Limits to Top Speed in Hummingbirds

Vertebrates such as hummingbirds can fly at high speeds. The limit that prevents hummingbirds from flying faster is the amount of forward-direct thrust. Thrust is affected by the kinematics of flight at high speed, such as the maximum wingtip velocity during the wing beat. The wingtip velocity hypothesis states that top speed is limited by purely kinematic factors at higher flight velocities. The muscle power hypothesis states that the size of the pectoralis muscle is a determining factor of the top speed a bird can reach. We tested 25 hummingbirds from 4 species (that vary in body size, wing length, and muscle size) to determine whether muscle size, body size, or wing length is correlated with limiting thrust. Top speed was determined by placing birds in a wind tunnel starting at 9.5 ms-1 and increased in increments of 0.03 ms-1every 10 seconds.A high-speed camera was used to capture wing kinematics at 9.5, 11, and 13 ms-1. In a separate assay, we measured the maximum load lifting capacity in still air. Top speed increased as calculated wingtip velocity increased across species. Hummingbirds that showed an increase in stroke amplitude at the highest airspeeds. Top speed showed a negative correlation with total body weight lifted during the load lifting exercises. This could indicate that wing kinematics alone are responsible for the amount of thrust produced and limits how fast a hummingbird can fly during forward flight