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Unmanned Aerial Vehicles (UAVs) are proliferating in both the civil and military markets. Flapping wing UAVs, or ornithopters, have the potential to combine the agility and maneuverability of rotary wing aircraft with excellent performance in the low Reynolds number flight regimes. The purpose of this paper is to present new free flight experimental results for an ornithopter equipped with single degree of freedom compliant spines. The compliant spines are designed and optimized in terms of mass, maximum von-Mises stress, and desired wing bending deflections. The spines are inserted in an experimental ornithopter wing leading edge spar, in order to achieve a set of desired kinematics during the up and down strokes of a flapping cycle. The ornithopter is flown at Wright Patterson Air Force Base in the Air Force Research Laboratory Small Unmanned Air Systems (SUAS) indoor flight facility. Vicon® motion tracking cameras are used to track the motion of the vehicle for four different wing configurations. The effect of the presence of the compliant spine on the wings and body kinematics, as well as the leading edge spar deflection during free flight is presented in this paper. Several metrics were used to evaluate the vehicle performance with various compliant spine designs inserted in the leading edge spar of the wings. Results show that passively morphing the wings, via adding compliance in the leading edge spar, does not require additional power expenditure and is beneficial to the overall vertical and horizontal propulsive force production. [ABSTRACT FROM AUTHOR]