In contrast to the upright trunk in humans, trunk orientation in most birds is almost horizontal (pronograde). It is conceivable that the orientation of the heavy trunk strongly influences the dynamics of bipedal terrestrial locomotion. Here we analyze for the first time the effects of a pronograde trunk orientation on leg function and stability during bipedal locomotion. For this, we first inferred the leg function and trunk control strategy applied by a generalized small bird during terrestrial locomotion by analyzing synchronously recorded kinematic (3D x-ray videography) and kinetic (3D force measurement) quail locomotion data. Then, by simulating quail gaits using a simplistic bio-inspired numerical model which made use of parameters obtained in in vivo experiments with real quail, we show that the observed asymmetric leg function (reflected in left-skewed ground reaction force and touchdown lift-off limb asymmetry) is necessary for pronograde steady-state locomotion. In addition, steady-state locomotion becomes stable for specific morphological parameters. For quail-like parameters, the most common stable solution is grounded running, a gait preferred by the quail and most of the other small birds. We hypothesize that stability of bipedal locomotion is a functional demand that, depending on trunk orientation, constrains basic hind limb morphology and function.