Burst muscle performance predicts the speed, acceleration, and turning performance of hummingbirds

Abstract

Despite recent advances in our understanding of animal flight, the biomechanical determinants of maneuverability in birds are poorly understood. It is thought that maneuverability is influenced by morphological features such as body mass, wing size, and wing shape, as well as by physiological traits such as muscle capacity. This hypothesis has not been evaluated for any animal because large numbers of measurements of free flight maneuvers from the same individuals have been lacking. We recorded a large number of flight sequences for 20 Anna's hummingbirds (Calypte anna) in a flight chamber to determine if an individual's maneuvering performance is 1) repeatable across trials, 2) associated with morphology, burst muscle capacity, or both, and 3) influenced by the presence of a competitor. Using a multi-camera tracking system, we analyzed performance metrics based on body position and orientation. Most measures were highly repeatable. Burst muscle capacity was associated with most performance metrics, such that birds with higher burst capacity flew with faster velocities, accelerations, and rotations, and performed more demanding complex turns. Wing morphology predicted only a few performance metrics, such that birds with higher wing aspect ratio had higher centripetal acceleration and performed more arcing turns. In the presence of a competitor, birds exhibited faster changes in pitch and altered the types of complex turns used, but surprisingly, they had lower horizontal accelerations. Collectively, these results indicate that burst muscle capacity is a key predictor of maneuverability, and that body angular velocity and arcing turns are associated with competition in flight.