Feathers are subject to large aerodynamic forces during flight and must therefore be stiff and strong, yet also light. Published estimates of Young's modulus of the feather keratin composite vary widely in the literature, and it is suggested that this is predominantly due to test methodology rather than true interspecific differences. The factors influencing the Young's modulus of keratin are discussed, together with their functional implications. Damage resistance must also be good; feathers that are lost or broken can only be replaced infrequently, but regularly, at moult. Feather shafts appear to be very strong for their weight. The possible role of the medullary foam in preventing buckling failure is still unclear; medulla removal only appears to have a minimal effect on both the stiffness and strength of whole sections of the rachis. Melanic keratin has properties that make it more competent at resisting abrasive wear than nonmelanic keratin.