A small digit on a bird's wing, usually bearing feathers, that helps with flight control and landing.
From Latin alula, diminutive of ala (wing), literally meaning 'little wing.' This tiny structure is a vestigial thumb that birds retained for aerodynamic purposes.
The alula is proof of evolution—birds inherited it from dinosaur ancestors who had actual thumbs, and it's only a few tiny feathers now, but it dramatically improves landing and slow-speed flight.
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