Limits to sustainable avian flight performance

我们建议利用最新的电子数据记录技术，对野生斑头雁的飞行生物学、迁徙生理学和能量学进行首次详细的科学研究。最后，我们将解决的问题是，在高海拔地区，鸟类可持续飞行性能的极限在哪里，以及体重的影响是什么？特别是，大型物种在飞行过程中如何应对空气密度降低、氧气供应不足和温度下降的综合影响？人们认为，只有少数大型鸟类能够在高海拔地区长时间拍打翅膀飞行，而这方面的研究很少。最著名的物种是斑头雁（Anser indicus），它们每年两次飞越喜马拉雅山的高原地区，这是对身体最具挑战性和最令人印象深刻的鸟类迁徙之一，有些种群在中国的高海拔繁殖地和印度北部的低地越冬区之间迁徙。尽管它们有非凡的飞行表现和非常有趣的生理和行为，但它们所需要的空气动力学和生理适应能力都没有得到很好的理解。我们将使用微型GPS跟踪设备提供飞行过程中的详细位置和高度，以便我们能够根据地理地形和环境条件确定它们的路线。这也将使我们能够测量它们在山上迁徙时的爬升速度。对于这样一种大型鸟类来说，斑头雁的迁徙是例外的，因为空气动力学和生物力学的考虑表明，随着鸟类体重的增加，飞行性能应该会恶化。因此，体重在2.5 ~ 3.5 kg左右的斑头雁，即使在海平面上，也应该只有维持爬升飞行的边际物理能力，而且由于空气密度的降低，这种能力会随着海拔的升高而变差。通过使用3轴加速度计，我们将能够计算作用在鸟类身体上的净空气动力，并监测随高度变化和爬升飞行过程中翅拍频率和相对翅拍幅度的变化。由于在青藏高原恶劣的环境条件下，尤其是环境温度低和氧气供应不足，他们的飞行在生理上很难维持任何一种运动，因此他们的飞行也很引人注目。然而，斑头雁的飞行高度在4000米至8000米之间，那里的氧分压约为海平面的50%，温度可低至-20摄氏度。我们将测量斑头雁在不同高度飞行时的心跳频率，并根据它们的最大预期能力估计在爬升飞行过程中所花费的最大努力。为了说明斑头雁的显著迁徙飞行，大约90%的鸟类在陆地上的迁徙发生在2000米以下，大多数发生在1000米以下，这远远低于斑头雁的一些主要繁殖湖的水平（4200米至4718米）。我们预计，喜马拉雅山脉的地理屏障应该会迫使这些相对较大的鸟类接近它们的心脏、肌肉、呼吸和空气动力学能力的极限。事实上，这一提议将解决这样一个假设，即这些迁徙的爬升飞行可能只有在有利的上升气流的帮助下才有可能，这是由于天气锋面或地形反射。电子数据记录仪的最新发展现在可以测量自由飞行的鸟类的飞行行为的物理和生理方面，而不是被圈养条件限制的动物。自由飞行的斑头鹅将提供一个独特的机会来研究这种被推向极限的相对较大的鸟类的飞行生物学。

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.