It was by observing birds rather than insects that we yearned, and, eventually, learnt (how) to fly. Even so, the wings of our most remarkable jets are really no match for the wings of birds. Aircraft wings are stiff, thin, metallic (or composite material) sheets, which only provide lift: thrust for forward motion and the generation of lift is separately provided by jet engines or propellers. A bird’s wing ingeniously combines both: providing power for motion and lift for flight.
Feathers, which first developed to provide insulation, are what separate birds from every other living creature. A bird has several different types of feathers for different purposes — one of the most important, of course, being those which enable it to fly. A bird’s flight feather is basically an aerofoil — long and slightly cambered, so that when you whiz one through the air horizontally, the air moves faster on its top surface than its lower surface, causing it to automatically rise. In addition, the stiff outer (leading) edge of the feather as compared to the relatively softer, more pliable inner (trailing) edge causes the feather to twist during a downward motion, taking on the shape of a small propeller and providing thrust — or forward motion. I actually tried this: I held up a feather stiffly and horizontally, and brought my arm down -— I was astonished to find my hand and arm automatically moving forwards.
The bird’s wing may be compared to the human arm, complete with elbow, wrist and fingers. It’s attached to the breastbone (the sternum) by massively powerful pectoral muscles, which are responsible for the power (down) stroke of the wings; a smaller, less powerful muscle provides the power for the “reloading” upstroke, for which the bird bends its wings at the “elbow” to reduce resistance. These muscles, anchored to the sternum, may be heavier than the bird’s skeleton. The “primaries”, around a dozen, long splayed-out feathers at the tips of the wings — and the equivalent of “fingers” — provide both propulsion and lift; it is these feathers that turn into tiny propellers as they whoosh down. The soft “secondary” and “tertiary” feathers attached to the trailing edge of the wing are the main providers of lift.
If you’ve observed a pigeon or any bird come in to land, you might have noticed a couple of stray feathers suddenly sticking out from the leading edge of the wings, just ahead of the primary feathers. This is the “alula” — rather uncharitably called the bastard wing, and is basically the “thumb” equivalent of the hand. Its purpose is to reduce the drag which builds up untenably, as the bird slows before touchdown. Air currents build up in front of the wing causing turbulence as the bird slows down, and when the alula opens up, it allows the air a smooth, swift escape. It’s rather like the confusion roiling in front of, say, a cinema theatre, where only one door is open and everyone is pushing to get in. Then, if all the doors are suddenly opened, the crowd moves in quickly and easily. The deeply-slotted primary feathers of birds that like to soar — eagles and vultures — do the same thing by allowing air currents to slide through them easily, enabling them to soar with languid grandeur. We have adapted this for aircraft, too: watch the wings of a plane just before touchdown — the wing slats open, smoothing the airflow over it, reducing drag.
The difference in air pressure between the upper and lower surfaces of the wing — which is responsible for lift — actually poses a bit of a problem at the wing tips. Here, air tries to “leak” from the higher pressure area underneath the wing, to the lower pressure area above. The air current at the top of the wing deflects a little inwards and that beneath it, deflects outwards. These two currents of air cross each other at the trailing edge of the wing, forming a series of small vortices which combine to form one large vortex at the wing-tip, which is called the induced drag. This maximises just before stalling, when the pressure difference between the upper and lower surfaces is at its maximum, and the angle of attack (the angle the wing is set to the horizon) is greatest.
One way of reducing induced drag is to make the wings very long and tapering, so there is less area at the wing tips for induced drag to form. Long distance flyers like gulls and albatrosses have such wings, as do gliders. Swifts and falcons have long, swept-back tapering wings for fast, long distance flying.
Broad, stubby wings can also provide more lift, having a comparatively lower weight to support — but they cause more surface friction — and are not tenable at low speeds, and are, thus, suited more to swift, short-distance flyers. They are good for birds such as woodland raptors (small hawks and owls, for example) which need to twist and turn at high speeds between trees and branches while pursuing prey, and their victims who need to evade them!
The power for the wings is provided by the muscles. There are two main types, burning different fuels. Birds, like partridge and chicken — game birds — have white flight muscles (hence, white meat), which burn the carbohydrate glycogen that provides explosive bursts of energy, but being anaerobic, fatigues rapidly. Pigeons and other long-distance flyers burn fat, which needs a constant supply of oxygen from the blood. Their muscles are consequently darker — hence the “red” meat. Eagles and other soaring birds, though, have the best of both worlds.
Wings really mean everything to most birds: watch a baby bird industriously exercise its wings, while jumping up and down in its nest; watch its first nervous take-off (with much encouragement from its parents) and the inevitable crash landing. Then, just a day later, watch the exultant way in which it zooms around and you’ll know you have seen one of life’s wonders. So, for God’s sake, don’t put a bird in a cage.