Fig. 1. The relationship between the frames of reference of the instruments during the experiments (A, level; B, inclined; C, vertical) and the global frame of reference. The force plate is sensitive in the normal plane (perpendicular to the plate) and in the parallel plane (plane of motion). The accelerometers are sensitive to acceleration in the dorso-ventral (D-V) plane and the anterior-posterior (A-P) plane, relative to the bird's back. During inclined WAIR (B), weight support is no longer measured from a single axis. Both during a single wing beat and across different inclines, the changing orientation of the bird's back requires that the accelerometers be analyzed from within the bird's frame of reference (i.e. the A-P and D-V planes).