Wingtip feathers in birds are aerodynamically efficient because of their torsional flexibility.
"Nature, by contrast, takes a less disdainful attitude toward torsion--in some applications adequate resistance matters, but in many others function depends on having sufficient torsional flexibility. A bird's wingtip feathers must twist in one direction during the upstroke of the wings and in the other direction during the downstroke to keep the local wind striking the wing at an appropriate angle to generate lift and thrust…The turning could be done at the base, with a completely inflexible feather; the aerodynamics are improved and material saved if the local flow forces twist the feather by just the right amount." (Vogel 2003:382)
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