Abstract: As a primary bird structure for suppressing flow separation and maintaining high lift, alula has been widely recognized for its control effects. However, the influence of alula on aerodynamic performance and the underlying mechanisms for high-lift wings with large aspect ratios remain underexplored. This study investigates the impact of alula on the lift characteristics and upper-surface flow field structures of wings using aerodynamic force measurements and stereo particle image velocimetry (SPIV) in a wind tunnel. The results show that alula can maintain high lift over a broad range of angles of attack (AoAs), although it reduces the maximum lift coefficient. The maintenance of high lift is primarily attributed to the control of separated flows by the alula. The oblique flow induced by the alula injects energy into the low-speed flow on the wing's upper surface and interacts with the wingtip vortex to enhance its downwash effect. At certain cross-sections, the flow on the wing's upper surface is influenced by the enhanced wingtip vortex near the leading edge and by the oblique flow near the trailing edge, ultimately forming a "reacceleration" flow structure. By altering the spanwise position of the alula, it is found that the alula at the mid-span of the wing induces the strongest oblique flow and achieves the best separation flow control effect. This study provides guidance for the lift-enhancement design of low Reynolds number aircraft, such as UAVs and MAVs.