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. These muscles, anchored to the sternum, may be heavier than the bird’s skeleton. 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. The bird’s wing may be compared to the human arm, complete with elbow, wrist and fingers. 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. 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. 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. 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. Feathers, which first developed to provide insulation, are what separate birds from every other living creature.
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