The shafts of feathers and petioles of leaves protect from wind by having non-circular cross sections.


"In cross section, feathers look like grooved petioles upside down. Again, that makes functional sense. If an elongated structure must have a groove to raise EI/GJ ('twistiness-to-bendiness ratio'), the groove should be on the side that's loaded in tension. That location won't increase the structure's tendency to buckle, since tensile loading is nearly shape-indifferent. A leaf blade bends its petiole downward; its aerodynamic loading bends a feather upward--leaf blades hang from the ends of their petioles; flying birds hang from bases of their wing feathers." (Vogel 2003:385)

Comparative Biomechanics: Life's Physical World, Second EditionOctober 23, 2016
Steven Vogel

Journal article
In vivo strains in pigeon flight feather shafts: implications for structural designJournal of Experimental BiologyJanuary 11, 1998
Corning WR; Beiwener AA