Turbulence explains the accelerations of an eagle in natural flight

Abstract

Significance Although soaring birds spend a significant portion of their lives navigating turbulent environments, the role turbulence plays in their lives is unclear. This holds especially when turbulent air currents fluctuate on similar timescales as avian flight behaviors. Against a backdrop of complicated behavior and aerodynamics, we find that a simple linear model almost fully explains the interactions between an eagle and turbulence, and that this interaction, in turn, almost fully explains the accelerations of the eagle’s body throughout its airborne life. The turbulent nature of the atmosphere is not merely a subtle nuance to which the eagle adjusts but rather a fundamental characteristic that leaves its imprint on all modes of flight. 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.