An Approach to the Functional Analysis of Bill Shape

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Abstract
The combination of cranial kinesis and Mm. protractor pterygoidei serve primarily to distribute the force on the bill resulting from pounding on a tree to the base of the skull, and it does not serve to absorb this force by stretching as was proposed. Stretch of M. protractor pterygoidei decreased the force of impact of the bill against the tree. Whether the bill is in the resting position or fully protracted at the moment of impact; the mechanism by which Mm. protractor pterygoidei remains in isometric contraction and the importance of the reported reduction in mobility of the upper jaw. These points are quite minor compared with the area of agreement; their resolution will probably require difficult experimentation. The term "resilient rigidity" may be the most apt description of the mechanism of shock-absorption by the redistribution of force.