Meeting Abstract
Hummingbirds differentially modify hovering flight kinematics depending upon body size and the type of challenge imposed. Flight in hypodense air is accomplished with increases in wingbeat frequency and stroke amplitude. Increases in angular velocity of the wing may be facilitated by lower air density and, thus, the costs of accelerating the wing and overcoming drag. Hovering failure occurs at an amplitude of 180° suggesting a morphological or mechanical constraint. Conversely, sustained weight lifting is achieved by increasing stroke amplitude (up to 160°), with only relatively large species increasing wingbeat frequency. We hypothesize that metabolic constraints play a greater role in limiting performance during weight lifting. We predict that aerobic hovering scope available for flight, calculated as the ratio of oxygen consumption rate near failure to unweighted, normodense hovering flight, would be higher during weight lifting than during air density reduction trials, as weight lifting would illicit metabolic rates closer to maximal limits. Hovering metabolic rates were recorded from four species of hummingbird at three elevations in Brazil while undergoing sustainable weight lifting trials. Metabolic rates increased with increasing elevation and mass, but max-recorded rates were invariant across elevations within each species. Aerobic hovering scope was greater during weight lifting than during previously reported air density reduction trials. Thus, in support of our hypothesis, sustained weight lifting is more limited by constraints on metabolic power of lift production while failure during hypodense flight trials occurs when metabolic rate is still lower than can be achieved.
