapes result in very different lives.

Body plans, size, and flight

Bald eagles are built for soaring, with long, broad, slightly-rounded wings, large wing slots, and broad, wedge-shaped tails. They tend to hold their wings flat in flight, migrate during the day, and use wind corridors – geographical features that concentrate and amplify wind –  whenever possible. A stiff tail wind will send migrating eagles aloft in the thousands, especially over surfaces with little opportunity for thermal soaring. The bald eagle body plan and low-aspect wings – i.e. large, broad wings relative to its overall surface area – are most suited to low-angle, low-energy soaring flight.

Peregrine falcons are built for speed and maneuverability. Their long, narrow, pointed wings and long tails are shaped for diving, twisting, and turning in flight. Where eagle flight is flat, peregrines often fly in a series of arcs as they dive, dash and pursue other birds. The peregrine body plan and high-aspect wings – narrow, pointed wings relative to its overall surface area – is most suited to high-speed, high-energy flight.

Bald Eagle Silhouette Falcon Silhouette
Bald Eagle Peregrine Falcon
Bald eagle and peregrine shapes reflect very different lifestyles
Bald Eagle, Turkey Vulture, or Osprey?

Body plans and flight styles are helpful in identifying similarly-sized birds of prey at a distance. Is the bird large and flying flat, holding its wings in a vee and tilting as it flies, or M-shaped? If you are in northeastern Iowa, the first is probably an eagle, the second is probably a turkey vulture, and the third is probably an osprey.

Still not sure about bird ID? Some more helpful questions: Are the bird’s wings large or small in proportion to the rest of it? Are they pointed or rounded? Is the bird’s tail long or wedge-shaped? Is the bird flapping and gliding, diving, hovering, or quartering low over a field? Did you see it in the woods or in the open? Combining field marks with a knowledge of bird behavior, shape, and range will improve your bird ID skills immensely. 

Silhouettes of birds of prey in flight

Silhouettes of birds of prey in flight from

Body plans, hunting, and prey base

All birds of prey have keen vision, talons, and curved beaks to help them hunt, kill, and eat prey. But not all beaks, talons, and hunting styles are the same. The peregrine falcon’s speed and maneuverability makes it uniquely suited to catching birds in flight, while a bald eagle’s size, strength, powerful feet, and long talons make it an excellent generalist hunter. Both birds take full advantage of their very different body plans when it comes to catching and eating prey.

Peregrine Falcon in a stoop A Bald Eagle Catching Fish
Peregrine Falcon Bald Eagle
Peregrine Falcons: High speed, high angle striking

A peregrine falcon’s speed and maneuverability allow it to fly high, dive steeply, and hit prey so hard that the force of impact severs its prey’s spinal cord. A peregrine begins its dive by rolling, cupping its wings around its body, and tucking in its feet, yielding an aerodynamic raindrop that slices through the air. Special cone-shaped bones in its nostrils – an adaptation unique to peregrine falcons – allow it to breath while diving at speeds of over 200 miles per hour. As the falcon approaches its prey, it extends its feet, brakes sharply, and snatches it out of the air with its long, slender toes and sharp talons. If hitting a bird doesn’t kill it, peregrine falcons use their tomial tooth – a special notch in their beaks that bald eagles don’t have – to sever their prey’s spine. While a peregrine’s feet are strong and quick – great for grabbing and slashing attacks – they don’t have the crushing strength of a bald eagle and their diet is largely restricted to other birds.

Bald Eagle: Low speed, low angle snatching

A bald eagle’s large size, soaring flight, and strong feet allow it to take a wide variety of prey. Compared to a peregrine falcon, its speed of attack is slower, its angle of attack is lower, and it usually kills with its feet. As we’ve seen at the fish hatchery, eagles swoop shallowly over the retaining pond, braking heavily as they plunge their feet into the water and pull out trout. Without stopping, they fly into a tree, on to the bluff, or into the nest, crushing or stabbing the trout with their powerful feet and sharp talons. Although they have spicules – rough bumps that help them grip slippery fish – bald eagles don’t specialize in any one kind of prey, and their size, strength, powerful feet, and fishing ability let them catch and eat a wide variety of prey.

Turkey Vulture: No flapping required!

A turkey vulture “V” is more correctly referred to as a dihedral. Turkey vultures are masters of soaring without flapping as they ride the wind in search of carrion. As wind strikes one wing or another, tipping the vulture right or left, one wing tips high and the other tips low. Wind flows under the low wing, pushing the vulture and righting it until it tips again. This allows them to exploit the smallest of air currents as they soar lowly and slowly through the sky. While they sacrifice some maneuverability, their food – carrion – doesn’t require agility to catch.

Again, learning about body plans and flight behavior helps us understand birds and identify them in the field. I find it to be more helpful than looking for hard-to-see features if a bird is far away.

Hashtag #musing: could body plans impact gregariousness? Off their breeding grounds, bald eagles are quite gregarious. Their flashy colors, large, visible body plans, feeding habits, and wind-seeking behavior often bring them into proximity with other eagles. On or off their breeding grounds, peregrine falcons are loners. While there can be multiple peregrines in a site with an abundance of food, they aren’t gregarious. Perhaps their more solitary behavior is driven in part by a body plan that results in a restricted prey base (leading to serious food competition), no real benefit to stealing or attempting to steal food, and less congregation around important dynamic and thermal soaring points.

Did you know?

Harriers also engage in dihedral flight, but their food – small mammals, reptiles, amphibians, and birds – requires considerably more agility. Compare their body plan with that of a turkey vulture. Dihedral flight allows them to soar very slow and very low, but they have a long tail and wings more like a falcon, which helps them roll and twist when needed. Visit Cornell’s website to learn more about them!

People often compare birds to aircraft, with mixed results. Aircraft can’t change their shape, but birds can and often do change shape as they fly! I chose silhouettes that I thought best represented each bird overall, but birds might adopt different flying styles under different conditions, even if they can’t change their overall body plan.

Things that helped me learn about this topic