Meeting Abstract
Diverse bird species exhibit a flap-bounding pattern in flight, wherein the bird periodically interrupts flapping by folding its wings and torpedoing through the air. According to theory, this flight style is aerodynamically inefficient, and, thus, its functional significance is unclear. Two hypotheses (fixed gear; Rayner, 1985; minimal activation-deactivation; Usherwood, 2016) predict that the major avian flight muscle, the pectoralis, should be used in a fixed manner to maximize efficiency (mechanical output:metabolic input). However, previous empirical research reveals that flap-bounding birds vary contractile behavior of their pectoralis across flight speeds and modes, and that wing kinematics provide a useful, non-invasive method for estimating this variation. Body mass fluctuates dramatically within birds as a function of migratory status, reproductive state and feeding rate. Also, wildlife researchers frequently add transmitters to birds to study movement patterns. The effects of variation in mass upon flap-bounding behavior and the implications for the muscle-based hypotheses are unknown. To begin to reveal the effects of mass upon flap-bounding, we added 10% of body weight to zebra finch (Taeniopygia guttata; n = 5) flying at 10 m s-1 in a wind tunnel. Consistent with predictions of both muscle-function hypotheses, the addition of weight did not cause significant changes in average wingbeat amplitude, wingbeat frequency, percent time in downstroke or downstroke velocity. Furthermore, weighted birds showed a significantly greater proportion of time spent flapping. However, within-individual differences in amplitude and downstroke velocity varied by up to 50% of the unweighted mean, which calls into question the degree to which the wingbeat can be considered “fixed”.
