Video of non-planar wing morphing from How pigeons couple three-dimensional elbow and wrist motion to morph their wings.

Wing skeletal motion is non-planar and deviates from the anatomical plane. In the ventral view, the wing bones appear to follow a traditional four-bar ‘drawing parallels’ pattern. However, when we rotate to the anterior view, we see the wing bones move out of plane, demonstrating that the overall motion is non-planar and therefore a 2D ‘drawing parallels’ model is insufficient. Skeletal motion is shown in the reference frames shown in figure 2.