Fig. 1. Types of cranial kinesis in birds. (A) Prokinesis, (B) distal rhynchokinesis, (C) central rhynchokinesis. Skull outlines are given in lateral view. Black triangles indicate the main area of rotation, multiple triangles in a single element indicate the boundaries of an bending zone. The triangle in A indicates the nasal–frontal articulation. Arrows indicate the lateral (nasal) bar of the upper bill, which is incomplete in C. Ligament is shown in gray. (Adapted from Zusi, 1984.)