| The purpose of this study was to determine the force production strategies, aerodynamic and anatomical mechanisms, and ecological and evolutionary significance of the maneuvering flight of birds, with a particular focus on slow, flapping flight. Because unsteady aerodynamics dominate this type of flight, two types of maneuvering performance are herein distinguished: Intrinsic maneuvering performance, which, under a steady-state assumption, is a function of wingloading; and facultative maneuvering performance, which is a function of a bird's ability to develop the high mass-specific power needed to fly slowly, coupled with the ability to simultaneously produce force asymmetries to effect a turn.; Using in vivo bilateral strain gauges on the insertion point of the major downstroke muscle, the pectoralis, the patterns of force asymmetry developed as pigeons (Columba livia) negotiated an obstacle course were examined. Pigeons were found to effect turns by producing a sequential series of force asymmetries, rather than by creating a single force asymmetry to initiate a bank and then flying through the turn using symmetrical downstroke force production.; To determine the aerodynamic mechanisms--and hence, the underlying anatomical mechanisms--by which pigeons produced these observed force asymmetries, the three-dimensional wing kinematics of pigeons were described. Rather than using asymmetries in angle of attack or wing surface area, pigeons used differential downstroke velocities to create changes in body position. The angular momentum of these changes in body position were then arrested--in the same downstroke--by reversing the downstroke velocity asymmetry.; The ability to maneuver in slow flight is suggested to be the adaptive prerequisite for the enormous diversification of birds. Such a radiation would depend largely on the mass-specific power performance of birds. Such performance may be compromised by features (long, high aspect ratio wings) that allow high efficiency in flight-in particular, efficiency in intrinsic (fixed wing) maneuvering performance. A study of the acceleration performance of a group of coursing aerial insectivores suggests that their highly efficient flight morphologies indeed compromise their mass-specific power output, and probably limit their facultative maneuverability. Because of their stilted, one-gait flight, this compromise is greatest in the Apodidae--a biomechanical and evolutionary irony, given their recent divergence from hummingbirds. |
