Bats and birds have evolved very different ways of flying. Birds have stiff feathers projecting from lightweight, fused arm and hand bones; bats have flexible wings of membranes stretched between elongated fingers. While birds use the strong downstroke of their wings to fly, bats support their weight on the upstroke, too, by twisting their wings backward. Bats tend to be more agile in flight, while many birds can fly with greater speed.

More info and transcript at BirdNote.org. 

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