The Elements of Evolution (23-8) Wings & Flight

Wings & Flight

There is a surprising diversity in avian wings, the shape of which affects how a bird flies and how far it can fly at a stretch. Whereas round wings aid maneuverability, long wings facilitate sustained flight. Wings further reflect lifestyle in how birds use their wings: to flap, glide, or dive.

Birds that migrate long distances have long, thin wings. Most such birds spend part of their lives in the higher latitudes. Bird wings generally become shorter and rounder the closer to the equator a bird lives. Such wings abet flying in forests and amid thick brush, when ready lift and maneuverability are essential.

Like the forelimbs of amphibians, reptiles, and mammals, avian wings are tripartite: an upper arm (brachium), forearm (antebrachium), and hand (manus). The upper arm and forearm are elongated to afford the attachment of the muscles and flight feathers needed to fly.

The ratio of total wing area to body weight – wing loading – determines the ease with which a bird can fly. As avian wings are more curved above than below, air flows faster across the top as a bird flies. This lessens the pressure above the wing (Bernoulli’s principle), providing the lift which keeps a bird aloft.

Flight is a matter of counteracting weight with lift and drag with thrust. Avian wings and bodies are superbly designed for what birds can do on the wing.

The feathers on the wing tips can be compared to the spread fingers of a hand. The wing tip generates several small air vortices instead of one large vortex. It requires more energy and costs more to lift off when only one large wing tip vortex is generated. ~ Swedish evolutionary ecologist Anders Hedenström

For forest birds, the most important aspect of flight is the ease with which wings can deliver lift (as opposed to thrust, which only becomes significant in sustaining flight). The emphasis on lift meant evolutionary measures to reduce weight. The most common approach has been to lessen overall density while increasing volume: minimal bodies amid an impressive halo of feathers. Although a lightweight feathery cloud reduces density, it does so at a cost increasing air resistance (drag), which ups the energetic cost of fast or sustained flight.

Lower density comes in handy in a fall. Perching birds such as the wood duck or bufflehead, which roost in trees, depend upon the fluff that protects their young when they drop from the nest. Some fledglings must drop 10–20 meters when they mere balls of down, long before they can fly.

Weight is partly reduced via temporary reductions in soft tissues. Reproductive organs do not grow until a bird reaches sexual maturity, and then shrivel after breeding season. There is even evidence that part of a songbird’s brain shrinks when such birds are not singing.

Avian skeletons have been more permanently trimmed via simplification, which often involves fusion. Articulating joints are relatively heavy, as they contain fluids and pads of connective tissue. A reduced number of such joints is especially widespread among forest birds. Many avian fused joints are in the head, replaced by flexion areas of exceptionally thin bone. The best known of these flexion areas connects the upper beak to the braincase.

There are exceptions. Parrots need a strong skull to peel tough-skinned fruits, and so have retained many independent bones in their heads. Many other birds ingest food whole, and so may dispense with unneeded flexion complexities.