Turbulence explains the accelerations of an eagle in natural flight

Abstract

Turbulent winds and gusts fluctuate on a wide range of timescales from milliseconds to minutes and longer, a range that overlaps the timescales of avian flight behavior, yet the importance of turbulence to avian behavior is unclear. By combining wind speed data with the measured accelerations of a golden eagle (Aquila chrysaetos) flying in the wild, we find evidence in favor of a linear relationship between the eagle’s accelerations and atmospheric turbulence for timescales between about 1/2 and 10 s. These timescales are comparable to those of typical eagle behaviors, corresponding to between about 1 and 25 wingbeats, and to those of turbulent gusts both larger than the eagle’s wingspan and smaller than large-scale atmospheric phenomena such as convection cells. The eagle’s accelerations exhibit power spectra and intermittent activity characteristic of turbulence and increase in proportion to the turbulence intensity. Intermittency results in accelerations that are occasionally several times stronger than gravity, which the eagle works against to stay aloft. These imprints of turbulence on the bird’s movements need to be further explored to understand the energetics of birds and other volant life-forms, to improve our own methods of flying through ceaselessly turbulent environments, and to engage airborne wildlife as distributed probes of the changing conditions in the atmosphere.

The implications of costly airflows for space-use and movement decisions in birds

“…The fact that turbulence generally followed an approximately-linear relationship with the proxies (particularly for birds but also in some cases for the ultralight proxies), means that relative changes in turbulence should be straightforward to approximate. The simple relationship is also in line with previous work that identified a linear correlation between the turbulence spectrum and the spectral composition of acceleration recorded onboard a golden eagle in gliding flight (Laurent et al, 2021).…”

“…The impact of atmospheric turbulence on flying animals represents an important frontier (Bomphrey and Godoy-Diana, 2018, Laurent et al, 2021, Quinn et al, 2017, as the effects on flight energetics and route selection are far less studied than those of wind Alerstam, 1995, Liechti, 2006). Turbulence is broadly defined as a measure of rapid changes in wind velocity at small scales and is driven by mechanical forcing or surface heating.…”

“…It is clear from flying in aircraft that turbulence causes changes in altitude and body motion, suggesting that high-frequency data from animal-attached loggers could also be used to provide insight into the level of turbulence experienced by animals in flight. Indeed, Laurent et al (2021) showed that the vertical displacements of the body accelerations of a single 5 kg golden eagle (Aquila chrysaetos) were similar to those of the motions of the turbulent air.…”

Honeybees modify flight trajectories in turbulent wind

“…For example, mosquito's prefer not to fly in turbulent conditions [1], possibly associated with the high accelerations in the flow [2,3]. Recent studies have investigated the behavior of small organisms such as planktonic gastropod larvae in turbulence [4] or of larger birds such as a golden eagle in atmospheric turbulence [5]. The ability for animals and especially insects to navigate turbulent flows while in flight has been studied to determine how they are capable of maintaining flight stability and control [6].…”

An Instrumented Golden Eagle’s (Aquila chrysaetos) Long-Distance Flight Behavior

“…While not specific to the golden eagle, this assumption is supported by studies of GPS-instrumented Andean condors, with the finding that the condor flapped its wings for only 0.8% of the migration flight [ 35 ]; (b) The eagle did not dive, especially at or near 2000 m AGL. Any rapid and short-lived descents were likely due to turbulence-scale (<10 m) vertical velocities [ 36 ]; and (c) The eagle is migrating and not foraging.…”