An Approach to the Functional Analysis of Bill Shape

Abstract

Several methods are suggested by which the influence of force, size, and weight on the shape of the avian bill can be analyzed. Torque analysis allows determination of the external forces acting on the bill, including the stresses on the nasal-frontal hinge and on the quad- rate. This method allows inquiry into the factors of external force and size on the shape of the bill. Using torque analysis, the forces on a wood- pecker bill and on a finch bill are described. Trajectory analysis allows investigation of the distribution of forces within the bill and how the bone is arranged best to withstand these stresses. It is suggested that the curvature of the dorsal surface of the upper jaw corresponds to the lines of trajectory. This method allows insight into the factor of weight, assuming that bone is present where stress is greatest, and into the outer shape of the bill. The analysis of external forces suggests that the kinetic structure of the avian skull may have some important functional properties that are not associated with movement of the upper jaw. One of these properties may be distribution of the major stresses on the upper jaw to the ventral part of the brain case rather than to the dorsal and lateral parts, as would be the case in the akinetic condition. The heavy bone of the base of the brain case can resist larger stresses than could the thin roof and sides of the brain case. This distribution of stress to the base of the brain case may also have an important part in the shock absorbing function of the kinetic mechanism.