Muscle function and energy use during bird flight

Understanding the manner in which the metabolic and mechanical power requirements of bird flight change as a function of speed represents a critical step to identifying the morphological features upon which natural selection may have acted to reduce the energy required for flight. Existing empirical measurements of avian power requirements have shown one of two relationships with flight speed. Birds either require high power outputs to fly at fast and slow flight speeds but are more economical at intermediate speeds, a U-shaped power curve, or they have power requirements that are independent of speed. Unlike birds, all other airborne objects exhibit only U-shaped power curves. The experiments described in this dissertation explore the long-standing question of whether some species of birds have developed mechanisms to reduce the energetic requirements of flight.; To address this issue and to examine the differences in muscle function that are implied by different power curves, I selected four avian species whose power curves were expected to differ qualitatively. I trained budgerigars (36.7 +/- 2.1 g), cockatiels (80.5 +/- 5.6 g), magpies (188.1 +/- 4.1 g) and kestrels (125.5 +/- 6.7 g) to fly in a variable speed wind tunnel and obtained flight measurements of energy liberation and muscle function. I found that the metabolic and mechanical power curves of these species were speed dependant, despite their musculature using different functional strategies to supply this power.; Finally, by comparing the rates of metabolic power consumption to mechanical power production, I obtained the first in vivo measures of efficiency for vertebrate muscle. These results challenge the existing understanding of the relationship between metabolic and mechanical power for birds in flight and vertebrate muscle in general. These results further reveal that published theoretical values for muscle efficiency underestimate the ability of muscle by a factor of two, and that birds actively lengthen their muscles to obtain higher than predicted values of efficiency.