The morphology of the primary feather shaft from the pigeon wing has been examined and its salient features noted. The cortex (outer wall) of the shaft appeared to be the most significant feature in relation to bending behaviour and was analysed quantitatively. A model that simulated bending of the shaft was made using this analysis and upon comparison of simulated results with observed bending behaviour it is shown that the shape and size of the cortex does indeed account for the majority of bending behaviour. The model does not include torsional effects and the effects of the pithy medulla and the transverse septa, but the magnitude of these effects is considered to be small in comparison with that of the cortex considered in simple bending. Differences in the shape of the cortex in the outermost primary and those proximal to it are shown to account for different mechanical properties and possible reasons for this are given. The shape and size of the cortex, as measured by its second moment of area, is shown to have some relation to the body weight of the bird.