" The flaps mimic a group of feathers, called covert feathers, that deploy when birds perform certain aerial maneuvers, such as landing or flying in a gust. Biologists have observed when and how these feathers deploy, but no studies have quantified the aerodynamic role of covert feathers during bird flight ":

https://www.sciencedaily.com/releases/2024/10/241028164339.htm

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