We investigate how the barb of bird feathers is changed along both the rachis and barb.To investigate the microstruc-tures and the mechanical behaviors of barbs,a series of barbs are manually cut from an eagle's primary feather to observe the cross sections.A λ-like cross section with a tiny hook is observed at the right feet at each section.Afterwards,a measurement of the setup system is developed to evaluate the leakage ratio of a feather followed by a numerical predicting approach based on the CFD method.It is found that the air leakage increases linearly against the pressure,and the predicted results coincide well with the experimental results.Finally,the influences of leakage of the flight feather on both steady and unsteady aerodynamics are studied.