Department of Biosciences, College of Science, Swansea University, Swansea SA2 8PP, UK
CEFE UMR 5175, CNRS, Université de Montpellier, Université Paul-Valéry Montpellier, EPHE, 1919 route de Mende, 34293 Montpellier Cedex 5, France.
J Exp Biol. 2018 Dec 3;221(Pt 23):jeb174995. doi: 10.1242/jeb.174995.
Many large birds rely on thermal soaring flight to travel cross-country. As such, they are under selective pressure to minimise the time spent gaining altitude in thermal updrafts. Birds should be able to maximise their climb rates by maintaining a position close to the thermal core through careful selection of bank angle and airspeed; however, there have been few direct measurements of either parameter. Here, we apply a novel methodology to quantify the bank angles selected by soaring birds using on-board magnetometers. We couple these data with airspeed measurements to parameterise the soaring envelope of two species of vulture, from which it is possible to predict 'optimal' bank angles. Our results show that these large birds respond to the challenges of gaining altitude in the initial phase of the climb, where thermal updrafts are weak and narrow, by adopting relatively high, and conserved, bank angles (25-35 deg). The bank angle decreased with increasing altitude, in a manner that was broadly consistent with a strategy of maximising the rate of climb. However, the lift coefficients estimated in our study were lower than those predicted by theoretical models and wind-tunnel studies. Overall, our results highlight how the relevant currency for soaring performance changes within individual climbs: when thermal radius is limiting, birds vary bank angle and maintain a constant airspeed, but speed increases later in the climb in order to respond to decreasing air density.
许多大型鸟类依靠热气流翱翔飞行来进行跨地区迁徙。因此，它们受到了选择压力，需要尽量减少在热上升气流中上升的时间。鸟类应该能够通过仔细选择倾斜角度和空速，将位置保持在热气流核心附近，从而最大限度地提高爬升率；然而，这两个参数的直接测量值很少。在这里，我们应用一种新的方法，使用机载磁力计来量化翱翔鸟类选择的倾斜角度。我们将这些数据与空速测量值相结合，为两种秃鹫物种的翱翔包络进行参数化，从而可以预测“最佳”倾斜角度。我们的研究结果表明，这些大型鸟类在初始爬升阶段，即热上升气流较弱且狭窄时，通过采用相对较高且保守的倾斜角度（25-35 度）来应对上升高度的挑战。倾斜角度随着高度的增加而减小，这与最大化爬升率的策略基本一致。然而，我们研究中估计的升力系数低于理论模型和风洞研究预测的值。总体而言，我们的研究结果强调了在单个爬升过程中，翱翔性能的相关货币如何发生变化：当热半径有限时，鸟类会改变倾斜角度并保持恒定的空速，但在爬升后期，速度会增加，以应对空气密度的降低。
