At a glance, swallows and swifts, both graceful fliers, look much alike. But swifts — like this Chimney Swift — have longer, slimmer wings and short bodies, enabling them to glide for long periods. Their glides are punctuated by rapid, stiff bursts of wing-beats. Swallows, on the other hand, flex and flap their wings. Why do swifts have such a peculiar, stiff wing stroke? Picture a bird wing, with two halves jointed in the middle. Swift wings have a short inner “half” and a long outer half. This causes the stiffness.

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