), but these were not included in the present analysis. A pair of 9.5-mm-diameter spherical retroreflective markers was fixed to either end of each perch to identify the perch axis.We used Vicon Nexus 2.7.6 software to reconstruct the positions of the markers within the flight volume, using a coordinate system corresponding to the principal axes of the flight hall.
A centripetal lift force is necessary to produce the convex trajectory that these assumptions imply, and a tangential drag force is assumed to dissipate kinetic energy throughout the flight. The integrated drag losses must be exactly balanced by the bird’s integrated aerodynamic power output , net of any transfer of gravitational potential energy.
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