ALTSHULER, Douglas L.; DUDLEY, Robert; California Institute of Technology; University of California, Berkeley: The physiology and biomechanics of bird flight across elevations.

Many birds fly at high altitude, either during long-distance flights or by virtue of residence in high elevation habitats. Among the many environmental features that vary with altitude, five may have significant consequences for the biomechanics and physiology of bird flight: wind speed, temperature, humidity, oxygen availability, and air density. During migratory flights, birds select flight altitudes that minimize energy expenditure but for the nocturnal roosting flights of swifts, air temperature exerts a strong influence on altitude selection. Oxygen partial pressure decreases substantially to as little as 26% of sea-level value at the highest altitudes at which birds migrate. Accordingly, birds exhibit numerous adaptations in pulmonary, cardiovascular, and muscular systems for coping with severe hypoxia. Decreases in air density impose increased aerodynamic costs by limiting lift production. This effect has been studied in the hovering flight of hummingbirds, which occur throughout high elevation habitats in the western hemisphere. Across elevations and among taxa, hummingbirds compensate for hypodense air morphologically through increases in wing size, and kinematically with increased stroke amplitudes. These compensatory mechanisms lead to constant power requirements in hovering across elevations, but decrease the reserve capacity for powering flight.