SUMMARY We used a combination of high-speed 3-D kinematics and three-axis accelerometer recordings obtained from cockatiels flying in a low-turbulence wind tunnel to characterize the instantaneous accelerations and, by extension,the net aerodynamic forces produced throughout the wingbeat cycle across a broad range of flight speeds (1–13 m s –1 ). Our goals were to investigate the variation in instantaneous aerodynamic force production during the wingbeat cycle of birds flying across a range of steady speeds, testing two predictions regarding aerodynamic force generation in upstroke and the commonly held assumption that all of the kinetic energy imparted to the wings of a bird in flapping flight is recovered as useful aerodynamic work. We found that cockatiels produce only a limited amount of lift during upstroke (14% of downstroke lift) at slower flight speeds(1–3 m s –1 ). Upstroke lift at intermediate flight speeds (7–11 m s –1 ) was moderate, averaging 39% of downstroke lift. Instantaneous aerodynamic forces were greatest near mid-downstroke. At the end of each half-stroke, during wing turnaround,aerodynamic forces were minimal, but inertial forces created by wing motion were large. However, we found that the inertial power requirements of downstroke (minimum of 0.29±0.10 W at 7 m s –1 and maximum of 0.56±0.13 W at 1 m s –1 ) were consistent with the assumption that nearly all wing kinetic energy in downstroke was applied to the production of aerodynamic forces and therefore should not be added separately to the overall power cost of flight. The inertial power requirements of upstroke (minimum of 0.16±0.04 W at 7 m s –1 and maximum of 0.35±0.11 W at 1 m s –1 ) cannot be recovered in a similar manner, but their magnitude was such that the power requirements for the upstroke musculature(minimum of 54±13 W kg –1 at 7 m s –1 and maximum of 122±35 W at 1 m s –1 ) fall within the established range for cockatiel flight muscle (<185 W kg –1 ).

SUMMARY The avian pectoralis muscle must produce a varying mechanical power output to achieve flight across a range of speeds (1-13 m s -1 ). We used the natural variation in the power requirements with flight speed to investigate the mechanisms employed by cockatiels ( Nymphicus hollandicus ) to modulate muscle power output. We found that pectoralis contractile function in cockatiels was generally conserved across speed and over a wide range of aerodynamic power requirements. Despite the 2-fold range of variation in muscle power output, many aspects of muscle performance varied little: duration of muscle shortening was invariant, and overall wingbeat frequency and muscle strain varied to a lesser degree (1.2-fold and 1.4-fold,respectively) than muscle power or work. Power output was primarily modulated by muscle force (accounting for 65% of the variation) rather than by muscle strain, cycle frequency or changes in the timing of force production relative to muscle strain. Strain rate and electromyogram (EMG) results suggest that the additional force was provided via increasing pectoralis recruitment. Due to their effect on the transformation of muscle work into useful aerodynamic work, changes in wing position and orientation during the downstroke probably also affect the magnitude of muscle force developed for a given level of motor recruitment. Analysis of the variation in muscle force and airflow over the wing suggests that the coefficients of lift and drag of the wing vary 4-fold over the speed range examined in this study.

SUMMARY Birds and bats are known to employ two different gaits in flapping flight,a vortex-ring gait in slow flight and a continuous-vortex gait in fast flight. We studied the use of these gaits over a wide range of speeds (1-17 ms -1 ) and transitions between gaits in cockatiels ( Nymphicus hollandicus ) and ringed turtle-doves ( Streptopelia risoria )trained to fly in a recently built, variable-speed wind tunnel. Gait use was investigated via a combination of three-dimensional kinematics and quasi-steady aerodynamic modeling of bound circulation on the distal and proximal portions of the wing. Estimates of lift from our circulation model were sufficient to support body weight at all but the slowest speeds (1 and 3 ms -1 ). From comparisons of aerodynamic impulse derived from our circulation analysis with the impulse estimated from whole-body acceleration,it appeared that our quasi-steady aerodynamic analysis was most accurate at intermediate speeds (5-11 ms -1 ). Despite differences in wing shape and wing loading, both species shifted from a vortex-ring to a continuous-vortex gait at 7 ms -1 . We found that the shift from a vortex-ring to a continuous-vortex gait (i) was associated with a phase delay in the peak angle of attack of the proximal wing section from downstroke into upstroke and (ii) depended on sufficient forward velocity to provide airflow over the wing during the upstroke similar to that during the downstroke. Our kinematic estimates indicated significant variation in the magnitude of circulation over the course the wingbeat cycle when either species used a continuous-vortex gait. This variation was great enough to suggest that both species shifted to a ladder-wake gait as they approached the maximum flight speed (cockatiels 15 ms -1 , doves 17 ms -1 ) that they would sustain in the wind tunnel. This shift in flight gait appeared to reflect the need to minimize drag and produce forward thrust in order to fly at high speed. The ladder-wake gait was also employed in forward and vertical acceleration at medium and fast flight speeds.

SUMMARY The role of syringeal muscles in controlling the aperture of the avian vocal organ, the syrinx, was evaluated directly for the first time by observing and filming through an endoscope while electrically stimulating different muscle groups of anaesthetised birds. In songbirds (brown thrashers, Toxostoma rufum , and cardinals, Cardinalis cardinalis ), direct observations of the biomechanical effects of contraction largely confirm the functions of the intrinsic syringeal muscles proposed from indirect studies. Contraction of the dorsal muscles, m. syringealis dorsalis (dS) and m. tracheobronchialis dorsalis, constricts the syringeal lumen and thus reduces airflow by adducting connective tissue masses, the medial (ML) and lateral (LL) labia. Activity of the medial portion of the dS appears to affect the position of the ML and, consequently, plays a previously undescribed role in aperture control. Under the experimental conditions used in this study, full constriction of the syringeal lumen could not be achieved by stimulating adductor muscles. Full closure may require simultaneous activation of extrinsic syringeal muscles or the supine positioning of the bird may have exerted excessive tension on the syrinx. Contraction of m. tracheobronchialis ventralis enlarges the syringeal lumen and thus increases airflow by abducting the LL but does not affect the ML. The largest syringeal muscle, m. syringealis ventralis, plays a minor role, if any, in direct aperture control and thus in gating airflow. In parrots (cockatiels, Nymphicus hollandicus ), direct observations show that even during quiet respiration the lateral tympaniform membranes (LTMs) are partially adducted into the tracheal lumen to form a narrow slot. Contraction of the superficial intrinsic muscle, m. syringealis superficialis, adducts the LTMs further into the tracheal lumen but does not close the syringeal aperture fully. The intrinsic deep muscle, m. syringealis profundus, abducts the LTMs through cranio-laterad movement of a paired, protruding half-ring. The weakly developed extrinsic m. sternotrachealis seems to increase tension in the ipsilateral LTM but does not move it in or out of the syringeal lumen.