Limits to sustainable avian flight performance

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We propose to undertake the first detailed scientific studies into the flight biology, migratory physiology and energetics of bar-headed geese in the wild using the latest electronic dataloggering technology. Ultimately, we will address the question of where are the limits to sustainable avian flight performance at high altitudes and what is the effect of body mass? In particular, how do larger species cope during flight with the combined effects of reduced air density, low oxygen availability and decreased temperature? Only a few species of larger birds are thought to be able to sustain long periods of flapping flight at high altitudes and these have received little study. The best known species is the bar-headed goose (Anser indicus) which performs one of the most physically challenging and impressive avian migrations by flying twice a year through the high plateau areas of the Himalayas, with some populations travelling between high altitude breeding grounds in China and lowland wintering areas in northern India. Despite their extraordinary flight performance and immensely interesting physiology and behaviour, neither the aerodynamic or physiological adaptations required to perform such feats are well understood. We will use miniature GPS tracking devices to provide detailed position and altitude during the flights so that we can identify their route in relation to the geographical topography and environmental conditions. This will also allow us to measure their rates of climb when migrating through the mountains. The bar-headed goose migration is exceptional for such a large bird as aerodynamic and biomechanical considerations suggest that as birds increase in body mass flight performance should deteriorate. Thus, bar-headed geese with a body mass of around 2.5 to 3.5 kg should only have a marginal physical capacity to sustain climbing flight even at sea level, and this ability should get worse as altitude increases due to the decrease in air density. By using 3-axis accelerometry we will be able to calculate the net aerodynamic forces acting on the body of the birds and monitor any changes in wingbeat frequency and relative wingbeat amplitude in response to changes in altitude and during the climbing flight. Their flights are also remarkable due to the physiological difficulties of sustaining any kind of exercise while coping with the harsh environmental conditions of the Tibetan plateau, especially the low ambient temperatures and the reduced availability of oxygen. Nevertheless, bar-headed geese have been recorded to fly between 4,000 m and 8,000 m, where partial pressures of oxygen are around 50% that of sea-level and temperatures can be as low as -20 C. We will measure the heart beat frequency of the birds during flights at different altitudes and estimate the maximum efforts expended during climbing flights in relation to their maximum expected capabilities. To place the remarkable migratory flights of the bar-headed goose in context, some 90% of avian migrations over land occur below 2000 m and the majority below 1000 m, which is well below the level of some of the main breeding lakes of the bar-headed goose (4,200 m to 4,718 m). We anticipate that the geographical barrier of the Himalayas should force these relatively large birds to fly close to the limits of their cardiac, muscular, respiratory and aerodynamic abilities. Indeed, this proposal will address the hypothesis that these migratory climbing flights may only by possible with the assistance of favourable up currents of air due to weather fronts or topographical reflections. Recent developments in electronic dataloggers now make it possible to measure both physical and physiological aspects of flight behaviour in free-flying birds rather than in animals constrained by captive conditions. Access to free-flying bar-headed geese would provide a unique opportunity to study the flight biology of a relatively large bird pushed to the extremes of its performance.