Fig. 3. Selected kinematic and mechanical variables in the rufous hummingbird, Selasphorus rufus, as a function of take-off treatment. (A) Peak vertical and horizontal ground-reaction forces (N) and peak accelerations (m s^–2). Vertical forces are net after subtracting g. (B) Velocity (m s^–1) at the end of leg thrust and take-off. (C) Wingbeat frequency (Hz; inverse of wingbeat duration) during the first and second wingbeat in the take-off. N=6 hummingbirds for autonomous and escape and N=4 for aggressive; error bars represent ± S.D.