[aerohead]

Each feather on a bird is attached at feather follicles, which contain nerve endings, leading to the brain.
As a bird 'fledges', it's brain 'learns' how to morph its wings and tail, to accomplish it's flight dynamics ( like learning to crawl, walk, and run ).
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Birds are 'most aerodynamic ' ( lowest drag ) during 'bounding flight', or' 'stooping,' where the feet and feathers are held as close as can be to the body, which then takes on the form of a 'streamline body of revolution'. A 'bomb'. Perrigrine Falcon at 244-mph ( 392-km/h ).
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Automobiles would experience no benefit with the inclusion of 'feathers.' And their danger to cyclists and pedestrians would never allow them to make it through the certification process.
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Aft-body length requirements for lowest drag are rooted in the adverse pressure gradient / flow separation relationship. No cheating possible.