Bird flight energetics and the mechanical performance of the muscles that power and control flight

Although bird flight is the most energetically costly form of locomotion, it remains one of the most economical, allowing birds to cover relatively large distances and occupy a wide range of niches across all continents. This makes the study of flight energetics important for understanding bird behavioural ecology, especially for migratory species that must carefully manage metabolic energy expenditure during long journeys. According to aerodynamic theory, mechanical power output during flight follows a U-shaped relationship with flight speed. This power is predominantly generated by the pectoralis muscle, responsible for wing depression during the downstroke, while other muscles in the wing itself serve to control the shape of the wing, which is important in slow, manoeuvring, take-off, and landing flights. This thesis primarily examines the metabolic input and mechanical output of the main flight muscle during level, steady flight, and concludes with an exploration of the function of the scapulotriceps muscle, an intrinsic wing muscle, during various flight modes. Measurements of metabolic power using respirometry exhibited a U-shaped power-speed curve. The measurements of metabolic power were used to validate acceleration data measured using back-mounted accelerometers, as an indirect measure of energy expenditure. The findings suggest that accelerometry provides a proxy that can reliably estimate flight metabolic power during free flight. The strategies used to modulate pectoralis power output were assessed using in vivo recordings of muscle strain and activation patterns. These data were then replicated in a novel in situ work loop preparation to estimate force and power output directly, where no U-shaped relationship was found. Finally, the mechanical function of the scapulotriceps was investigated in situ, confirming its hypothesized role in elbow support during wing flexion. This work demonstrates that accelerometry can be utilized as a robust proxy for accurate estimation of metabolic power in the field and also provides the first in situ evaluation of the function of the scapulotriceps muscle across different flight modes.