Vultures respond to challenges of near-ground thermal soaring by varying bank angle

Abstract

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 Gyps 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.

Automated extraction of bank angles from bird‐borne video footage using open‐source software

“…In the past, footage obtained from animal‐borne cameras was mainly considered useful in the public domain or for educational purposes with low scientific potential (Moll et al ). However, more recently, back‐mounted cameras have been used on captive Gyps vultures to study the effect of bank angle on soaring flight (Williams et al ). Video cameras have also been used in conjunction with IMUs and ground‐based video to estimate the body and tail movements of a Steppe Eagle ( Aquila nipalensis ) in flight (Gillies et al ).…”

“…Although inertial measurement units (IMUs), including accelerometers, magnetometers, and gyroscopes, have been used to study the flight behavior of several species of seabirds (Boyd et al , Bograd et al ), data from these devices do not come without their own challenges. Accelerometer data are not appropriate for studying the posture of dynamic‐soaring birds due to the increased gravitational pull while banking (Williams et al , ), and although magnetometer data can be used to infer average bank angle of soaring birds (Williams et al , ), the data are complex and difficult to use (Williams et al ). Gyroscopes measure angular velocity (which can be used for bank‐angle measurements) at a very fine scale, but are subject to drift, require very high sampling rates (40–100 Hz), have high power consumption, and are extremely sensitive to changes in temperature (Ettinger et al , Williams et al ).…”

Certainty and integration of options in animal movement

“…Even in passive movement there is quantitative information [25]. For a soaring bird to exploit an updraft it must adopt a specific bank angle and turning radius which, along with the bird's own vertical speed and according to the laws of physics, reflect the strength (vertical velocity) of the underlying updraft [26,27]. Importantly, these active and passive behaviours cannot be false or dishonest because movement can only ever be the combined result of locomotion and the environmentfor instance, 'soaring' animals (birds or aquatic species exploiting underwater updrafts) would otherwise drop in altitude or depth if they displayed soaring behaviour but without the support of an updraft [18,28].…”

Soaring styles of extinct giant birds and pterosaurs

“…For thermal soaring, it is also unknown how much updraft animals experience at a given turning radius or the distance between thermals. Recent advances in tracking technology have made it possible to record details of the motion of birds in dynamic and thermal soaring (35)(36)(37)(38)(39).…”