Nearly all species of modern birds are capable of flight; therefore mechanical competency of appendages and the rigidity of their skeletal system should be optimized. Birds have developed extremely lightweight skeletal systems that help aid in the generation of lift and thrust forces as well as helping them maintain flight over, in many cases, extended periods of time. The humerus and ulna of different species of birds (flapping, flapping/soaring, flapping/gliding, and non-flying) have been analyzed by optical microscopy and mechanical testing. The reinforcing structures found within bones vary from species to species, depending on how a particular species utilizes its wings. Interestingly, reinforcing ridges and struts have been found within certain sections of the bones of flapping/soaring and flapping/gliding birds (vulture and sea gull), while the bones from the flapping bird (raven) and non-flying bird (domestic duck) did not have supporting structures of any kind. The presence of these reinforcing structures increases the resistance to torsion and flexure with a minimum weight penalty, and is therefore of importance in flapping/gliding birds. Vickers hardness testing was performed on the compact section of the bones of all bird species. The data from the mechanical testing were compared with microstructural observations to determine the relevance behind the reinforcing structures and its mechanical and biological role. Finite element analysis was used to model the mechanical response of vulture ulna in torsion.
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