TOBALSKE, B.W.*; ALTSHULER, D.L.; POWERS, D.R.: Take-off mechanics in hummingbirds (Trochilidae)

Among birds species studied to date, leg forces contribute proportionally more to initial take-off velocity than lift from the wings. Hummingbirds use their wings in a different manner compared with all other flying birds, and, as members of the Apodiformes, hummingbirds possess relatively small hind limbs. We undertook this study to test whether take-off in hummingbirds is accomplished using primarily the legs or wings and to evaluate whether take-off performance varies according to motivational state. During take-off in rufous hummingbirds (Selasphorus rufus, 3.3 g, n = 6), we measured vertical and horizontal ground-reaction forces using an instrumented perch. We used high-speed video (1000 Hz) to study wing and body kinematics. Birds took to flight within a mesh corridor (2 x 1 x 2 m) for one of three reasons: to feed, to respond to a startling human motion, or to chase a conspecific away from a feeder. Consistent with their unique anatomy, hummingbirds initiated flight differently than other birds. Hummingbirds beat their wings 2 – 4 times before their feet were off the perch. Their legs contributed 65% of velocity during take-off. In contrast, other species are only halfway through their first down stroke as their feet leave a perch, and forces transmitted via their legs contribute 85 -100% of take-off velocity. There was a significant effect of motivational state on take-off performance in hummingbirds. For example, take-off velocity was greater during startle and chase take-off (0.78 and 0.80 m/s, respectively) than during feeding take-off (0.54 m/s). Although performance during take-off to feed appeared distinct, differences between startle and chase take-off were generally not statistically significant. (Supported in part by Murdock 99153)