Flight and Anatomy

 

Northern Mockingbird

Birds amaze us with their ability to fly. With the aid of a camera we can isolate some of the intricacies of movements that they combine into coordinated flight.

Great Egret taking flight, Folly Cove

Getting into the air typically requires a burst of propulsion from the bird's legs and wings. The necessary musculature is concentrated near the center of its body for aerodynamic reasons, and the power translated by rope-like tendons to working parts in the limbs.

Generating lift

Of course overpowering gravity requires strength and energy, but the cross-sectional shape of the wing contributes inherently to lift, a principle of physics that lowers pressure above a curved surface and raises pressure below it as air rushes past in forward motion. A bird's wing is pulled upward into this 'dynamic vacuum' by the passing air simply as a result of moving forward. It has to generate that forward motion either through its own effort, or by plunging with its wings outstretched, or by borrowing the force of the wind. In takeoff it may be able to blend all three phenomena. The upward force of lift operates mainly on the broader, stiffer, more cambered parts of the inner wing with its relatively fixed feathers. This principle of lift has been carefully applied to aircraft design.

Blue Jay in forward stroke

In their power stroke birds reach down and forward with primary feathers closed to force air past their wings for lift as well as for propulsion. Even in level flight their bodies, when flapping, are oriented on a somewhat up-facing diagonal in the direction of thrust.

Black-capped Chickadee

Feathers are controlled to various degrees by muscles at their point of attachment to the skin and skeleton. The primaries, located past the bend in the wing in the area analogous to our hands, are controlled individually by rotating, spreading, and altering angles. Within a fraction of a second they loop through the figure-eight cycle of a wingbeat.

Great Cormorant

This dexterity extends to the tip of the wing with "fingertip" subtleties of guidance.

Cooper's Hawk

The Accipiter group of hawks have evolved long tails to facilitate their twistings and turnings in pursuit of prey through woodlands, as distinct from the broad-winged Buteos that excel at soaring over open ground and swooping out of the sky.

Eastern Wood-pewee

Most birds lose aerodynamic lift in an upright flight posture. They can maintain verticality only briefly and with great effort.

Ruby-throated Hummingbird

Hummingbirds can "stand still" in the air with propeller-like wings cycling as much as 80 times per second supported by rapid metabolism and a heart rate exceeding1,000 beats per minute. In proportion to their size they use a vast amount of calories while feeding.

Only tiny but well-muscled creatures hover in this way, supported by extraordinary respiratory and circulatory systems.

Bufflehead landing

All birds need a technique for returning to earth, mobilizing their wings and legs in something of a reversal of takeoff maneuvers. While water landings can be relatively cushioned and random, learning to come in precisely on a perch demands fair practice from juveniles.

Barn Swallow feeding its young
on a Halibut Point quarry cliff

Evolutionary success for birds has balanced opposing factors such as flexibility and durability, lightness and strength, individuality and reproduction. Respiratory sacs dispersed within hollowed bones reduce lung size to conserve core space for denser organs. They get similar advantage by processing food with lightweight beaks and a gizzard rather than heavy teeth away from their center of gravity. 

These features and many more can be explored in a fine online encapsulation How Birds Fly by the University of Wisconsin, or in the encyclopedic essays offered by local resident Chris Leahy in The Birdwatcher's Companion.