Avian wing morphology : intra- and inter- specific effects on take-off performance and muscle function in controlling wing shape over the course of the wing stroke

Take-off is how a bird initiates flight and is important for predator evasion and therefore survival. A birds’ take-off ability is affected by the mechanical power available from the flight muscles, the size and shape of the wings, the mechanical control of the wings and therefore the wing beat kinematics, and the body weight that needs to be supported. How a birds’ body weight, size, shape and dynamic motions of their wings interact with the air flow during flight, will determine flight performance and trade-offs in wing morphology are likely due to different morphologies being optimal for different types of flight, impacting on the ecology and behaviour of birds. Understanding the power required to overcome drag and support an animals’ body weight during flight is only one side of the flight equation. The mechanical performance of the flight muscles determine the power available for creating thrust and lift, by powering the wing stroke, as well as controlling the shape and the postural position of the wings. The mechanical work and power output has been found to vary with flight speed when studying the main depressor and pronator muscle, the pectoralis. However, the mechanical performance in terms of work and power production, and therefore the mechanical function, is not known for the intrinsic wing muscles. The shape and position of the wings during the wing beat are determined by the muscles of the wing, affecting the wing beat kinematics and flight performance. Whether the mechanical function of the wing muscles varies with flight type is also unknown. Determining the aerodynamic power required and the muscular power available for different types of flight, were examined separately. The effects of intra- and interspecific variation in wing morphology and wing beat kinematics on take-off performance were examined. Two high-speed digital cameras were used to track the position of a birds’ centre of mass and wing positions spatially and temporally as they took off to determine flight velocities, accelerations and wing beat kinematics so as to calculate the aerodynamic power required. The relationship between the power used to increase the rate of the potential and kinetic energy of a birds’ bodies’ centre of mass and the birds’ wing morphology and wing beat kinematics were tested. The effects of phylogeny were included in inter-specific comparisons. The mechanical function of two intrinsic wing muscles; the biceps brachii and the scapulotriceps, were investigated in vitro using the work-loop technique. The muscles were stimulated with strain and activity patterns that had previously been measured in vivo during take-off, level and landing flight. This meant that the mechanical work and power output, and therefore the muscle function, could be determined. Whether the biceps and scapulotriceps mechanical function varied relative to flight mode was also investigated. Body mass did not influence take-off performance intra-specifically but some wing morphological and kinematic traits did scale isometrically inter-specifically. However, when examining take-off performance both within and between species; large, broad wings favoured improved take-off ability by reducing the power needed to generate lift. Short wings were also beneficial as this correlated with higher wing beat frequencies which improved take-off. Larger species had lower induced power requirements than expected for their size and therefore could devote more energy to moving their body’s centre of mass. Even when differences due to phylogeny were accounted for, wing morphology was important for take-off performance and affected the wing beat kinematics, altering the aerodynamic forces generated and loads acting upon the wings, impacting on take-off performance. The biceps brachii and scapulotriceps co-contract and are involved in the postural position of the elbow, flexing and extending the elbow respectively during muscle shortening and therefore affecting the wing shape during the wing beat cycle. Both muscles also act as elbow stabilisers and decelerators during active lengthening of the muscles, doing negative work. Net work is near zero for the biceps and positive for the scapulotriceps. The work done by the biceps is less than the work done onto the scapulortriceps, suggesting the involvement of other muscles or wing inertia in providing the energy recovered by the scapulotriceps when lengthening. The mechanical power output of these muscles is stereotypical, regardless of mode of flight.