Magpie mechanical power output II: Sonomicrometric measurement of DPC excursion.

(In prep)

The Flight Laboratory's previous investigation into the mechanical power output of magpies (Dial et al. 1997) assumed that the excursion of the wing accurately represented the excursion of the humerus at the delto-pectoral crest - the movement of which being a critical component in the calculation of mechanical work and, hence, power. In the present study, we sought to infer the excursion of the DPC by measuring the fiber length changes of the prime mover of the humerus, the pectoralis, using sonomicrometry rather than wing kinematics. Strain gauges fixed to the delto-pectoral crest were again used to measure the force of the pectoralis contraction. Mechanical work (change in fiber length x force of contraction, producing a workloop) over time is power. For purposes here, uncalibrated strain gauge data and fiber length changes were used to calculate relative work and power at different flight speeds. The change in 'power' over the full range of flight speeds is shown in the second figure.

The figure on the left is overall fiber length strain for the three magpies by flight speed. Strain (DL/L), or total change in fiber length over resting length, for the magpies at low speeds (0 - 4 m s-1) were similar to those reported for pigeons in slow flight by Biewener et al. (In press). The figure on the right represents 'power' for three magpies flying over their full range of speeds. Uncalibrated relative power is normalized by calculating the power at each speed relative to maximum, which always occurred at hovering.

Magpie mechanical power output over a range of speeds as assessed by the kinematic method and the sonomicrometric method. Kinematic power data are from Dial et al. 1997.

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