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Fig. 1. Types of cranial kinesis in birds. (A) prokinesis, (B) distal
rhynchokinesis and (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 nasalfrontal articulation. Arrows indicate
the lateral (nasal) bar of the upper bill, which is incomplete in C. Ligament
is shown in grey. (Adapted from Zusi,
1984.)