Emulating periodic main wing movement of a bird for generating lift and thrust remains a significant challenge in developing a robotic bird.The sequences of main wing motion are comprised of flapping,folding,bending,and twisting.In this paper,we concentrate on the flapping and folding motion,and design two wing mechanisms based on a 4-bar linkage structure:one is only for Flapping Motion (FM) and the other is for simultaneous Flapping and Folding Motion (FFM) during a wing stroke.We derive relationships between length and angle of links to analyze kinematic characteristics of the mechanisms and conduct an optimization to select the length parameters of links that allow maximization of the flapping angle.We run a simulation to confirm the performance of the optimized parameters by examining physical properties,and fabricate two wing mechanisms accordingly.In particular,the folding motion is achieved without using an additional actuator.Force measurements to investigate a lift profile of each mechanism and their quantitative comparison of the performance of both types confirm the benefits of the folding motion in the perspectives of wing frequency and lift.We expect that our kinematic formulation,design procedures,and comparative measurement results can help develop a wing mechanism to create a truly biomimetic robotic bird.