Kinematic stages of the display dive of the male Anna's hummingbird, as indicated by high-speed video and sound recordings. Also see videos in the electronic supplemental materials. (a,b) Composite images from high-speed videos of birds diving towards and then past the camera. Consecutive images are 0.0125 s apart (a) or 0.01 s apart (b). See table 1 for descriptions of the five (1–5) stages of the dive kinematics. (c) A sound spectrogram of the sounds produced during the course of the dive.

Position, velocity and acceleration profiles of diving Anna's hummingbirds. 1–5 indicate the kinematic stages of the dive (figure 1, table 1). (a) Two inertial coordinate systems were used in the analyses of diving male Anna's hummingbirds: a static, global coordinate system with the Y_global axis opposite to gravity (g), X_global horizontal in the direction of the dive (solid lines), and Z normal to X and Y; quasi-bird-based coordinate system (dotted lines) in which the X_bird and Y_bird axes were rotated by Θ° about the Z axis, so the X_bird axis was parallel to the bird's instantaneous velocity (V_bird), and tangent to the bird's XY trajectory (dashed line). This coordinate system was inertial (the origin did not move). (b) Bird trajectories at the bottom of the dive. Solid black line: a representative trajectory. Faint lines: all 26 individual dive trajectories. (c) Average velocity of diving Anna's hummingbirds in X_global, Y_global and overall speed (black line) over the course of the dive (n = 9 sets). Bars are ±1 s.d. Dives were aligned temporally at the instant Y velocity = 0. (d) Average accelerations of diving Anna's hummingbirds in X_bird and Y_bird (n = 9). X_bird acceleration is the bird's tangential acceleration and corresponds to changes in speed (black line) shown in (c). The Y_bird acceleration is centripetal acceleration and corresponds to changes in direction that cause the curved trajectory evident in (b).